Forrest W. Christian wrote:
> 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

Something that will complicate your design here is the fact that you are 
stacking 15 cells.  As the cells age they will charge at differing 
rates.  This has the effect of hiding any end-of-charge behavior.  For 
example, nicads peak and then droop slightly as they become full.  If 
all were in sync, it would be easy to detect as the effect would be 15 
times larger than for a single cell.  But when they get old, they all do 
this at differing times and the effect is washed out.  And for cells 
that lose capacity in old age, they charge quicker since they hold less 
and then are subject to overcharging while the good cells are still 
coming up.

Nate and others advice of a removable pack and oem charger is good 
advice although probably more expensive.

BTW is there any good reason to choose NiMH?  Lithium technologies beat 
NiMH in just about every category.
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