Similar to alkaline batteries, rechargeable 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 rechargeable battery are reversible, allowing them to be recharged again and again.
Container - A steel can that houses the cell's active components, and conducts electronic current during charge and discharge. - Electrodes - Where the reduction/oxidation (redox) electrochemical reactions take place.
- Positive electrode - Nickel hydroxide, Ni(OH)2, is the positive electrode.
- Negative electrode - A hydrogen-absorbing metal alloy (or metal hydride, abbreviated MH) is the negative electrode.
- Separator - A non-woven polymer "cloth" that electrically insulates the electrodes from each other, forcing electrons to flow through external circuitry to get from one electrode to the other. Pores in the separator hold the electrolyte.
- Electrolyte - Potassium hydroxide (K+ and OH- ) dissolved in water is the electrolyte. The electrolyte carries the ionic current inside the battery.
CHARGING THE BATTERY
Before a rechargeable battery can be used, it must be charged. NiMH batteries are built in a discharged state.
Just like an alkaline battery, NiMH 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, batteries include components called separators and current collectors.
When a NiMH battery is charged, the 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 charger, the 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 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 DEVICE
During discharge, all of the electrochemical reactions that happened during charge happen again – in reverse.
