The first drawing, I show you what not to do! So you have your battery we will call number 1 Then the battery you’ll add the number 2 The negative (less) The positves terminals (most) What he should not do is to connect the batteries in series as in the drawing; For that if you plug in series, the voltages of the two batteries additionnent.Soit 12v + 12v=24volts! So too strong tension, it is important not to be connected in series! Now this is what you really have in connection Here is our number 1 and 2 battery with the poles – and + The good connection to do is connect in parallel. As you will have 12 volts output voltage. And if you have two batteries of 80 Ah, you will have an output voltage of 12V and 160Ah; For the cable section take 10mm2 By cons there is a logic to do for wiring … If you simply like wire drawing, a battery will discharge more quickly than the other. So a different wiring system is to have a perfect balance; Here’s what to do. Disconnect the negative original battery 1 and attach a domino above to join the negative battery terminal 2 Then, the negative terminal 2 cable again towards the terminal # 1 To the positive terminal 1 leave the original cable on the terminal and just add a cable from terminal 1 to terminal 2. This wiring is balanced, there is the same resistance between the two poles. I pulled the chair to access the original battery. Disconnect the 2 terminals Installing the battery No. 2 behind the No. 1 or try to find a place not too far. So here is the negative 1, I’m going to remove the lug and cable to a domino to go to the negative terminal 2. For aesthetics I will go through the floor, make holes etc … Allow the hardware before 3 cables, dominoes etc … The son 10 mm2 flexible because not use rigid copper in a vehicle My cables have all gone through the floor Then connect the cables I connect the positive 1 still on # 2. Then remove the original neutral 1 to wire with another cable to go to negative 2. Here is the domino wiring Then wire the battery # 2 Here is the battery side 1 Review the beginning of the video design for the wiring of the battery # 2. Wiring is completed I added fuses to add options later as taken etc … I hope the video will help you in your work Visit please all my videos or subscribe for free !!! Type BRICOZAVTOURS in the youtube search to find me.