Battery Selection for an NB-IoT Motor Control Project

Task Overview

The goal is to select the most suitable type of rechargeable battery for a pack rated at approximately 24 V with a capacity of about 10 Ah. The battery will be used very infrequently to power a DC motor: the motor is started roughly once every few months and runs for 15–20 minutes, after which the battery remains idle with no load for long periods.

Key requirements are high energy density and long service life, with a strong emphasis on retaining charge and preserving usable capacity during extended storage. Weight and size are not critical, but higher stored energy is preferred.

The battery must also operate reliably across a wide temperature range: –10 °C to +70 °C, including extreme storage and operating conditions.

Battery Technologies Under Consideration

The following rechargeable battery types are being evaluated:

• Lithium-ion (Li-ion) based on metal oxide chemistries:

  • NMC (Nickel Manganese Cobalt)

  • NCA (Nickel Cobalt Aluminum)

  • LCO (Lithium Cobalt Oxide)

• Lithium Iron Phosphate (LiFePO₄, LFP)

• Lithium Titanate (LTO)

• Nickel–Metal Hydride (NiMH)

• Sealed Lead–Acid (AGM and Gel types)

For each battery type, the following aspects will be reviewed:

• Chemistry and operating principle

• Key characteristics (energy density, self-discharge rate, storage conditions)

• Charging requirements

• Safety considerations

• Suitability for infrequent use with long idle periods

A summary comparison table of the main parameters is provided at the end.

Lithium-Ion Batteries (NMC / NCA / LCO)

Chemistry and operating principle:Lithium-ion batteries in this group use a graphite anode and a metal-oxide cathode that contains lithium combined with transition metals such as cobalt, nickel, manganese, or aluminum. During charge and discharge, lithium ions shuttle between the cathode and anode through an intercalation process.

The main subtypes differ in cathode composition:

• LCO (LiCoO₂) – the original lithium cobalt oxide chemistry

• NMC – nickel manganese cobalt oxide

• NCA – nickel cobalt aluminum oxide

The proportions of metals vary between these chemistries, affecting capacity, cycle life, and safety characteristics. Despite these differences, their basic operating principle remains the same.

These batteries use an organic electrolyte and a separator. All lithium-ion cells of this type require protection against overcharge and over-discharge, typically implemented through a battery management system (BMS). Directly connecting cells without proper monitoring and protection is unsafe.

Energy density:Very high—this is one of the main advantages of Li-ion. Typical gravimetric energy density is about 150–250 Wh/kg (some modern cells reach ~300 Wh/kg). Volumetric energy density is also high, typically 300–700 Wh/L.

For reference, NCA cells (e.g., Tesla/Panasonic-type cells) are around ~260 Wh/kg, while typical NMC cells are roughly ~150–200 Wh/kg. This high energy density means lower weight and smaller volume for the same capacity.

In the context of a 24 V, 10 Ah pack (~240 Wh), a lithium-ion pack could be significantly lighter and more compact than most alternative chemistries.

Self-discharge:Lithium-ion batteries have relatively low self-discharge. After charging, they typically lose about ~5% in the first 24 hours, then around ~1–2% per month at room temperature. However, the BMS/protection circuit may add extra drain—often on the order of ~3% per month if it remains continuously connected.

As a result, over several months of storage the total capacity loss is usually modest, which is a strong advantage for infrequent-use systems.

Note: Self-discharge increases significantly at higher temperatures (roughly doubling for every +10°C). For example, at +60°C, monthly self-discharge of a fully charged Li-ion pack can reach ~35%, versus roughly ~4–20% at 25°C depending on state of charge. This means that in hot conditions, a Li-ion battery left at 100% charge can lose a substantial portion of its stored energy over just a couple of months.

Storage life without recharging:Under moderate conditions (around 20°C), lithium-ion batteries can be stored for 6–12 months without recharging thanks to their low self-discharge, while retaining most of their stored energy. However, storing them at 100% state of charge for extended periods, especially at elevated temperatures, accelerates aging. Manufacturers typically recommend storing Li-ion batteries at ~40–60% charge for long-term storage.

At low temperatures (near 0°C), degradation slows down, but charging below 0°C is not recommended, as it can cause lithium plating on the anode. Storage at –10°C is generally acceptable (self-discharge is minimal), but charging at that temperature should be avoided.

The upper limit of +70°C exceeds the typical operating range of most Li-ion chemistries (usually up to about +60°C). Prolonged exposure to 70°C significantly reduces battery life. In controlled tests, a fully charged Li-ion battery stored at +60°C can lose around 40% of its capacity in just three months.

In practical terms, Li-ion batteries can be stored for several months without recharging if kept in a cool environment. In hot conditions (above 50°C), however, recharging or thermal management may be required within 1–2 months, otherwise substantial capacity loss and accelerated degradation will occur.

Charging complexity:Charging lithium-ion batteries requires a controlled CC/CV (Constant Current / Constant Voltage) profile. The process begins with constant current until the cell reaches its maximum voltage—typically ~4.2 V per cell for NMC, NCA, and LCO chemistries. After that, the charger holds the voltage constant while the current gradually tapers down, usually to about ~0.05C, at which point charging is terminated.

Accurate voltage control is critical. Charging above 4.2 V per cell creates a safety risk and accelerates degradation. In multi-cell packs, cell balancing is also necessary to prevent overvoltage on individual cells. For this reason, a battery management system (BMS)—or at minimum voltage protection and balancing circuitry—is required.

Lithium-ion batteries must not be kept on indefinite constant current or constant voltage after reaching full charge. Once charging is complete, the current must be cut off. These batteries are not tolerant of continuous trickle charging.

