Theory

June 20, 1999
There are two types of batteries. Primary and Secondary. Primary batteries are non-rechargeable and must be disposed of when fully used up. These are carbon-zinc, alkaline, lithium, mercury, just to name a few. Secondary batteries are rechargeable and can be recharged numerous times before they eventually go bad. They are Nickel Cadmium (NiCad), Nickel-Metal Hydride (NiMH), Lead acid (wet), Lead acid (gelled), Lithium Ion (LiOn), and a few other specialized batteries for industrial operations. A battery is normally defined as a group of cells. Each type of cell has a voltage under 3 volts. Carbon-zinc and alkaline are nominally 1.5 volts. NiCad and NiMH around 1.25 volts. Lead-acid 2.1 volts. Lithium 3 volts. Cells are stacked in "series" to obtain a larger voltage. For the sake of discussion I will focus on the secondary, AA type cell that is used in most portable battery powered equipment. Penlight flashlights, digital cameras, GPS units, CB/FRS radios, FM walkie-talkies, portable CD players, AM/FM/SW radios, all use AA penlight cells.

The charging process is where an external power supply is applied in some fashion to the cell(s) at a rate fast enough for convenience, but not so fast as to overheat the cell, in order to bring them up to useful purposes.

The specified charge rate for most batteries is approximately 10% of the capacity of the cell's output for approximately 8 to 10 hours. A slower rate would be inconvenient but possible. A faster rate of charge, under controlled conditions can bring a cell up to full charge in about an hour. A cell's life is reduced if the cell overheats and "cooks out". This is the drying out of the electrolyte within the cell.

Careful monitoring of the temperature by the charge controller, and the monitoring of the state of charge by the controller while charging will prolong the life of the cell. "Smart" chargers as they are referred to, do just that and they may also send pulses of power to the cell rather than a steady current flow. This has a tendency to allow the electrodes in the cell to build up a uniform "coating" on them (especially wet cells), and between pulses, monitor the batteries condition to see if the charge is "taking". Once the charge controller senses a fully charged battery, it shuts off and provides no further charging. Some controllers however, may provide a continuous "trickle" charge to keep the battery "topped off" while in the charger for extended periods of time. Other smart chargers also have a "conditioning" mode that discharges the cells completely and then charges them up to full. This is especially important for new cells that haven't been used yet. They're able to provide full capacity after about 8-10 cycles of charge.

There are also two theories about NiCads. One is that they can have a "memory efffect". That is, if one keeps the battery fully charged without "cycling" it to full discharge (keeping it topped of or shallow discharges), it will remember that it cannot provide its full capacity and discharges in a short period of time. The other theory is that crystals form in the electrolyte if shallow charged and causes the cell to short. Hard charging till the cell gets a little hot burns the crystals out and allows for full capacity over its service life. See NiCad controversy for a discussion on this.