02 July 2007

Charger for NiMH 9.6V pack

I quickly got fed up of removing all 8 batteries from the container just to recharge them. So why not building a generic charger for NiMH battery packs?

At home I had two chips that work as smart chargers with charge detection: U2400B and MAX713. The former is old and supports NiCd only. The latter is mainly for fast charging (C/2 and higher current), which is not something I am looking for right now.

So I did some searches and looks like NiMH can be properly charged at a constant C/10 current for 15 hours: I need a constant current source and a timed AC plug.

I looked at newsgroups and I found an active thread it.hobby.elettronica about a simple charger. Giorgio was kind enough to provide me a FidoCAD diagram of a current generator for 260 mA.

At home I had a BD646 which took place of the TIP1x5 Darlington PNP. I also swapped the 3V Zener for two red LEDs in series as suggested by Giorgio. NB: don't forget LEDs will be forward polarized!

Will add the 6.8ohm/1W resistor soon, and let you know what happens!