Cars, trucks, RVs, and motorhomes run dual 12-volt batteries for various reasons. Depending on how you wire a two-battery 12-volt system, the result can be a 12-volt system or a 24-volt system ...

A 9 Volt battery has about 500 mA of capacity typically, ... If you wire this circuit incompetently you can get hot on both terminals, and it still appears to work correctly.