A nickel metal hydride (or NiMH) battery is a type of rechargeable battery similar to a nickel-cadmium battery (NiCd or NiCad) but which does not contain expensive (and environmentally risky) cadmium. This is why they are sometimes called the most environmentally friendly battery type. NiMH batteries tend to have a higher capacity than NiCads, suffer far less from voltage depression as they empty, but also have higher internal resistance than NiCds and suffer from larger voltage droops when supplying higher currents. This is difference in internal resistance is easily corrected by adding an extra cell to boost the voltage back to NiCd levels (along with the increased capacity of NiMh). However, when compared with lithium polymer batteries they have a lower energy density and a higher self-discharge rate. In addition to their use in radio controlled models, NiMH batteries are used with high drain electronic devices like digital cameras and other common electronic devices. For high torque power tools and other devices that require fast discharge rates (ie. large current draws), NiCad can outperform NiMH due to the diffences in internal resistance.
When fast-charging, it is advisable to charge the NiMH batteries with intelligent microprocessor-controlled battery chargers to prevent over-charging with large currents, which could damage the battery. Modern NiMH batteries contain catalysts to immediately deal with gases developed as a result of over-charging without being harmed (2 H2 + O2 ---catalyst--> 2 H2O). This however only works with over-charging currents of up to C/10 h (nominal capacity divided by 10 hours). As a result of this reaction, the batteries will heat up considerably, marking the end of the charging process. Some quick chargers (one hour or less) include fans to keep the batteries cool.
Some equipment manufacturers consider that NiMH can be safely charged in simple fixed (low) current chargers with or without timers, and that permanent over-charging is permissible with currents up to C/10 h. In fact, this is what happens in cheap cordless phone base stations and the cheapest battery chargers. Although this may be safe, it may not be good for the health of the battery. According to the Panasonic NiMH charging Manual (link below), permanent trickle charging (small current overcharging) can cause battery deterioration and the trickle charge rate should be limited to between 0.033×C per hour and 0.05×C per hour for a maximum of 20 hours to avoid damaging the batteries.
Long term maintenance charge of NiMH batteries needs to be by low duty cycle pulses of high current rather than continuous low current in order to preserve battery health.
Brand new batteries, or batteries which have been unused for some time, need "reforming" to reach their full capacity. For this reason new batteries may need several charge/discharge cycles before they operate to their advertised capacity.
Care must also be taken during discharge to ensure that a cell in a series battery (for example the common arrangement of four AA cells in series in a digital camera) does not become totally flat and then reverse charged. This reverse charge can cause irreparable damage to the cell. Usually the device (e.g. camera) detects the safe end of discharge voltage of the series cells and shuts itself down, thus saving the cells from damage.
A single cell driving a load generally cannot become reverse charged, simply discharging down to 0 volts. This does not cause irreparable damage, and in fact if performed periodically and followed by a full recharge will help to maintain the cell's capacity and performance.
Nickel metal hydride batteries have a high self-discharge rate of approximately 30% per month and more. This is higher than that of NiCd batteries, which is around 20% per month. The self-discharge rate is highest for full batteries and drops off somewhat for lower charges. The rate is strongly affected by the temperature at which the batteries are stored. Recommended long time storage charge is around 40%.
Common penlight-size (AA) batteries have nominal capacities C ranging from 1100 mA·h (3960 coulomb|C) to 2700 mA·h (9720 C) at 1.2 V; the true useful capacities can be significantly lower and depend on the discharge rate. NiMH batteries have an alkaline electrolyte. The specific energy density for NiMH material is approximately 60 W·h/kg (220 kJ/kg), with a volumetric energy density of about 100 W·h/L (360 MJ/m³).
NiMH battery technology was developed by Michigan-based Ovonic Battery, a division of ECD Ovonics (www.ovonic.com), the company founded in the 1950s by physicist Stanley Ovshinsky. NiMH batteries were made available to the public in 1983.
The "metal" in a NiMH battery is actually an intermetallic compound. Many different compounds have been developed for this application, but those in current use fall into two classes. The most common is AB5, where A is a rare earth mixture and/or titanium and B is nickel, cobalt, manganese, and/or aluminum. Higher-capacity "multi-component" electrodes are based on AB2 compounds, where A is titanium and/or vanadium and B is zirconium or nickel, modified with chromium, cobalt, iron, and/or manganese .
Any of these compounds serves the same role, reversibly forming a mixture of metal hydride compounds. When hydrogen ions are forced out of the potassium hydroxide electrolyte solution by the voltage applied during charging, this process prevents them from forming a gas, allowing a low pressure and volume to be maintained. As the battery is discharged, these same ions are released to participate in the reverse reaction.
- Panasonic NiMH charging manual (PDF)
- "The Nickel-based Battery" at BatteryUniversity.com
- Battery Care & Tips
- NiMH Battery Handbook
- Steve's Digicams - Rechargeable Battery Information
- NiMH Batteries and Chargers - Knowledge Base
- R/C Battery Clinic
- Batteries in a Portable World
- How to charge Nickel Metal Hydride Batteries.
- Battery Chargers and Charging Methods