Why does a cummins diesel truck have 2 batteries?

There are two main reasons

1st--there are fast acting double grid heaters in the intake that can pull up to 120 amps while starting.

2nd- reason in order to build up cylinder pressure ( heat is used to ignite the fuel); and build up fuel pressure; the engine needs to be able to turn a min. of 250 rpm's while cranking---with out dropping below 10.0 volts ( 9.8 volts min voltage needed while cranking the computerized systems in order for it to function, and the min voltage needed to pull and hold in the shut down solenoid on the non-computerized ones ). Because this is a 12 volt system and the amount of amp draw from the battery-- it was easier to put in two batteries then one huge one.

No civilian Dodge Ram Cummins Diesel has ever been a 24 Volt systems, never. It has 2 batteries ran in parallel (12 volts).

It has 2 batterys for mulitple reasons. Like most other diesels.

1. Glow plugs, or in the case of DI engines heater grid. Huge amounts of current are drawn before and after the start of the engine.

2. Like Redrepair stated, it uses heat to ignite fuel. So the engine has to spin at a minium RPM. This require huge amounts of current.

3. Battery replacement , you can replace your batterys without losing any stored info/memory in truck. Whatever that info that depends on constant 12volt source. When your replace batterys in these you can do one at a time. It sounds silly but dodge considered going to one bigger battery at a time to reduce production costs, but decided against it and this was one of the reasons.

4. Electrical grounds are achieved easier, if you notice underneath the hood grounds are ran to both batteries. This reduces the number of wires going to one terminal, thus reducing the chance of a bad ground. Grounds are critical, espically in the new dodges (47RE on up tranys), a faulty ground cause your transmission to lose 1 and 2. I can go on and on about what a bad ground can do.

Now any decent single 65 series battery will crank over a normal operating cummins even in the winter. I'm still using orginal batterys in my 98. No problems starting and it's below zero. And I'm pretty sure the batterys got to be getting weak and one new 65 series battery will probably spank both of my orginal dodge batteries. My Cummins is running good, that's kind of the catch there. A Cummins that has seen better days (or a un-broken in engine) will still start, it'll just take 2 batteries!

Diesel Batteries

Two batteries for the power needed to turn that high compression diesel motor over to get it started. Once started, the batteries are just along for the ride. They will be charged, as needed, by the alternator/charging system while the vehicle is running. In essence, a diesel motor does not need electricity to run, unless is uses a computer or an electronic fuel pump.

The alternator should generate the electrical requirements for the vehicle, not the battery(ies) while the vehicle is running (any vehicle). The batteries are just for starting power.

if your truly stuck on swapping the engine.. the easiest way to do this is to buy a truck with the cummins motor you want to swap in your gmc. that way you have all the mounts, fuel system, electronics, transmission components, 4X4 components, ect.. you'll basically shell out both vehicles and swap the cummins truck into your GMC, you'll need a few partners and someone who can weld very well. and if you feel that you can get your money back, put the V8 you dont want into the previous cummins truck. i did a v6 swap into a 4 cyl honda civic with a friend, after we did that we put the civics 4cyl into the v6 accord and sold it for the 4 cylinder price. so in return we only spent about 2 grand in buying all the stuff we need to do the swap.

Probably because diesel engines have high compression. So they require more starting power to overcome the pressure - thus the two batteries.........

It has two batteries because it takes so much power to start, and then run, that one normal battery would drain while trying to start the engine. There is so much power being drained from the battery during operation that the alternator can not keep one battery charged enough for optimum operation of all the electronics that are on board, hence at least two batteries are required for normal operating.

it has 2 batteries because the starter uses 24 volts.

hw do you know witch battery is going bad

The real answer is because it's 24V system.