Overall, lithium-ion charging systems are more complex and typically more expensive than those used for lead-acid or NiMH batteries. However, they are widely available, and suitable charging controllers are common in modern electronic systems.

Safety considerations:Lithium-ion batteries carry a higher safety risk if operated outside their specified limits. They are sensitive to overheating, overcharging, and short circuits. Metal-oxide cathodes contain oxygen, which can be released at high temperatures and sustain combustion. Cases of thermal runaway are well documented—when an overheated or damaged Li-ion cell ignites.

This risk is more pronounced in chemistries with higher cobalt content, such as LCO, NCA, and NMC. For this reason, these batteries must always be used with proper protection against overvoltage, overcurrent, and excessive temperature.

When operated correctly, lithium-ion batteries are generally safe and widely used in billions of devices. However, among the battery types considered here, they present the highest fire risk under fault conditions.

Lithium-ion cells are also sensitive to deep discharge below ~2.5 V per cell, which can permanently damage the cell and, in some cases, create safety hazards during subsequent recharging.

In applications with infrequent use, special attention must be given to storage conditions. Prolonged storage at high temperature and full charge increases the risk of swelling, capacity loss, or failure.

Suitability for long-term storage (infrequent use):Moderate.

On the positive side, NMC/NCA/LCO lithium-ion batteries have low self-discharge. After 3–6 months of storage in a cool environment, they will retain most of their charge, meaning the system is likely to remain ready for operation. They also provide high energy output in a relatively compact package.

However, there are drawbacks for infrequent-use scenarios. Calendar aging occurs even without cycling—capacity gradually declines over time, especially if the battery is stored at full charge and/or high temperature. At +70°C, lithium-ion batteries degrade rapidly and may fail within a matter of months.

For rare-use applications, the preferred approach is to store Li-ion batteries at moderate temperature and partial charge (~50%), then recharge to 100% before operating the motor. This requires deliberate charge management.

In addition, the typical service life of Li-ion (particularly NMC-based systems) is about 5–10 years, even without heavy use, due to electrolyte and material aging. This limits long-term durability.

Overall, lithium-ion batteries can be used for infrequent motor activation if storage conditions are controlled. However, under sustained +70°C exposure or when maximum long-term robustness is required, other chemistries (such as LFP or LTO) may be more suitable.

Lithium Iron Phosphate (LiFePO₄, LFP)

Chemistry and operating principle:LiFePO₄ is a type of lithium-ion battery that uses lithium iron phosphate (LiFePO₄) as the cathode material. The anode is typically graphite, as in other Li-ion chemistries.

LFP stands out for its high thermal and chemical stability. The phosphate crystal structure is very stable and does not readily release oxygen when heated, which significantly improves thermal safety. The nominal cell voltage is about ~3.2 V, with a typical charge voltage of ~3.6–3.65 V per cell.

The operating principle is the same as other lithium-based systems: lithium ions shuttle between the cathode and anode during charge and discharge.

Due to its material properties, LiFePO₄ offers long cycle life and can tolerate relatively high discharge currents. This chemistry is widely used in power tools, stationary energy storage systems, and electric vehicles—particularly in applications where safety and long service life are priorities.

LiFePO₄ is also considered more environmentally benign, as it does not contain cobalt. Its main trade-off is lower energy density compared to NMC or NCA chemistries.

Energy density:Lower than NMC-based lithium-ion cells. Typical gravimetric energy density is in the range of 90–160 Wh/kg, with some of the best modern cells reaching ~160–180 Wh/kg. Volumetric energy density is generally around 250–350 Wh/L (approximately ~325 Wh/L on average).

For the same stored energy, a LiFePO₄ battery will therefore be roughly 1.5 times larger and heavier than an equivalent NMC pack.

In practical terms, a 24 V, 10 Ah (≈240 Wh) LiFePO₄ pack may weigh around 2–3 kg, depending on the specific cells used. A comparable NMC pack would typically weigh under 2 kg.

Despite this, LiFePO₄ still offers significantly higher energy density than NiMH or lead-acid batteries—typically 2–3 times higher Wh/kg than sealed lead-acid.

Storage life without recharging:Long. Thanks to its low self-discharge, a LiFePO₄ pack can often be stored for up to a year without recharging—especially if it is charged to 50–100% before storage. For maximum lifetime, manufacturers typically recommend storing at around 50% state of charge and at temperatures no higher than +25°C.

A major advantage of LiFePO₄ is its chemical stability. Its calendar life can exceed 10 years, and storage-related aging generally progresses more slowly than with NMC chemistries. High temperatures still accelerate aging (electrolyte decomposition, rising internal resistance), but LFP is known to tolerate +60…+70°C better than typical NMC/NCA/LCO lithium-ion cells.

That said, the commonly specified operating range is about –20…+60°C. Prolonged exposure near +70°C is not ideal—even if short-term exposure is unlikely to cause immediate failure, it will accelerate wear. At –10°C, storage and discharge are generally possible (available capacity will drop during discharge, then partially recover when warmed).

A key limitation remains the same as for other lithium chemistries: charging below 0°C is not recommended, as it can damage the anode.

Overall, LiFePO₄ can be stored safely for many months without recharging. For example, at +20°C, losses over four months are often only a few percent. In a rare-start application, this means the motor can remain connected to an LFP battery for 3–6 months and still have the pack close to fully charged when needed.

Charging complexity:The charging profile for LiFePO₄ is similar to other lithium-ion chemistries, but with different voltage levels. Charging is typically done using a CC/CV method: constant current up to about ~3.65 V per cell, followed by constant voltage until the current tapers down to the cutoff level.

