Or connect a group 24 with a group 27 and group 31 sized battery. Charging 12 volt batteries wired in parallel requires a 12 volt battery charger.

Paralleling 3 Lead Acid Batteries All About Circuits
Connect batteries in parallel for charging. We assume when you plan to connect your batteries in parallel you are using the same type age and size of batteries. How to connect lithium batteries in parallel and series. For secondary rechargeable batteries the stronger battery would charge the weaker one draining itself and wasting energy. Series connection increases battery voltageparellel connection increases battery capacity. Connect your positive and negative wires to one battery and run your application. Connect red battery clip or lug to the positive battery post of any of the batteries connected in parallel.
With no resistance to slow this charging process the charged units can overheat as they rapidly drain and the discharged battery can overheat as it attempts to charge at well above its design. The process is simple but as the capacity of the batteries increases depending on the number of batteries wired in parallel it can take twice the time to charge two 12 volt batteries and three times as long for three batteries. Take another wire and connect the negative post of the first battery to the negative post of the second battery. If you connect rechargeable batteries in parallel and one is discharged while the others are charged the charged batteries will attempt to charge the discharged battery. Connect the other end to the positive terminal of the second battery. Always use heavy duty wires when connecting batteries in parallel.
Or connect 2 old batteries with 2 brand spanking new batteries. Then connect the black battery clip or lug to the negative battery post of any of the batteries. B1 pos to b2 pos and b1 neg to b2 neg battery capacity x number of batteries battery bank capacity. Following this example where there are two 12v 200ah batteries connected in parallel we will therefore have a voltage of 12v volts and a total capacity of 400ah ampere hour. For example you would not connect a deep cycle battery with a starting battery. A battery word equation and special note.
The parallel connection of two identical batteries allows to get twice the capacity of the individual batteries keeping the same rated voltage.