While the engine compartment is often the first. Attach that cable to the negative terminal on battery number two.

How Car Electrical Systems Work How A Car Works
How to wire two batteries in a car. How to add a second battery to a vehicle step 1. Cars trucks rvs and motorhomes run dual 12 volt batteries for various reasons. Wiring multiple power sources in series will increase the available voltage. First we measure the voltage from each battery. Wiring two batteries in series. Use the second battery cable to connect the negative terminal on battery number one to the positive terminal on battery number two.
If you install a second battery in the caravan link the two together as shown in drawing two then connect the positive from your solar regulator to the positive on battery b and the negative from your solar regulator to the negative on battery a that way any voltage drops will be equal between the two batteries. Stacey walks you through different ways you can wire dual batteries in your vehicle and talks about what is the best way. Pick a location inside the car to mount the second battery. Connect one of the battery cables to the positive terminal on battery number one. Then we wire them in series by connecting the negative lead connected to aluminum foil to the positive lead of the other battery. Mount a battery tray in the chosen location using a tray that fits the second batterys size.
The total voltage for the series connection will be 24 volts. Depending on how you wire a two battery 12 volt system the result can be a 12 volt system or a 24 volt system.