A BMS is still required to monitor and protect the cells in the pack—especially for a ~24 V system, which typically uses 8 cells in series. Overall, the charger complexity is comparable to an NMC-based Li-ion system; the main difference is the target voltage.

Today, 4S LFP (12.8 V nominal) chargers are widely available as drop-in replacements for lead-acid systems and are relatively inexpensive. LFP cells also tolerate partial charge well and are less prone to thermal runaway; mild overcurrent is generally less critical. However, overvoltage above ~3.65 V per cell is still not acceptable, as it accelerates degradation.

Because LFP is common in industrial and energy-storage applications, there are many off-the-shelf BMS and charger solutions.

Trickle charging (keeping the battery permanently at full charge) is generally unnecessary and not recommended. After the pack reaches full charge, the current should be terminated. That said, LFP is typically more tolerant of float-style operation (e.g., buffer systems) than cobalt-based Li-ion chemistries. Compared to NiMH and lead-acid, the charging electronics are more complex, but LFP systems are generally very reliable.

Safety level:Low. LiFePO₄ is considered one of the safest lithium-based chemistries. The stable phosphate crystal structure does not readily release oxygen, which significantly reduces the risk of combustion. The cathode decomposition temperature is typically above 200°C, and even in cases of short circuit or overheating, an LFP cell usually heats up rather than ignites.

In nail penetration or thermal abuse tests, LFP cells generally do not catch fire, unlike many NMC-based cells. This makes them attractive for safety-critical applications such as electric buses and residential energy storage systems.

That said, proper operating limits must still be respected. Severe overvoltage can cause swelling and failure, and a short circuit can produce very high current and significant heating. However, the likelihood of thermal runaway is substantially lower than with cobalt-based lithium-ion chemistries.

LFP cells are also somewhat more tolerant of deep discharge. While discharging below ~2.5 V per cell is not recommended, the risk of immediate irreversible damage is generally lower compared to other Li-ion types.

Overall, the safety profile of LiFePO₄ is strong: low fire risk and no toxic cathode materials. The electrolyte is still organic and flammable, but the cathode itself is stable and non-toxic. Within the –10…+70°C range, no acute safety issues arise, although prolonged exposure near +70°C may accelerate aging.

Suitability for long-term storage:High.

LiFePO₄ is very well suited to infrequent-use scenarios. The reasons are straightforward:

• Extremely low self-discharge, so the battery retains most of its charge during long idle periods.

• Long calendar life, often exceeding 10 years, even with minimal cycling.

• No strict requirement to maintain a precise storage state of charge.

LFP tolerates storage in a charged state reasonably well. While storing at less than 100% charge reduces long-term degradation, it is not as critical as with NMC-based lithium-ion. If the system must remain ready for operation, the battery can be kept at ~90–100% charge, and after several months it will still retain nearly full capacity.

At –10°C, LFP remains functional, typically delivering ~60–80% of nominal capacity, which may still be sufficient for a 15-minute motor run. At elevated temperatures, LFP degrades more slowly than conventional lithium-ion chemistries. Although operation at +60°C or above still accelerates aging, the risk of swelling, failure, or thermal instability is significantly lower.

The primary drawback is lower energy density, resulting in greater weight and volume. However, if size and mass are not critical constraints, this disadvantage is often outweighed by the benefits.

Overall, LiFePO₄ is one of the strongest candidates for applications involving rare activation and long periods of storage in a charged state.

Lithium Titanate (LTO)

Chemistry and operating principle:LTO batteries are a specialized type of lithium-ion system in which the traditional graphite anode is replaced with lithium titanate (Li₄Ti₅O₁₂) in a spinel structure. The cathode is typically lithium manganese oxide (LiMn₂O₄) or sometimes NMC. The electrolyte is similar to that used in other lithium-ion batteries.

The nominal cell voltage is about ~2.4 V, with a full-charge voltage of approximately ~2.8 V.

The defining feature of LTO is its “zero-strain” anode structure. During charge and discharge, the anode undergoes almost no volume change. It does not form a conventional solid electrolyte interphase (SEI) layer in the same way graphite does, and lithium plating is effectively eliminated—even at high charge rates and low temperatures.

This results in several major advantages:

• Extremely long cycle life

• Ability to handle very high charge and discharge currents (often >10C)

• Excellent low-temperature performance

• Very high safety and thermal stability (stable up to ~250°C)

LTO batteries can be charged very rapidly—often within 10–15 minutes, depending on system design.

The trade-offs are significant:

• Very low energy density (less than half that of LiFePO₄)

• High manufacturing cost

Commercially, LTO has been available since around 2008 and is used in specialized applications, including industrial equipment, certain electric vehicle variants (e.g., specific Mitsubishi i-MiEV versions), and energy storage systems where extreme durability and wide temperature tolerance are required.

Energy density:Low. Typical gravimetric energy density is only ~50–80 Wh/kg, roughly comparable to older NiCd cells. Volumetric energy density is also modest, usually around ~100–180 Wh/L. In practical terms, this is on the order of about one-third of an NMC lithium-ion pack, due to both the lower specific capacity and heavier active materials.

As a result, a 24 V, 10 Ah (≈240 Wh) LTO pack would be relatively heavy. To store ~240 Wh, it may require roughly ~4–5 kg of cells, depending on the exact cell type and packaging. This is a major disadvantage, although in this project weight is not the primary constraint.

Self-discharge:Very low—comparable to other lithium-based batteries. In theory, the lack of a conventional SEI layer might suggest near-zero self-discharge, but in practice LTO cells still have some internal losses.

Typical estimates are around ~2–5% per month at 20°C. Some manufacturers state that over three months the loss is <10% at 20°C. A practical rule of thumb is ~1–2% per month under normal conditions.

