Parallel battery cables with a set of battery clamps.
A set of 50 cm long heavy duty cables allowing you to connect a second battery on your 12 volt system. The heavy duty brass battery clamps allow better connection to the battery terminals.
Connecting negative on your first battery to negative on your second battery and positive on your first battery to the positive on your second battery.
This allows you to have more battery storage allowing more power to readily available to you should you need it but keeps the voltage at 12 volt.