During charging, the positive active material is oxidized, producing electrons, and the negative material is reduced, consuming electrons. These electrons constitute the current flow in the external circuit. The electrolyte may serve as a simple buffer for ion flow between the electrodes, as in lithium-ion and nickel-cadmium cells, or it may be an active participant in the electrochemical reaction, as in lead-acid cells.

 

Battery charger

 

A solar-powered charger for rechargeable AA batteries

The energy used to charge rechargeable batteries usually comes from a battery charger using AC mains electricity. Chargers take from a few minutes (rapid chargers) to several hours to charge a battery. Most batteries are capable of being charged far faster than simple battery chargers are capable of; there are chargers that can charge consumer sizes of NiMH batteries in 15 minutes. Fast charges must have multiple ways of detecting full charge (voltage, temperature, etc.) to stop charging before onset of harmful overcharging.

Rechargeable multi-cell batteries are susceptible to cell damage due to reverse charging if they are fully discharged. Fully integrated battery chargers that optimize the charging current are available.

Attempting to recharge non-rechargeable batteries with unsuitable equipment may cause battery explosion^{[citation needed]}.

Flow batteries, used for specialised applications, are recharged by replacing the electrolyte liquid.

Battery manufacturers' technical notes often refer to VPC; this is volts per cell, and refers to the individual secondary cells that make up the battery. For example, to charge a 12 V battery (containing 6 cells of 2 V each) at 2.3 VPC requires a voltage of 13.8 V across the battery's terminals.

Non-rechargeable alkaline and zinc-carbon cells output 1.5V when new, but this voltage gradually drops with use. Most NiMH AA and AAA batteries rate their cells at 1.2 V, and can usually be used in equipment designed to use alkaline batteries up to an end-point of 0.9 to 1.2V^{[citation needed]}.

[edit] Reverse charging

Subjecting a discharged cell to a current in the direction which tends to discharge it further, rather than charge it, is called reverse charging; this damages cells. Reverse charging can occur under a number of circumstances, the two most common being:

• When a battery or cell is connected to a charging circuit the wrong way round.
• When a battery made of several cells connected in series is deeply discharged.

When one cell completely discharges ahead of the rest, the live cells will apply a reverse current to the discharged cell ("cell reversal"). This can happen even to a "weak" cell that is not fully discharged. If the battery drain current is high enough, the weak cell's internal resistance can experience a reverse voltage that is greater than the cell's remaining internal forward voltage. This results in the reversal of the weak cell's polarity while the current is flowing through the cells.^[3][4] This can significantly shorten the life of the affected cell and therefore of the battery. The higher the discharge rate of the battery needs to be, the better matched the cells should be, both in kind of cell and state of charge. In some extreme cases, the reversed cell can begin to emit smoke or catch fire.

In critical applications using Ni-Cad batteries, such as in aircraft, each cell is individually discharged by connecting a load clip across the terminals of each cell, thereby avoiding cell reversal, then charging the cells in series.^{[citation needed]}

[edit] Depth of discharge

Depth of discharge (DOD) is normally stated as a percentage of the nominal ampere-hour capacity; 0% DOD means no discharge. Since the usable capacity of a battery system depends on the rate of discharge and the allowable voltage at the end of discharge, the depth of discharge must be qualified to show the way it is to be measured. Due to variations during manufacture and aging, the DOD for complete discharge can change over time or number of discharge cycles. Generally a rechargeable battery system will tolerate more charge/discharge cycles if the DOD is lower on each cycle.^[5]