This is an excellent figure—orders of magnitude better than NiMH. Over six months, the pack might lose only ~5–10%, which is minor. Even accounting for higher losses at elevated temperatures, LTO will retain charge far better over several months of idle storage than NiMH or lead-acid.

Storage life without recharging:Very long. LTO batteries are known for retaining usable capacity over extremely long periods, even with minimal cycling.

First, they offer exceptional cycle life—typically ~6,000 to 20,000 cycles before a major capacity drop. In a rare-use scenario, degradation is dominated by calendar aging, and LTO has very low calendar aging because it avoids many of the side reactions associated with graphite anodes and tends to have fewer parasitic electrolyte reactions.

Second, LTO has outstanding temperature tolerance. Its typical operating range is very wide—roughly –40°C to +75°C—which fully covers the project requirement of –10°C to +70°C. Even at –30°C, these cells can still deliver around ~80% of nominal capacity, and they can be charged (slowly) below 0°C with far less risk of lithium plating than graphite-based lithium-ion cells.

At +70°C, LTO generally remains functional better than most other lithium chemistries. Thermal runaway is not a typical concern; aging does accelerate, but the battery continues to operate.

As a result, an LTO pack can often be stored for a year or more without recharging. In practical terms, a fully charged LTO battery left at room temperature may still be usable after a year, with only a small loss of charge. Even at +60°C, service life is often measured in years, though capacity will gradually decline over time.

The fact that the pack does not need to be topped up every few months is a major advantage. In principle, an LTO battery can be charged, stored for a year, and still start the motor without a pre-charge—assuming reasonable storage conditions.

Charging complexity:Similar to other lithium-ion systems. Charging requires a source with current limiting and a voltage cutoff of about ~2.80 V per cell. A ~24 V pack would typically use about 10 LTO cells in series (≈24 V nominal).

As with other lithium chemistries, a balancer and protection circuitry are recommended. LTO is somewhat more tolerant of imbalance, but it is still best to prevent cell-to-cell divergence.

LTO supports very fast charging—often 5–10C—but for maximum lifetime it is better to limit charge current to around ~1C. Overall, the charger is comparable in complexity to a standard lithium charger; the main difference is the voltage target.

Because LTO is less common, off-the-shelf chargers can be harder to find, although configurable industrial chargers and programmable power supplies are available.

One practical advantage is that LTO is more forgiving of overcharge and undercharge. There is no graphite anode, so lithium plating is not the same concern. If the pack is slightly overcharged (by a few hundred millivolts per cell), it typically will not fail catastrophically—aging reactions may accelerate, but the risk of an immediate thermal event is low. This means charging accuracy can be somewhat less strict than with conventional Li-ion chemistries.

LTO can also be stored at full charge with fewer negative consequences than graphite-based lithium-ion cells. Even so, for best longevity it is still preferable to follow a proper CC/CV profile and avoid keeping the battery continuously at maximum voltage for long periods (continuous float at 100% is generally not ideal for lifespan, even if it is safe).

Safety level:Minimal. Lithium titanate (LTO) batteries are considered among the safest rechargeable battery types available.

They are not prone to thermal runaway under normal conditions. The decomposition temperature is very high, and LTO cells do not ignite under nail penetration or moderate overcharge scenarios. Even in a short-circuit event, although significant heat may be generated, the cathode/anode combination is inherently more stable and not prone to combustion.

In safety testing, LTO cells demonstrate exceptional robustness. They can be charged at low temperatures without the risk of lithium plating, tolerate high discharge currents, and withstand mechanical stress better than most lithium chemistries.

As with any battery, extreme overheating or insulation failure can cause damage, and a short circuit can deliver very high current capable of igniting surrounding wiring. However, the LTO cell itself has very low flammability.

LTO also avoids toxic metals such as cobalt and lead. The electrolyte is similar to other Li-ion systems (organic and flammable), but because of the lower cell voltage and stable anode structure, the likelihood of electrolyte breakdown is reduced.

In terms of safety, LTO outperforms the other chemistries discussed here. For operation at +70°C, it is the least risky option and is specifically suited for high-temperature environments.

Suitability for long-term storage:Very high.

LTO is particularly well suited to applications that involve infrequent use, long service life, and wide temperature exposure. A fully charged LTO battery can be stored for a year and still remain operational. It does not require regular top-up charging due to its very low self-discharge.

Cold storage poses no problem; in fact, degradation slows significantly at low temperatures. LTO also handles high temperatures better than most other battery chemistries. While storing at +70°C is not ideal for any battery, LTO tolerates such conditions more reliably than conventional lithium-ion or lead-acid systems.

Cycle life is measured in tens of thousands of cycles, which, in a rare-use scenario, effectively eliminates cycle-related wear as a limiting factor. In practical terms, LTO systems can remain functional for 15–20 years.

The main drawbacks are high cost and low energy density, meaning more cells are required to achieve the desired capacity. If these factors are acceptable, LTO represents the most robust and durable option among the chemistries considered.

For applications requiring a power source that remains ready and is activated only a few times per year, LTO offers the highest level of reliability and confidence.

Nickel–Metal Hydride (NiMH)

Chemistry and operating principle:NiMH batteries are an evolution of nickel–cadmium (NiCd) technology. The positive electrode is nickel oxyhydroxide / nickel hydroxide (NiOOH / Ni(OH)₂), similar to NiCd, while the negative electrode is a metal alloy capable of absorbing and releasing hydrogen (a metal hydride). The electrolyte is an aqueous solution of potassium hydroxide (KOH).

