Why Choose Nickel-Metal Hydride (NiMH) Batteries?

Nickel-metal hydride (NiMH) batteries represent a significant advancement over their predecessor, nickel-cadmium (NiCd) batteries. The term "metal" in NiMH refers not to a singular metal, but rather to a complex alloy. These alloys, known as metal hydrides, are composed of various metals, and they are responsible for the battery’s ability to store and release energy. Among the most common metal hydride alloys used in NiMH batteries are the AB5 and AB2 structures. In AB5, the "A" part is a mixture of rare earth metals or titanium (Ti), while the "B" part consists of metals such as nickel (Ni), cobalt (Co), manganese (Mn), and sometimes aluminum (Al). In higher-capacity batteries, the alloys are often of the AB2 type, where "A" is typically titanium (Ti) or vanadium (V), and "B" is zirconium (Zr), nickel (Ni), or a mix of chromium (Cr), cobalt (Co), iron (Fe), and manganese (Mn).

These metal hydride compounds play a key role in storing and releasing hydrogen ions in the battery. During charging, the potassium hydroxide (KOH) electrolyte inside the battery releases hydrogen ions (H+), which are absorbed by the metal hydride alloy, preventing the formation of hydrogen gas (H2) and maintaining the pressure and volume inside the battery. During discharge, the hydrogen ions are released back through the opposite process, providing power to the device.

1. Weight Comparison

In terms of voltage, both NiMH and NiCd batteries have an average voltage of 1.2V per cell, whereas lithium-ion (Li-ion) batteries have a significantly higher voltage of 3.6V per cell. This higher voltage in Li-ion batteries means that, for the same power output, fewer batteries are needed, reducing both the weight and the size of the final battery pack. While the weight of a Li-ion battery is similar to that of a NiCd battery for a single cell, the NiMH battery tends to be heavier due to its lower voltage. Therefore, while NiMH batteries offer improved performance over NiCd batteries, they may result in slightly bulkier battery packs when compared to Li-ion solutions.

2. Memory Effect

The memory effect refers to a phenomenon where a battery "remembers" a partial discharge cycle and can lose its total capacity if it is consistently recharged before being fully discharged. NiMH batteries, like NiCd batteries, do suffer from a memory effect, although it is far less pronounced. The memory effect in NiMH is significantly lower than in NiCd batteries but still greater than that of lithium-ion batteries. To prevent or reduce this issue, it is recommended to perform a full charge-discharge cycle every few months, typically every three months. However, the memory effect in NiMH batteries is not as problematic as in NiCd batteries and may be ignored in many practical applications.

On the other hand, lithium-ion batteries are renowned for their almost negligible memory effect. Users of Li-ion batteries do not need to worry about partially discharging their battery before charging, making them more user-friendly. The simplicity and ease of use of Li-ion batteries contribute significantly to their popularity in modern electronic devices.

3. Self-Discharge Rate

Self-discharge is the natural phenomenon where a battery loses charge even when not in use. The self-discharge rate is an important factor in determining how often a battery needs to be recharged. NiCd batteries typically have a self-discharge rate between 15-30% per month, while NiMH batteries have a slightly higher self-discharge rate of 25-35% per month. In contrast, lithium-ion batteries are much more efficient in this regard, with a self-discharge rate of only 2-5% per month. This low self-discharge rate is one of the key advantages of lithium-ion batteries, as they retain charge much longer when not in use, reducing the need for frequent recharging.

4. Charging Methods

Both NiMH and Li-ion batteries require careful attention during the charging process, as they cannot tolerate overcharging. For NiMH batteries, the most effective charging method is using a constant current charging system with a cutoff mechanism (often referred to as the PICK CUT control), which stops charging once the maximum voltage has been reached. On the other hand, Li-ion batteries typically use a constant current, constant voltage charging method, which ensures that the battery is charged safely and efficiently. Charging NiMH or Li-ion batteries using chargers designed for NiCd batteries, such as those using the Delta-V (DV) control method, can be harmful to the batteries and may lead to reduced lifespan or potential damage.