AN417

Charge termination methods

Doc ID 2074 Rev 2

5/21

2

Charge termination methods

Basically, NiCd and NiMH batteries are charged by a constant current source (see Figure 1).

A battery charger is made of a constant current source controlled by a microcontroller which

monitors the battery voltage variation with its internal analog-to-digital converter

Figure 1.

Battery charger circuit diagram

As soon as the full capacity of the battery has been detected by the microcontroller, the

charging is stopped by turning the current off. Schematic examples of power converters

operating as current sources are given later. The same converter hardware can be used in

two different charging methods depending upon the appliance requirements.

2.1

The [-

Δ V] method

When a NiCd battery reaches full charge, its voltage decreases slightly (see Figure 2). The

negative delta voltage method [-

slope versus time becomes negative. This first charge termination technique is optimized to

fast charge a NiCd battery to its full capacity.

micro controller

cell voltage

I charge

battery

Vin

NiCd

or

NiMH

ST62xx