During charging, hydrogen is generated at the nickel electrode and absorbed by the metal alloy, forming a hydride. During discharge, the process reverses: hydrogen is released from the hydride and oxidized, reducing the nickel compound back to Ni(OH)₂.

The half-reactions are:

• Positive electrode:NiOOH + H₂O + e⁻ ⇌ Ni(OH)₂ + OH⁻

• Negative electrode:M + H₂O + e⁻ ⇌ MH + OH⁻

(where M represents the metal alloy)

Overall, the electrochemical behavior is similar to NiCd, but without toxic cadmium.

The nominal cell voltage is about ~1.2 V.

NiMH batteries are capable of delivering relatively high discharge currents (though typically somewhat lower than NiCd) and offer higher energy density than NiCd. They were widely used in the 1990s and 2000s—ranging from AA rechargeable cells to hybrid vehicle battery packs—before lithium-ion became dominant.

For a 24 V system, approximately 20 NiMH cells in series would be required (20 × 1.2 V ≈ 24 V).

Energy density:Moderate. The typical gravimetric energy density of NiMH batteries is about 60–120 Wh/kg. In optimized, lower-current industrial designs it can reach ~100–120 Wh/kg, while common low-self-discharge consumer cells (such as AA types) are closer to ~80 Wh/kg.

Volumetric energy density is usually ~250–400 Wh/L, since cylindrical NiMH cells can be packed efficiently (for example, AA Eneloop cells are around ~330 Wh/L).

Compared to lithium chemistries, NiMH generally has about half the energy density by weight. For a 24 V, 10 Ah (~240 Wh) pack, a NiMH solution—such as ~20 D-size cells rated around 10 Ah each—could weigh approximately 3–4 kg. This is heavier than a comparable LiFePO₄ pack (~2–3 kg) and significantly heavier than NMC lithium-ion (~1–2 kg).

However, NiMH still offers roughly twice the specific energy of sealed lead-acid, which explains its historical use in early hybrid vehicles (e.g., Toyota Prius), where it provided a lighter alternative to lead-acid, though heavier than lithium-ion.

Self-discharge:This is a major weakness of NiMH. Standard NiMH cells tend to self-discharge quickly: they can lose ~10% in the first 24 hours after charging, then roughly ~20–30% per month at room temperature. After 2–3 months, such a battery may be almost fully discharged on its own. High-power NiMH cells can have even higher self-discharge.

However, there is a class of low-self-discharge NiMH (LSD NiMH) cells, introduced around 2005, with design improvements (alloy formulation, separator changes, etc.). LSD NiMH loses charge much more slowly—often quoted as about ~15% per year or roughly ~1–2% per month. For example, common LSD NiMH cells (such as Panasonic Eneloop-type) can retain ~70–85% of their capacity after a year. That brings them closer to lithium-based performance, though typically still somewhat worse.

A key point is that NiMH self-discharge rises sharply with temperature. At +35…40°C, they can lose charge so rapidly that they may discharge completely within weeks. Self-discharge also increases with aging; older cells can “leak” so quickly that they may not hold charge even for a few days.

For this project, using NiMH would effectively require LSD NiMH cells—otherwise the pack will be near empty after a couple of months of standby. Even with LSD NiMH, periodic top-up charging is usually advisable, roughly every 3–6 months.

Storage life without recharging:Limited. Without top-up charging, standard NiMH cells do not last long in storage—capacity drops noticeably within a month. LSD NiMH performs better: it can be stored for several months, but it still typically needs a recharge every 6–12 months.

NiMH also tolerates long storage in a fully discharged state poorly. If a cell self-discharges to empty and remains that way for an extended period, it may suffer irreversible capacity loss, associated with electrode crystallization, corrosion, and memory-effect-like degradation. For long storage, NiMH is therefore usually kept charged and periodically topped up.

Within the required temperature range (–10…+70°C):

• At –10°C, NiMH can be stored, and cold conditions actually help by slowing self-discharge.

• At +70°C, storing NiMH is close to impractical. Self-discharge accelerates dramatically, and prolonged exposure can cause gas generation and possible venting or electrolyte loss. While some NiMH cells may specify operation up to ~+60…70°C, long-term exposure near +70°C is near the limit: plastics and seals can degrade, vapor pressure increases, and the safety vent may open.

In practice, if acceptable remaining charge is required, NiMH can be stored no more than ~1–3 months without recharging (even with LSD variants; standard NiMH is typically worse). For multi-month idle periods, NiMH would either need a top-up charge before use or storage at low temperature to reduce self-discharge.

Charging complexity:Relatively low. NiMH batteries can be charged with constant current (typically 0.1–1C) and the end of charge is detected using characteristic “delta” behaviors. In practice, two common approaches are used:

• Slow trickle charging at a very low current (around C/20 or less)

• Fast charging (about 1–2 hours) with –ΔV and/or ΔT/dt termination

When NiMH cells reach full charge, they often show a small voltage drop (negative delta-V) that smart chargers detect to stop charging. Temperature also begins to rise near full charge, so many chargers use temperature rate-of-rise (ΔT/dt) as an additional termination method.

Compared to lithium systems, NiMH charging is simpler and does not require extremely precise voltage control. NiMH cells can absorb some overcharge for a limited time through internal gas recombination reactions. This makes continuous low-current “maintenance” charging possible to offset self-discharge, which is convenient for standby applications.

However, heavy or prolonged overcharge still causes heating and reduces lifespan, so the charge current must be chosen appropriately.

For a 24 V pack (~20 cells in series), a dedicated charger is needed that can handle a total charge voltage of roughly ~28 V and reliably detect end-of-charge. Such chargers are common in industrial applications and hobby/RC equipment, so this is not unusual.

