More information needed.....
Is this a one off personal project or a proposed product for consumer use?
AA NiMh implies 2100 to 2800 mah cells. 2 amp charger minimum.
If they are to be charged in series at least 24 to 30 volts dc will need to 
be developed.

A quick & dirty solution for loose cells would be a kludge consisting of 5 
four cell camera battery chargers plugged into a power strip (squid is 
better) and a commercially built inverter.

You have $50 worth of batteries involved. If you need to charge them in 
series some sort of balancing charger will save money in the long haul.

Please note that the new NiMh technology that has the low self discharge 
rate also has a much higher internal impedance. Don't expect to be able to 
get the peak power levels of the older NiMh.
Great for cameras though!

John Ferrell  W8CCW

"All that is necessary for the triumph of evil is for good men to do 
nothing." -- Edmund Burke
http://DixieNC.US


----- Original Message ----- 
From: "Forrest W. Christian" <forrestc@imach.com>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Thursday, January 29, 2009 4:15 AM
Subject: [EE] Charging NiMH batteries


> I'll admit it.
>
> Any circuit which charges batteries scares me.  Perhaps I've put too
> much stock in those warnings on batteries and chargers, or seen the
> effect of an overcharged battery one too many times.   So, if a product
> design required charging batteries, I put it on the shelf for later,
> hoping that sometime I would conquer my fear.
>
> Well, I guess that there is no better time than now to conquer my fears.
>
> I have an application where I need to end up with a ~15 cell (18V) AA
> NiMH battery pack.  And I need to charge it from a automotive power
> source.   And preferably relatively quickly, like a 1 hour charger or
> similar.
>
> Oh, and I'd prefer to keep melting, flame, and explosions to a minimum. 
> :-O
>
> I've looked at a lot of charger circuits and I'm still scared.    What
> I'd really like is for someone (or someones) on the list to tell me how
> paranoid I am being (or not paranoid) and sort of give me some pointers
> of what to watch for.
> (Note I'm not suggesting LiIon... :)
>
> -forrest
> -- 
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