Forrest,

One thing you might need to consider is that if the NiMH cells are=20
charged as a pack they will become unbalanced in charge over time.

Mark Skeels
Engineer
Competition Electronics, Inc.
TEL: 815-874-8001
FAX: 815-874-8181
www.competitionelectronics.com

On 10/11/2010 11:13 PM, Forrest W Christian wrote:
>    I am finally getting back to a project which will require a
> rechargeable battery inside...  Because of cost, apparent reliability,
> ability to obtain replacements easily, etc., I'm likely going to end up
> with a battery made up of multiple NiMH AA cells.
>
> I would like to charge the pack from A roughly 12V source (I.E. be able
> to plug it into a automotive lighter outlet, which is typically
> somewhere between 12 and 14 volts).
>
> I need around 20Watt-hours.  This means I need around 8 good quality
> cells, or a 9.6V pack, which should be fairly easy to charge from 12V...
> however....
>
> My application also requires around 18VDC out - so a 9.6V pack would
> require a step-up DC-DC converter for the output....  So, a 15 or 16
> cell pack would eliminate that part, but perhaps make the charger
> circuit more complicated, depending on the charge versus load current
> requirements.
>
> I also prefer to avoid having to heatsink in any meaningful way the
> 'output stages' of the charge controller - that is, a linear charger
> probably isn't what I'm looking for.
>
> In an ideal world, there would be a switchmode charge controler that
> with the IC and a half-dozen fairly non-critical external components, it
> would magically take around 12V in and charge an 18V NiMH battery  pack
> reliably and correctly.   Unfortunately doesn't seem to exist.
>
> So, I guess my question is... did I miss a 'perfect one'?  And if not,
> what is your favorite 'pack charging' NiMH charge controller?
>
> -forrest
--=20
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