I should have mentioned that this needs to be put inside of a charger for
auto-negotiation.  I've been doing more research, and I'm thinking the way
to go may be to apply an AC signal (100-1K Hz), measure the AC voltage of
the signal across the battery, and calculate the battery impedance.  This
would be different between packs because of the parallelled resistances in
the 9-cell battery.  Something like this <
http://www.maxim-ic.com/appnotes.cfm/appnote_number/975>, but likely using
an Atmel ATtiny.

What do you guys think?

-Shawn


On 12/19/05, William Chops Westfield <westfw@mac.com> wrote:
>
>
> On Dec 19, 2005, at 3:13 PM, Shawn Wilton wrote:
>
> > I've been googling for how to detect the difference between a 3(series)
> > cell lithium pack and a 9(3x3) cell lithium pack.
>
> > I was thinking maybe create a dummy load, with a sense resistor and
> > determine how much current the pack would kick out, but that doesn't
> > seem to give a different value across the packs.
> >
> At any useful level, this would probably cause the pack's over-current
> prediction to kick in.  Most Li-ion cells will provide way more current
> than the over-current protection is designed to allow (it's probably not
> good for the cells, but it will happen!)
>
> You should be able to figure it out by seeing how much the cell voltage
> changes when a known amount of power is put into or out of the pack,
> since
> the Li-ion cell discharge curve is relatively linear.  Pump 5W into a
> "uncharged" 1000mAH pack and it should move 3x further up the charge
> curve than pumping the same energy into the 3000mAH pack..  But I'm not
> sure how long it would take to get meaningful numbers out, and one
> might get confused by partially charged packs and such...
>
> BillW
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--


Shawn Wilton (b9 Systems)
http://black9.com
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