Battery Charger

A battery charger is a device used to put energy into a cell or Battery by forcing an Electric Current through that cell or battery.

The charge current depends upon the technology and capacity of the battery being charged. For example, the current that should be applied to a 12 V Car battery under charge will be very different to the current for a Mobile Phone battery.

TYPES OF BATTERY CHARGERS

Simple

A simple charger works by connecting a constant DC Power source to the battery being charged. The simple charger does not alter its output based on time or the charge on the battery. This simplicity means that a simple charger is inexpensive, but there is a tradeoff in quality. Typically, a simple charger takes longer to charge a battery to prevent severe over-charging. Even so, a battery left in a simple charger for too long will be weakened or destroyed due to over-charging.

Timer

Output as simple charger, but the output is terminated after a pre-determined time. Timer chargers were the most common type for Ni-Cd cells in the late 1990s for example.

Often a timer charger and set of batteries could be bought as a bundle, and the charger time was set to suit those batteries. If batteries of lower capacity were charged then they would be overcharged, and if batteries of higher capacity were charged they would be only partly charged. With the trend for battery technology to increase capacity year on year, an old timer charger would only partly charge the newer batteries.

Intelligent

Output current depends upon the battery's state. An intelligent charger may monitor the battery's voltage, temperature and/or time under charge to determine the optimum charge current at that instant. Charging is terminated when a combination of the voltage, temperature and/or time indicates that the battery is fully charged.

For NiCd and NiMH batteries, the voltage over the battery increases slowly during the charging process, until the battery is fully charged. After that, the voltage ''decreases'', which indicates to an intelligent charger that the battery is fully charged. Such chargers are often labeled as a ΔV charger, indicating that they monitor the voltage change.

"15 minute" superfast

The VARTA -Rayovac group sell "15 minute" superfast chargers for special NiMH cells. The peculiarity of such chargers is that the charging process isn't only controlled in the charger, but also in the cells themselves. The special 15 minute cells are typically rated at 2000/2100 MAh and contain control circuitry that makes such fast charging times possible without damaging the cells' elements.
There are various models of 15 minute chargers. Most have a cooling fan to help keep the temperature of the cells under control. All of them are also capable of acting as a standard overnight charger if used with standard NiMH cells that do not have the special control circuitry.

"Short circuiting"

A short circuit (connecting the output terminals together) does not usually damage a simple battery charger. For that reason it is a suitable source of DC voltage for experimentation. It may, however, require an external capacitor/condenser to be connected across its output terminals in order to "smooth" the voltage sufficiently, which may be thought of as a DC voltage plus a "ripple" (AC) voltage added to it. To see the difference between connecting, and not connecting, a capacitor/condenser connect also a Voltmeter and/or an Oscilloscope across the output terminals.

Note that there may be an internal resistance connected to limit the short circuit current, and the value of that internal resistance may have to be taken into consideration in experiments.

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Battery Charger