Similar to alkaline batteries, NiMH rechargeable batteries batteries have four basic components - a positive electrode (cathode during discharge), a negative electrode (anode during discharge), a separator, and electrolyte. But that's where the similarities end. The chemicals reactions that occur inside a NiMH rechargeable battery are reversible, allowing them to be recharged again and again.
Charging The NiMH Rechargeable Battery
Before a NiMH rechargeable battery can be used, it must be charged. NiMH batteries are built in a discharged state.
Just like an alkaline battery, NiMH rechargeable batteries provide electrical power from a chemical reaction.
When a chemical reaction happens in a battery, electrons are either produced or consumed. This kind of reaction is called an electrochemical reaction. To make these electrons flow in a useful direction, NiMH rechargeable batteries include components called separators and current collectors.
When a NiMH rechargeable battery is charged, the battery charger pumps electrons out of the positive electrode. This causes the nickel in this electrode to oxidize, or change from Ni+2 to Ni+3. During this oxidation, hydrogen atoms leave the positive electrode and react with the electrolyte.
Simultaneously during charge, the charger pumps electrons into the negative electrode. This causes a reduction reaction to occur, and causes this electrode to absorb hydrogen from the electrolyte.
Based upon the type of battery charger, the battery charger will automatically stop pumping electrons out of the positive electrode into the negative electrode after most of the hydrogen has been removed from the nickel. The battery charger can tell when this happens in one of three ways: through a change in temperature, a change in voltage, or by a built-in timer.
Powering The Electronic Device
During discharge, all of the electrochemical reactions that happened during charge happen again – in reverse.
