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
>   
We use(ed) to use nimh and nicad in robot wars. We would typically suck 
a pack down 80% in 3 minutes
We would then give it about 15 minutes of forced air cooling (the cells 
would typically be 70C minimum, we saw 120c more than once, but that 
caused the cells to leak some)
then stick it on charge for 40 minutes or so at 5 amps or so (~3Ah 
cells) with the cooling running, let that charge finish, then do another 
charge at ~1A or so for another 20 minutes to "top it off"
after another 10 minutes or so they were cooled down enough to go back 
in the bot.

Needless to say we were fairly abusive of the packs.
For all that only one person had a major problem, his pack shorted every 
pair of battery's to the frame of his (aluminium) bot.
These cells were good for 200A pulse discharge. Lets just say his bot 
got rather warm. And more than a little smokey.
and then his battery compartment filled up a little with some boiling 
electrolyte.

We stopped the match and got an IR thermometer onto it, he managed to 
hit 160C or so then he pulled it all apart.
washed his bot out, (the electrolyte is pretty nasty stuff to metals), 
then rinsed his hands again with soap after I told him to ;->
He almost managed to keep his finger prints ;-> (after about 20 minutes 
of immersion, no permanent harm, keep it in mind if you want to pull off 
a bank job ;-P)

He still uses the pack as a dummy pack I believe for making sure his 
bots drive around etc, its pretty screwed in terms of supplying current.


My point is, you don't need to be that paranoid, these were C and larger 
cells with 100-200A pulse discharge and sometimes 10A(4C) charge rates
If your worried put a temperature sensor in the pack, Its also a good 
idea as a charge termination (if you can pick up the temperature spike 
as the cells are about to hit charged its actually a better measure than 
-dv/dt sensing)

We have mainly gone to lithiums now, in a similar environment, 
Personally we use A123 packs (taken from dewalt drills) because they 
don't do that whole "supply their own oxygen to a lithium fire" thing 
and are sposed to be more robust than the li-polys.
That said, the same guy who melted his pack last time uses li-polys. We 
are mainly concerned about physical damage to those, but personally I'd 
use one of those li-poly charging bags if i was running those.



A thought for you is to perhaps look at using an unmodified drill pack 
in your device if you can, that way people can easily buy batteries for 
it (they do die over time) and as a result can quick swap with a few 
spare packs.


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