Nickel Metal Hydride (NiMH) Battery Chemistry
A Nickel Metal Hydride (NiMH) battery is a rechargeable type of battery with a nominal voltage of 1.2V per cell. NiMH batteries use Nickel Oxide Hydroxide (NiOOH) at the positive electrode and hydrogen-absorbing alloy (hydride) at the negative electrode.
When compared with Nickel Cadmium (NiCd) batteries, NiMH batteries feature larger capacity, almost no memory effect, higher energy density, and support a larger number of charging/discharging cycles.
Published: September 9, 2021.
NiMH Battery Features and Specifications
NiMH batteries feature nominal voltage of 1.2 volts, open-circuit voltage up to 1.4 volts, drain voltage under light load up to 1.25 volts, and cutoff voltage down to 1.0-1.1 volts.
Because of this, NiMH batteries are very often used instead of Zinc-Carbon and Alkaline cylindrical batteries, like AAAA, AAA, AA, C-Cell, D-Cell, and similar.
Although Zinc-Carbon and Alkaline batteries feature 1.5V nominal voltage, most devices that accept 1.5V batteries work properly when powered by NiMH 1.2V batteries - Zinc-Carbon and Alkaline batteries feature cutoff voltages below 1.0 volts, lower than the cutoff voltage of the NiMH battery.
Also, NiMH batteries feature very low internal resistance and tolerate high-drain applications much better than Alkaline and especially Zinc-Carbon batteries, making them a preferred battery type (over Zinc-Carbon and/or Alkaline batteries) in high-drain applications.
However, most high-drain applications like flashlights, cameras and similar have switched from NiMH batteries to Lithium batteries.
NiMH batteries support a large number of charging/discharging cycles that vary from a couple of hundreds up to 2000 charging cycles, depending on the battery model, type of charger, battery age, temperature, and similar.
Older NiMH batteries suffered from memory effect, although much weaker when compared with the memory effect of the NiCd (Nickel Cadmium) batteries. Modern NiMH batteries practically have no memory effect.
However, it is a good practice to periodically fully discharge the NiMH batteries using a battery charger with a discharge function and then recharge it fully again.
Note: discharging the NiMH batteries below a certain level can cause polarity reversal and permanent damage to the battery.
Self-discharge rate depends on the temperature and battery model - for example, Panasonic Eneloop NiMH batteries after 10 years(!) of storage still retain 70% of the initial charge.
Since NiCd batteries contain toxic cadmium, NiMH batteries are preferred batteries for various tools, toys, game controllers, flashlights, electric vehicles, and similar.
However, lightweight lithium batteries are becoming more and more popular and affordable, making them a preferred choice for many applications where lightweight batteries with high energy density are required.
AAAA NiMH Battery
The following chart lists the most common chemistry types of AAAA batteries:
| Chemistry | Zinc-Carbon | Alkaline | Nickel-Cadmium | Nickel Metal Hydride | Lithium-Ion |
| Rechargeable | No | No | Yes | Yes | Yes |
| Typical Voltage | 1.5 V | 1.5 V | 1.2 V | 1.2 V | ~3.6 V |
| Typical Capacity | 300 mAh | 400 - 600 mAh | 200-400 mAh | 350 - 500 mAh | 170 mAh |
NiMH AAAA batteries feature a larger capacity than Zinc-Carbon batteries and a capacity very similar to Alkaline AAAA batteries. But, NiMH batteries may be recharged, while Zinc-Carbon and Alkaline AAAA batteries cannot.
Although lithium AAAA batteries are available, they are not directly compatible with other AAAA batteries due to the voltage difference.
AAA NiMH Battery
The following chart lists the most common chemistry types of AAA batteries:
| Chemistry | Common Name | Rechargeable | Typical Capacity (mAh) | Voltage |
| Zinc Carbon | R03, 24D | No | 500-600 | 1.5 V |
| Alkaline | LR03, 24A | No (Mostly No) | 850-1200 | 1.5 V |
| Li-FeS2 | FR03, 24LF | No | 1100-1300 | 1.5 V (1.8 V max) |
| Li-ion | 10440 | Yes | 350-600 | 3.6 - 3.7 V |
| NiCd | KR03, 24K | Yes | 300-500 | 1.2 V |
| NiMH | HR03, 24H | Yes | 600-1300 | 1.2 V |
| NiOOH | ZR03 | No | 1000-1200 | 1.5 V (1.7 V max) |
AAA batteries are larger and more common than AAAA batteries, and are available in more chemistries, each having its own pros and cons.
