Button cells are suitable for small electronic appliances such as wristwatches, microphones, handy remote controllers, electronic pocketbooks, electronic thermometers, car keys, and so on. These cells can be used at low temperatures. Button cells are usually divided into rechargeable and non-rechargeable ones. The English letters on a button-cell battery represent the type of battery while the number represents the size.
Why do we need Button Cell Charger?
NiCd charger would overcharge NiMH. Do not leave a nickel-based battery in the charger for more than a few days. The charge algorithm for NiMH is similar to NiCd with the exception that NiMH is more complex. Negative Delta V to detect full charge is faint, especially when charging at less than 0.5C. A mismatched or hot pack reduces the symptoms further.
In miniature circuits, the trend is away from expensive dry button cells and towards NiCd button types. These batteries are easy to maintain: however, they may be with a constant current for a certain fixed period, normally 14-16 hours.
The usual NiCd battery charger is generally not suitable for charging button cells, as their minimum charging current is too high for these cells. However, the button cell charger described here operates from a 9 V mains adaptor and can charge from one to five button cells.
Button Cell Charger Circuit Diagram:
D1 is a protection diode, while R1 and C1 decouple the supply line to charging processor IC1, an economy model of the well-known U2400B. D3 is a safeguard against polarity reversal of the button cells. R4 limits the charging current to 5 mA. During trickle charging, a current of 0.5 mA flows through R4 and R5. The value of C2 determines the charging time. After the mains adaptor is plugged in, the cells are charged at full current for a period shown in the table (T1 is ON): after that period trickle charging takes place (T1 is OFF). The trickle charging current is always 1/10 of the full charging current.