When connecting batteries in parallel try to only connect batteries of the same type, model, capacity, and age. For example, if you want to connect your Deep Cycle battery with another one to increase power output, make sure the second battery meets the mentioned criteria. AGM and Lithium batteries cannot be wired up in parallel together. If you have 2 or more batteries of the same criteria and you are ready to wire them in parallel, follow these steps.
The negative terminal of one battery is connected to the negative terminal of the next and so on through the string of batteries. The same is done with positive terminals, i.e. the positive terminal of one battery to the positive terminal of the next.
We would recommend purchasing a equalizer if you are connecting your batteries up in parallel / series. This will balance all the cells throughout the batteries and give your batteries a much longer life expectancy.
https://www.aussiebatteries.com.au/Battery-Equalizer-for-12V-24V-48V-Battery-Setups