The two batteries are for higher cranking amps, which are needed for the high resistance load that a diesel engine requires during starting. The batteries in your truck are wired in paralell, whereby the positive terminal of each battery are hooked to one another as well as the negative terminals.Click to see full answer. Also, how do you jump start a diesel battery with two batteries? JUMP STARTING A DUAL BATTERY DIESEL ENGINES ONLY Repair Guide Open the hood and locate the batteries. Position the donor vehicle so that the jumper cables will reach from its battery (must be 12 volt, negative ground) to the appropriate battery in the diesel. Shut off all electrical equipment on both vehicles. Secondly, why does my f350 have two batteries? Primary purpose of batteries is to power the starter motor for engine start up. Two batteries double the available power available for the starter. This is important because the 7.3L diesel is harder to start then the gas engines. In respect to this, do both batteries on a diesel? It takes a lot of battery power to start a diesel engine, which is why these trucks generally have two batteries. The batteries should be replaced in pairs, but because It’s expensive to replace two batteries at once you want to be sure you’re getting maximum life from them.Are diesel truck batteries in series or parallel?They’re connected in parallel which gives two batteries the same voltage as one but doubles the amps. Hooking them in series would double the voltage but keep the amps the same as one.