User manual

There are also chargers with thermal monitoring and deactivation (e.g. in rechar-

geable batteries of low-cost cordless screwdrivers). This works best with NiCd

rechargeable batteries, since they convert the energy owing in from the charger

into heat when fully charged. Thus, the electronics can understand that the rechar-

geable battery is now fully charged.

In simple mains chargers the charge current limitation of the constant current

charge is implemented by a resistor that is inserted between the mains unit and

the rechargeable battery and controls the charging current.

The resistor R1 is calculated with the formula: R = U/I. R is the resistance in Ohm,

U the voltage in volt and I the current in ampere. R1 should be sized so that the

charging current is suitable for the rechargeable battery.

Fig. 029: Principle circuit diagram constant current charger; R1 is the resistor

relevant for the charging current.

The charging method with constant current is very simple but has a few disad-

vantages: The rechargeable battery should be fully discharged and the charging

current should be about C/10 of the rechargeable battery capacity to balance out

inaccuracies by slight but harmless overcharging. If the rechargeable battery is

quick-charge-capable, the charging current may be higher if the charging time is

accordingly shorter as well.

Older NiCd rechargeable batteries may suffer from a memory effect if the rechar-

geable battery has not been fully discharged.