I should have clarified that battery charge process is done
separatedly (in other words, batteries are plugged in completely
charged already).

2008/2/19, Bob Axtell <engineer@cotse.net>:
> You cannot do this because you cannot charge either battery correctly.
> You MIGHT be able to do something with a latching relay, but you will need
> to control it and sort out the  charging circuit.
>
> It would be easier if you had two identical batteries.
>
> --Bob A
>
> 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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-- 
Ariel Rocholl
Madrid, Spain
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