From a circuit and control standpoint, NiMH is among the simplest options: it typically does not require active cell balancing, and it is more tolerant of slight overcharge than lithium. Even a crude charger (e.g., a resistor plus a voltage source) can sometimes work—something that is generally unacceptable for lithium-based packs.

Safety level:Low. NiMH batteries are not prone to fire. They use a water-based electrolyte, so they do not burn under overheating conditions (at most, the plastic casing may melt).

During overcharge, NiMH cells produce hydrogen and oxygen due to water electrolysis, which increases internal pressure. Each cell includes a pressure relief valve to vent excess gas. Severe overcharge or overheating may trigger venting. The released gas mixture can be flammable (due to hydrogen), so in theory, a confined space with many heavily overcharged cells could pose a gas explosion risk. In normal applications, this is unlikely.

The most typical failure modes are:

• Electrolyte leakage (alkaline KOH, corrosive)

• Loss of sealing

• Reduced capacity due to drying or internal damage

Fire risk is extremely low.

NiMH cells are thermally robust and can operate up to ~+70°C, although internal pressure increases at high temperatures. At low temperatures (e.g., –10°C), capacity drops significantly (often to around ~50%) and internal resistance rises, but this affects performance rather than safety.

Regarding misuse tolerance: NiMH handles deep discharge better than lithium systems (though it may cause cell reversal in weak cells within a pack), and it tolerates moderate overcharge, typically resulting in heat generation and gradual water loss rather than ignition.

Overall, NiMH is a very safe battery technology, especially compared to high-energy lithium-based systems.

Suitability for long-term storage:Low.

The main limitation of NiMH in this scenario is high self-discharge. If the system is activated only once every few months, a NiMH pack will likely be significantly discharged by the time it is needed—unless it is actively maintained.

There are two practical approaches:

1. Keep the NiMH pack continuously on trickle charge from the mains, ensuring it remains full.

2. Recharge the battery before each rare activation.

The first option is not always feasible, especially for autonomous or remotely stored equipment. The second requires planning and is unsuitable for emergency or immediate-start situations.

If self-discharge is ignored, NiMH itself can be reasonably durable: around ~500 cycles and 5–10 years of calendar life under proper maintenance. However, this assumes periodic charging and monitoring.

High temperatures accelerate water loss and aging. Storage at +70°C can significantly shorten lifespan, potentially damaging the battery within months. Cold storage (–10°C) improves longevity but reduces available discharge capacity.

In summary, NiMH is not well suited to rare-use applications because it requires regular maintenance charging. Its advantages are low cost, simplicity, and inherent safety. However, if maintenance is neglected, the battery may be empty when needed.

Low-self-discharge (LSD) NiMH improves the situation but does not fully eliminate the issue. For infrequent activation systems, lithium-based or lead-acid solutions are generally preferred.

Lead–Acid Batteries (AGM, GEL)

Chemistry and operating principle:Lead–acid batteries are the oldest type of rechargeable battery, in use since the late 19th century. They use a lead anode (porous lead) and a lead dioxide cathode (PbO₂) in a sulfuric acid electrolyte.

During discharge, both electrodes convert to lead sulfate (PbSO₄), and the electrolyte becomes more diluted as sulfuric acid is consumed. During charging, the reaction reverses: lead sulfate is converted back into metallic lead and lead dioxide, and sulfate ions return to the electrolyte, restoring acid concentration.

The nominal cell voltage is about ~2.0 V (around 2.1 V at rest), with a fully charged voltage of ~2.15–2.20 V and a fully discharged voltage of ~1.75–1.8 V. A 24 V system requires 12 cells in series.

AGM (Absorbent Glass Mat) and GEL are types of sealed lead–acid batteries (VRLA – Valve Regulated Lead Acid):

• In AGM, the liquid electrolyte is absorbed into fiberglass mats between the plates.

• In GEL, the electrolyte is immobilized as a silica-based gel.

Both designs are sealed and include pressure-relief valves, preventing acid leakage and minimizing maintenance (no need to refill water). The electrochemical reactions remain the same as in traditional flooded lead–acid batteries.

Lead–acid batteries are valued for their ruggedness and tolerance to abuse, but they have low specific energy and limited cycle life, especially under deep discharge conditions. They are commonly used where weight is not critical: automotive starter batteries, backup power systems (UPS), and budget-constrained solar installations.

Energy density:Low. Lead–acid batteries typically provide about ~30–50 Wh/kg, with volumetric energy density around ~60–100 Wh/L. These are the lowest values among the chemistries considered.

As a result, a 24 V, 10 Ah (≈240 Wh) lead–acid pack would weigh roughly ~5–8 kg.

For illustration, a common 12 V, 7 Ah AGM battery (UPS type) weighs about ~2.5 kg and stores around ~84 Wh, which is roughly ~34 Wh/kg. This low energy density makes lead–acid batteries bulky and heavy.

In this project, size is not a strict constraint, but 5+ kg is still substantially heavier than any lithium-based alternative. The main reason lead–acid remains widely used is cost—it is typically the cheapest option, with weight accepted as the trade-off.

Self-discharge:Moderate at normal temperatures. Lead–acid batteries typically lose about ~3–5% per month at +20°C. This is better than standard NiMH, though not as low as lithium-based chemistries. Sealed AGM and GEL types are often closer to ~3% per month when new.

However, self-discharge increases sharply with temperature. A common rule of thumb is that the rate roughly doubles for every +10°C rise. That means:

• Around +30°C → ~10% per month

• Around +40°C → ~20% per month

• Around +50°C → ~40% per month

• Near +60…70°C → losses can become significant on a weekly or even daily basis

There is also a slightly elevated self-discharge immediately after charging (often ~1–2% in the first day).

