How to connect two 12 volt batteries together
There are a few ways to connect batteries together, really depends on what voltage or capacity your requirements are. Drilling right down to the basics, In a 12 volt configuration where more capacity is needed you would connect two batteries (or more) in parallel. For a 24 volt configuration you would just connect two 12 volt batteries in series to achieve the required voltage. Either of these can be done easily by connecting the terminals in the correct way.
How to connect 2 x 12 volt batteries in parallel
Using a jumper cable, connect between the negative terminals of both batteries. Using another jumper cable connect between the positive terminals of both batteries.
To use the parallel connection on an appliance, connect the positive and negative leads of the appliance you are using to the corresponding terminals on ONE of the batteries.
You can use either battery, but make sure you connect both leads to the same battery. Your appliance will now have use of the combined capacity of the two batteries.
How to series 2 x 12 volt batteries to achieve 24 Volts
First step to getting a 24 volt configuration is to connect a jumper cable between the Positive terminal of the first battery to the Negative terminal of the other battery.
Next step is to connect the Negative terminal of the first battery to the Negative lead from the 24 volt appliance the batteries will be powering.
Connect the Positive terminal of the second battery to the Positive lead of the appliance, the batteries are now in a series configuration and the 24 volt appliance can be powered.
How to connect 4 x 12 volt batteries in series parallel
Step 1. Connect the negative terminals of battery set 'A' together.
Step 2. Connect the positive terminal of battery set 'A' to the negative terminals of battery set 'B'.
Step 3. Connect the positive terminals of battery set 'B' together.
You can now connect the negative terminal of ONE battery in set 'A' to the appliance and the positive terminal of ONE battery in set 'B'.