What Happens If You Overcharge a NiMH Battery?

Nickel-metal hydride batteries (NiMH) offer several advantages over their cadmium nickel counterparts including: higher energy density, safer low temperature performance and lower voltage drop during discharge. However, like all batteries they do have a limit to their charge and discharge cycles and overcharging will cause damage to the cell reducing its cycle life and capacity. Overcharged batteries can emit hydrogen gas that may explode in high concentrations if the battery is not properly ventilated. Using a charger with a predetermined termination current can prevent overcharge and overheating.

NiMH batteries have a lower self-discharge than alkaline and as such can be left in a charge for longer periods than their alkaline counterparts. This is not ideal as the batteries will lose some capacity due to their own internal chemical processes, and a trickle charge will also reduce their cycle life. It is important to use a charger with a low trickle current and to remove the batteries as soon as the charge cycle has finished.

When a NiMH battery is charged it generates heat especially during the early stages of charging. This is due to ohmic polarization which causes the internal impedance of the cell to increase as the battery charges causing a voltage drop. This voltage drop is proportional to the overall impedance of the cell and the rate of current (charge/discharge) the cell is subjected to. This makes it important to have a charger with the capability to monitor the internal cell voltage to ensure that the termination current is not being applied too quickly to the battery.

Battery overcharge may be caused by excessive heat or a short circuit in the positive terminal. The most common reason is excessive current from a poorly designed or calibrated charger. This can be caused by a charger that does not detect the end of a charge cycle or by a charger that is set to terminate at a very high current, which is not suitable for NiMH battery chemistry.

Using a timed termination method is one of the best ways to prevent overcharge and protect the cells, this requires the charger to have a thermistor to monitor the cell temperature, and a good quality analyzer that can read these signals correctly at low current settings. Another option is to use a -dv/dt termination, however this can cause miss terminations at low current settings and can lead to overcharge of partially discharged batteries.

The most common symptom of an overcharged battery is that it will emit hydrogen gas, a highly flammable substance with the unpleasant odor of rotten eggs. If the battery is not ventilated it can explode if the concentration of the gas is over 4%. A battery that has been overcharged will develop a swollen case and can rupture. It will also have a reduced capacity and cycle life and can suffer from permanent damage to the cell. It is recommended to always use a predetermined termination setting and never exceed the battery manufacturer’s maximum charge current.