In cool storage conditions, a lead–acid battery can retain usable charge for several months (for example, losing ~20–30% over six months at moderate temperature). At +70°C, however, the battery may discharge almost completely within 1–2 months, and high temperature will also accelerate sulfation and long-term degradation.

At low temperatures (e.g., –10°C), self-discharge slows significantly—often below ~1–2% per month. However, available discharge capacity is reduced in cold conditions due to slower chemical reactions (capacity generally recovers when warmed).

Storage life without recharging:Limited, due to the need for periodic recharging.

Although lead–acid batteries self-discharge relatively slowly at moderate temperatures, they do not tolerate being left discharged. If stored in a low state of charge, sulfation occurs: lead sulfate crystals form on the plates and may become irreversible. This permanently reduces capacity.

For this reason, lead–acid batteries should either:

• Be kept on continuous float charge (e.g., ~13.6 V for a 12 V battery), or

• Be periodically recharged to compensate for self-discharge

Top-up charging is typically recommended at least every 3–6 months, ideally every 3 months, during storage.

In the context of this project:

• At room temperature, a sealed AGM battery can be left without charging for up to about 3 months, after which recharging is advisable.

• At +70°C, storage without charging becomes impractical. Within weeks, charge will drop significantly, and sulfation may begin—especially at warmer regions of the plates. High temperature also accelerates grid corrosion and electrolyte drying, sharply reducing lifespan.

If the battery is maintained on continuous float charge (as in UPS or emergency systems), it can remain installed and ready for many years. However, in an autonomous system without continuous charging, a lead–acid battery would require manual maintenance every few months.

At –10°C, a fully charged lead–acid battery can be stored for much longer—potentially up to a year—because self-discharge slows significantly. A fully charged battery will not freeze easily, as concentrated electrolyte remains liquid down to approximately –60°C. Cold storage therefore extends maintenance intervals, but the battery must not be left discharged, as that will quickly damage it regardless of temperature.

Charging complexity:Minimal. Lead–acid charging is simple and well understood.

A typical profile is two-stage:

1. Bulk charge (constant current) at about 0.1–0.2C until the voltage reaches approximately ~2.4 V per cell (for a 12 V battery, about ~14.4 V).

2. Float stage at about ~2.25–2.30 V per cell (for 12 V, around ~13.6 V).

At float voltage, the battery becomes fully saturated and the current naturally drops to a very low level. It can remain at this float voltage for extended periods without issue.

Lead–acid chargers can be very simple. Even a basic voltage-limited power supply can work adequately, as the battery chemistry tolerates moderate imprecision. AGM and GEL types prefer controlled voltage to avoid excessive gas generation, but they are still relatively forgiving.

Overcharge leads to electrolysis of water, producing gas. However, AGM batteries allow partial gas recombination internally, and excess pressure is vented through safety valves. A brief overcharge typically reduces electrolyte reserve slightly but does not cause immediate failure or fire risk.

Balancing is generally not required for small series-connected lead–acid systems. Voltage differences between cells are modest, and periodic full charging tends to equalize them. In larger systems, an occasional equalization charge (slightly elevated voltage) may be used, but for a 24 V, 10 Ah pack, a standard charger is sufficient.

Overall, lead–acid batteries are the simplest to charge. Inexpensive chargers are widely available, continuous float maintenance is easy to implement, and even basic or homemade circuits can function reliably—provided the battery is not chronically overcharged or left deeply discharged.

Safety level:Moderate. The fire risk is low, but there are specific considerations.

Lead–acid batteries do not burn—the electrolyte is water-based and nonflammable. Even under short-circuit conditions, the battery may heat significantly, but it will not ignite (the plastic casing may melt in extreme cases).

During charging—especially overcharging—hydrogen and oxygen gas are produced. In sealed VRLA (AGM/GEL) batteries, these gases normally recombine internally. However, if charging current is too high, the pressure relief valves will open and hydrogen will be released. In confined spaces, accumulated hydrogen can form an explosive mixture. Explosions have occurred when a spark (for example, while disconnecting a terminal) ignited hydrogen trapped near the battery.

Therefore, lead–acid batteries should be installed in well-ventilated areas, and sparks should be avoided during servicing.

Another safety aspect is sulfuric acid. In AGM and GEL batteries it is immobilized, but under severe overcharge, overheating, or physical rupture, electrolyte leakage can occur. Sulfuric acid is highly corrosive and can damage nearby equipment or cause injury. Additionally, lead is toxic and requires proper disposal.

Under normal operation, VRLA batteries are thermally stable and do not present combustion risks. At –10°C and +70°C, the primary concerns are performance degradation and accelerated aging rather than fire. However, prolonged exposure to +70°C may increase gas venting and eventually dry out the battery, reducing lifespan.

In summary: the risk of fire is minimal, but there is a moderate risk related to gas release and acid exposure. Proper ventilation and installation practices are required. Lead–acid batteries are often considered robust and predictable—they are more likely to fail gradually than to create a sudden hazard.

Suitability for long-term storage:Moderate to low without maintenance; moderate with maintenance.

If the system allows continuous float charging (for example, from mains power or a solar panel), a lead–acid battery can remain in standby service for an extended period. Many backup systems operate this way, keeping the battery permanently on float charge and ready to deliver current when required. In such conditions, service life is mainly limited by time and temperature—typically 3–5 years at 25°C, and significantly less at higher temperatures (for example, at 40°C, lifespan may drop to ~1–2 years).

If left unattended without periodic charging, however, a lead–acid battery may suffer irreversible capacity loss due to sulfation, even though its self-discharge rate is moderate. Leaving an AGM battery uncharged for a year carries a high risk of severe capacity degradation or incomplete recovery upon recharge—especially in hot environments.

