Cheap and easy:

Use a diode to pass the current from the smaller battery.  It won't
contribute (much) to the load until the main battery has a voltage
less than the backup battery minus the diode voltage drop.

-Adam

On 2/19/08, Ariel Rocholl <foros@arocholl.com> wrote:
> I would like to connect two batteries in parallel to service a RF
> receiver. Both are Lipo batteries but battery A is high capacity and
> main battery whereas battery B is reserve low capacity battery.
>
> The average consumption of the RF receiver is about 2A, battery A is
> 5A/h and battery B is 1.5A/h.
>
> I do not see an easy way to:
>
> 1) have battery A depleted then (and only then)
> 2) switch to battery B till it is depleted too.
>
> I would assume a large capacitor plus a very fast switching transistor
> (IGBT?) may help to make it transparent to the load to a certain
> extent.
>
> But perhaps anyone knows a better way to do this. Any help really
> appreciated, I wouldn't like to reinvent the wheel but, except for low
> current ICs in Maxim, I didn't find anything else doing this already.
>
> TIA
>
> --
> Ariel Rocholl
> Madrid, Spain
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