AAA NiMH batteries offer good capacity combined with a large number of charging/discharging cycles and good discharge characteristics.
However, the actual capacity of the AAA NiMH batteries differs depending on their internal construction (high-drain, low-drain, multi-drain) and intended use.
AA NiMH Battery
The following table lists the most common AA battery chemistries and their features, including their most important feature, AA battery voltage:
| Chemistry | Common Name | Rechargeable | Typical Capacity (mAh) | Voltage (V) |
| Zinc Carbon | R6, 15D | No | 600 - 1600 | 1.5 |
| Alkaline | LR6, 15A | No (Mostly No) | 1800 - 2700 | 1.5 |
| Li-FeS2 | FR6, 15LF | No | 2700 - 3300 | 1.5 (1.8 max) |
| Li-ion | 14500 | Yes | 600 - 2000+ | 3.6 - 3.7 |
| Li-SOCl2 | (14505) | No | 2400-2700 | 3.5-3.6 |
| Li-MnO2 | CR AA | No | ~2000 | 3.0 |
| NiCd | KR6, 1.2K2 | Yes | 600 - 1000 | 1.2 |
| NiMH | HR6, 1.2H2 | Yes | 700 - 2800 | 1.2 |
| NiOOH | - | No | 2200 - 2700 | 1.5 (1.7 max) |
| NiZn | ZR6 | Yes | 1500 - 1800 | 1.6 - 1.65 |
AA NiMH batteries are very popular AA rechargeable batteries, commonly found in remote controls, toys, game controllers, some flashlights, wireless devices, and similar.
Panasonic Eneloop AA NiMH battery, when charged with a proper charger can withstand up to 2000 charging/discharging cycles.
D-Cell NiMH Battery
The following comparison chart lists some of most common chemistries of the D-Cell batteries:
| Chemistry | Zinc-Carbon | Alkaline | NiCd | NiMH |
| Typical Label | R20, 13D | LR20, 13A | KR20 | HR20, B006 |
| Rechargeable | No | No (Mostly No) | Yes | Yes |
| Typical Capacity | 8 Ah | 12-18 Ah | 2-6 Ah | 8-12 Ah |
| Nominal Voltage | 1.5 V | 1.5 V | 1.2 V | 1.2 V |
As one can see, the capacity of the NiMH D-Cell battery is comparable with the capacity of the alkaline D-Cell Alkaline battery. When the Alkaline D-Cell battery is discharged using higher currents, its actual capacity drops rather quickly.
Since the NiMH battery can be recharged many times, D-Cell NiMH batteries can be a rather cost-effective solution for cycling applications based on the D-Cell batteries.
9V NiMH Battery
The following comparison chart lists some of the most popular 9-volt battery chemistries with their most important features and specifications:
| Battery Type | Rechargeable | Typical Capacity | Nominal Voltage |
| Zinc-Carbon | No | 400 mAh | 9V |
| Alkaline | No (Mostly No) | 550-600 mAh | 9V |
| Lithium | No | 1200 mAh | 9V |
| NiCd | Yes | 100-120 mAh | 7.2, 8.4, 9.6V |
| NiMH | Yes | 175-300 mAh | 7.2, 8.4, 9.6V |
| Lithium Polymer | Yes | 500-550 mAh | 7.4V (11.1V) |
| Lithium-Ion | Yes | 600-650 mAh | 7.4V (11.1V) |
| LiFePO4 | Yes | 200-320 mAh | 9.6V |
9V NiMH batteries have a nominal voltage of 9 volts, however, internally they may consist of 6, 7, or 8 cells connected in series in order to achieve 7.2, 8.4, or 9.6 volts, respectively.
Most 9V NiMH batteries feature 7 cells connected in series providing an actual voltage of 8.4 volts that slowly decreases down to the cutoff voltage of 7.0-7.7 volts, making NiMH 9V batteries compatible with 9V Carbon-Zinc, Alkaline, and non-rechargeable lithium (Li-MnO2) batteries.
NiMH batteries are also commonly used in other dimensions like C-Cell, Sub-C cells, 6V lantern batteries (5 NiMH cells in series), etc.
With a good discharge rate, affordable price, good energy density, NiMH batteries are still a very popular rechargeable battery type.
However, with advancements in technology and especially chemistry, lithium rechargeable batteries are replacing NiMH batteries in many applications.