In article ,
Harry Bloomfield wrote:
charles laid this down on his screen :
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.

Because they realised that diodes just did not and could not work?

They did work - we used them for at least 15 years, running technical
equipment off the second battery.

The only way you can make diodes work effectively, is by increasing the
input voltage by 0.6 to 0.7 volts to take care of the forward voltage
lost across the junction.

the relay was better.

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from KT24 in Surrey, England