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So your coach doesn’t have two starting batteries? Unusual for a diesel.
If you want to connect two 12 volt batteries for your starting batteries you will want to hook them up in parallel and not series. When you connect in series you double the voltage. When you connect in parallel you double the amperage.
Your coach batteries should be two to more 6 volt batteries connected in series to get you 12 volts to the coach. You double the voltage.
Message Body : My diesel motorhome uses a D size battery for starting. I would prefer to use two car batteries in series – easier to handle,more readily available etc. What is your opinion?