You could use FETs or even low power relays. Don't do any fancy switching, 
since you can overlap the batteries for a small instant for the switching.

Use either a comparator or a lm3914 (IIRC) as voltage level detector. Control 
the relays as inverse of each other and use a 555 timer to overlap the 
batteries for some small time (1 sec?). You could probably just put a few 
hundred uF cap in parallel with the first relay so it stays latched a second or 
two after the switching. QED. ;)


Ariel Rocholl 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
> 
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