Many boats carry more than one marine battery. You can connect marine batteries together in two different ways depending on your needs. For example, should you need to combine two 12-volt batteries in order to achieve a 24-volt battery power source, then you should connect them in series. Another situation might dictate that you have a 12-volt power source, but need the combined capacity of two batteries. This would require that you connect the batteries in parallel. Either of these can be done easily by connecting their positive and negative terminals in the correct way.
Connect batteries together in series by first running a jump lead between the negative terminal of one of the batteries and the positive terminal of the other battery.
Connect the positive terminal of the first battery to the positive lead from the 24-volt device the batteries will be powering. Connect the negative terminal of the second battery to the negative lead of the device, and you will have successfully connected the batteries in series and will have 24-volt power to the device.
Connect batteries in parallel by first running a jump lead between the negative terminals of both batteries.
Connect the positive terminals of the two batteries with a jump lead.
Complete the parallel connection by connecting the positive and negative leads of the device you are using to the corresponding terminals on one of the batteries. Use either battery, but be sure to connect both leads to the same battery. Your device will now have access to the combined capacity of the tow batteries.