1. Why do trucks with diesel engines have two batteries?

2. Are the batteries wired in series or in parallel?

3. When one battery fails, do they both have to be replaced?

4. Why does my F250 need a 200 Amp alternator?

 

Wow, eight views and no answers. I guess they aren't such easy questions.

 

I'm not an expert, and I didn't look it up...but here's my educated guess...

I know that diesel has a higher flashpoint, ie it takes more energy/heat to ignite that gas. So, logic tells me it would take more "spark" per se to ignite it to start the truck. So, that likely explains the need for some serious battery action.

Then, in turn the higher output alternator insures the twin batteries are properly charged for future use.

As for them being wired in series or parallel? My guess would be in series for the wiring config.

Hope that helps!

 

The diesels have two batteries for a number of reasons. First, the much higher compression ratio puts a heck of a strain on the starter and electrical system in general. You also have a set of glow plugs to heat up on cold mornings. As for series or parallel, series would give you 24 volts. Your truck uses a 12 volt system. You can replace either battery all by itself. As for the 200 amp alternator, it's partially for the stuff your truck already has on it, plus Super Duty owners tend to add all sorts of other power robbing stuff (like snow plows) that require even more juice.

 

I thought the reason for the two Bat was you needed 24 volts to start the truck. Am I wrong about this?

 

Thanks guys! I appreciate the responses. I don't know much about diesels, except that they do not have spark plugs. The combustion is created by injecting the fuel into the combustion chamber at a very high compression. At first, I thought that the second battery was there to run the block heater, but then I found out that the block heater has to be plugged into an electrical outlet. I guess a second battery is necessary just to turn the engine over witht he high compression. Thanks again.

 

Yep. You're wrong.

 

The second battery is actually there for a number of reasons - as I mentioned earlier. Turning over a large displacement engine with a ton of compression is just one of them. I'm glad we didn't get into the reason for dual alternators on some diesels.

 

I am with F250SDDiesel. I thought it was a 24V start, with a 12V operating system. The heavy construction equipment I used to work on was set up this way.

 

Try wiring your batteries up in series and see what happens when you try and start the truck. Trust me, you're going to see alot of sparks and smoke. In order to do a 24/12 system you must have a conversion block under the hood, of which there is none. There is no such thing as a 24/12 system in ANY light duty truck. That is only used on heavy equipment.


The comment saying that you can only replace one battery is NOT correct. If your batteries are getting weak and you only replace one the 2nd battery will cause the new battery to deteriorate to its level. I see this quite a bit where someone puts a new battery on and it everything is good for a short time. After a few months the new battery starts to die and the person usually gets pissed off wondering why their brand new battery died. If you've got 1 weak battery replace both of them, doing just one is throwing money out the window.

 

First off, nice truck it looks great.

The answers you are getting are correct,

1. Since your truck requires so much power (Watts) to crank over the motor (high compression, large displacement, heavy rotating mass), as well as warming up the glow plugs when you turn the key on. Watts = Volts * Current, or W=E*I. Since your truck does only use a 12 VDC system you are going to need twice as many Amps.

2. Parallel, end of story

3. As one battery goes bad it can pull down the other, so you make the call

4. See answer 1. You need a lot of juice to power the system and charge those batteries.

Casey

 

I had a feeling that was the case, which is why I asked. It's just like when alkaline batteries in a flashlight go dead...you replace all of them at once. I have been considering putting a pair of Odyssey Batteries in the truck. I will probably wait until the batteries die before I drop $500 on a pair of batteries though.

 

Thanks, I knew you guys could help me out.

 

Hmm, I always thought the second battery on a 7.3 was just for the glow plugs. Guess you learn something new everyday

 

I agree that is true if that is the case - two weak batteries. However, if you have two strong batteries, and one fails due to some other reason, such as an internal short or loss of electrolyte, it doesn't necessarily mean they both have to be replaced. The other battery may be unaffected. A load test on each one is a common way to see if they are both weak.