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I use a 1969 912 as my daily driver in an urban environment. As I understand, 911s of that vintage had two 12 volt batteries - one in each front corner - connected in parallel. 912s had a single battery.
I have installed two batteries for peace of mind. 912s have dynamos (generators) not alternators and these do not charge the battery as efficiently at low revs (idle). Also I notice at night, with lights and wipers on, the charge light glows very faintly at idle.
I use standard automotive batteries. I had no trouble finding ones to fit. Just be careful that the terminals/connectors do not touch the body. I have also installed an isolation switch. Otherwise it is a bit of a pain to have to disconnect two batteries when you want to isolate the electrics.
Finally, it is probably prudent to have both batteries fully charged before connection. I am not an expert but I avoid connecting a pair of batteries in parallel if they are not in a similar charge state.
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