Philip Pemberton wrote:
> So far the best idea I've come up with is to have swappable battery holders 
> and a voltage divider. The pulldown resistor is in the charger, the input 
> resistor is in the battery holder. A five-pin connector connects the pack to 
> the charger (2 pins for +/-, one pin for sense, two pins for telling the 
> charger how many cells the holder is designed for.

Can't you tell that just from the value you set for the reference voltage?
Seems to me 3 pins is all you need since the number of cells also determines
the reference voltage you want.

> Or I could redesign the voltage amp using a digital potentiometer or 
> something. 

Or something. 1% resistors attached to tri state pins on PIC to change
values in binary. If you're clever about the R values you can probably
get away with ONE pin. Low, Tristate, High for  1, 2, 4 cells.

> Divide it down as if there was a four-cell pack connected, then use 
> the digipot to vary the gain of an op-amp stage... Decisions, decisions.

See above.
You can use diodes to give you different R values for pull up and
pull down, if you don't mind their change in voltage drop with temperature
(which can be compensated by a matching diode elsewhere in the circuit).

R
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