In article , Harry Bloomfield
wrote:
Brian Gaff has brought this to us :
If it were a low power use I'd say isolate them by diodes and charge
one at a time but in this case, either get a battery capable of
starting the vehicle on its own, make sure but batteries are
switchable and keep both charged and use them in rotation.

You cannot isolate batteries via diodes, because it would create a
difference of 0.7v due to the voltage lost across the diode. 0.7v is a
lot of difference in battery voltage terms, much more than the
difference between a fully charged and a flat battery.


You certainly can isolate batteries via diodes. Lucas used to make a unit
-about the same size as an ignition coil specifially for this purpose. Our
tecnical survey vehicles used them in the 70's and '80s. After then, a
hefty relay, which closed when the alternator was producing enough volts
was used.

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from KT24 in Surrey, England