For infrequent-use applications, lead–acid is suitable only if regular maintenance charging is feasible. It must either be kept continuously connected to a charger or recharged every few months.

If maintenance is acceptable, lead–acid is a low-cost solution. It can deliver the required motor current and retain sufficient charge for a few months of standby under moderate conditions. However, long-term durability is limited. Even without cycling, plate corrosion and aging—accelerated by high temperature—mean that replacement is typically required after 5–6 years.

Advantages include simplicity, low cost, and tolerance of rough handling. Disadvantages include weight, the need for periodic charging, and shorter service life in standby applications compared to lithium-based systems.

Comparison of Key Characteristics

For clarity, the main characteristics of the evaluated battery types are summarized below:

Battery Type	Specific Energy (Wh/kg)	Volumetric Energy (Wh/L)	Self-Discharge (%/month at 20°C)	Cycle Life (to ~80% capacity)	Operating Temperature Range
Li-ion (NMC/NCA/LCO)	150–250	300–700	~2–3%	~500–1000 cycles	−20…+60°C (up to +70°C short-term)
LiFePO₄ (LFP)	90–160	~250–350	~2–3%	~2000–5000 cycles	−20…+60°C (higher reduces lifespan)
Li-titanate (LTO)	50–80	~100–150 (low)	~1–3%	6000–20000 cycles	−40…+75°C (very wide range)
NiMH (standard)	60–120	~250–400	~20–30%	~500 cycles	−20…+70°C (operational)
NiMH (low self-discharge)	~50–100	~200–350	~1–5%	500–2000 cycles	−20…+70°C
Lead–acid (AGM/GEL)	30–50	~60–100	~3–5%	~300–500 cycles	−20…+50°C (+70°C significantly reduces lifespan)

Notes on the table:

• The values shown are approximate and may vary depending on manufacturer and specific cell design.

• Cycle life depends strongly on depth of discharge (DoD). The figures provided are typical estimates for about ~80% DoD (deep cycling conditions).

• The temperature range reflects typical operating limits. Long-term storage at extreme temperatures can accelerate degradation, even if operation remains technically possible.

• For NiMH, characteristics are listed separately for standard and low self-discharge (LSD) variants, as their storage performance differs significantly.

Conclusions and Recommendations

Based on the analysis, the following conclusions can be drawn regarding the suitability of each battery type for a 24 V / 10 Ah system with rare motor activation and exposure to temperature extremes:

Lithium-ion (NMC/NCA/LCO)

These offer the highest energy density but are sensitive to long-term storage at elevated temperatures. They can be used for infrequent operation, provided storage conditions are controlled—ideally at moderate temperature and partial state of charge. Prolonged storage at +70°C significantly accelerates degradation. A proper BMS is mandatory for safety.

Conclusion: Suitable where compactness is critical, but less robust for long-term idle use in high-temperature environments compared to other lithium chemistries.

LiFePO₄ (LFP)

A strong overall balance of energy density, very low self-discharge, and long service life. It is inherently safer than cobalt-based lithium-ion and can remain unused for many months without significant charge loss. It tolerates –10°C well and handles elevated temperatures better than NMC/NCA, although sustained +70°C will still reduce lifespan. Requires a BMS, but fire risk is minimal.

Conclusion: An excellent fit for this application. It maintains readiness during long idle periods and can deliver many years of service with minimal maintenance.

Lithium Titanate (LTO)

The most durable and temperature-resilient option. LTO tolerates deep cold and sustained high temperatures (up to around +70°C) better than other chemistries. It has extremely low self-discharge and negligible calendar aging compared to alternatives. It can effectively be charged and left unused for extended periods with minimal loss.

The trade-offs are higher cost and lower energy density (greater weight and volume).

Conclusion: The most reliable solution under extreme conditions. Particularly justified if high temperatures are frequent or continuous and long-term unattended operation is required.

NiMH

Not well suited for long storage without maintenance due to high self-discharge. Even low-self-discharge (LSD) variants require periodic recharging. Elevated temperatures accelerate degradation significantly.

Conclusion: Only practical if regular maintenance charging is acceptable. Otherwise, reliability at the moment of use is uncertain.

Lead–acid (AGM/GEL)

Can function reliably if kept on continuous float charge. Advantages include low cost, simple charging, and strong current delivery. However, it is heavy, sensitive to prolonged storage without recharge, and particularly vulnerable to high-temperature aging. Sustained +70°C exposure severely shortens service life.

Conclusion: Feasible only if maintenance charging is guaranteed. Not ideal for autonomous, unattended systems exposed to heat.

Final Recommendation

For rare, short-duration motor activations with long standby periods, lithium-based chemistries with extended calendar life are the most appropriate:

• LiFePO₄ is the most practical and balanced choice. It offers long life, safety, low self-discharge, and reasonable cost. A 24 V / 10 Ah LFP pack will be slightly heavier than NMC but far more suitable for long idle storage. It is recommended when operating temperatures may occasionally be high but generally remain below ~60°C.

• LTO is the optimal solution for the most demanding conditions—especially where +70°C exposure is frequent, maintenance is impossible, and maximum long-term reliability is required. Despite higher cost and lower energy density, it provides unmatched durability and temperature tolerance.

Conventional Li-ion (NMC/NCA) should be selected only if compactness and weight are primary constraints and storage conditions can be controlled. NiMH and lead–acid options are less suitable due to maintenance requirements and reduced long-term reliability in this use case.

Overall recommendation:For a balanced, durable solution — LiFePO₄.For extreme temperature and maximum longevity — LTO.