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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