charging 40maH backup battery

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#1
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Hi guys,
maybe slightly offtopic, but I hope someone can give me an advice.
My device is a cash register that works 9-20, then turned off overnight. While powered off, CPU (mega128) is fed from small 40 maH NiMH battery; CPU sleeps in powersave mode and wakes up every second from Timer0 interrupt to keep real time/date. While powered on, backup battery gots 1ma trickle charge.
The question is: Does anybody have practical experience in using NiMH battery for such purposes? What charging algorithm is the best (maybe not the best, but just suitable)?
Different sources found with google give different recomendations...

Cats never lie. At least, they do this rarely.

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To safely charge a NiMH battery without any hardware monitoring, charge it at its 10% capacity. For a 40mAH one, this is 4mA.

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I know that C/10 is often cited as the safe trickle charge rate for NiMH batteries, but I have read stuff on the web that disputes this, and suggests that damage may occur if NiMH batteries(or cells) are subjected to long-term charging at this rate. I would suggest that micklecat does some Googling first. I have used C/16 for trickle charging, dropping to C/64 after 24 hours. Of course, if this is just for private/hobby use, then ignore me. I'm talking about a commercial product.

Four legs good, two legs bad, three legs stable.

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Did you consider using supercaps instead ? These are much easier to use and can be charged quickly without problems. I think they are much more suitable for your application

Patrick

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Micklecat,

A few weeks ago I've been playing with the LM3647 from National to build 10 NiCd-chargers. It's a universal battery charger IC for NiCd, NiMH an Li-ions as well.
Very simple to use and costs something close to nothing.

Maybe you can draw some usefull information out of this link:
http://www.national.com/ds/LM/LM3647.pdf

Hope this helps, :D

Regards,

- - big bang - -

P.S.: it's only available in SMD package but I don't think that wouldn't be a problem

Take a good look around. Admire our world, our galaxy, the whole universe. Then you just feel there must be a
'Theory of everything'

Maybe, someday, the human race is ready to discover it.

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Thanks all,
unfortunately, supercaps are not suitable because of retention time requirements; and using charger chip is impossible too because of price increase ($2,4 here in Kiev). Cat will find his own path... probably, smart charging with measuring battery voltage with Mega's ADC. Anyway, thanks all.

Cats never lie. At least, they do this rarely.