My 2012 Ford F-350 Dually died this morning. It has the original two batteries from the factory. I have it hooked up to battery tender while not in use connected to the passenger side battery. I jumped the passenger side battery and I was able to start the diesel engine. When I got it to the Ford Dealer to get it checked out, I was told that only the passenger side battery require replacing after testing both batteries. The driver side battery was fine,...I was told by the Dealer. They only replaced the passenger side battery, and the truck seems to start fine now.My newb questions are,1) Why does my truck have two batteries?2) What does each of the batteries do?3) If one is a back up battery that is "fine", then how come the truck could not start?4) Should I have gotten both batteries replaced?I don't want to be stranded while towing. Please give me some recommendations.Thanks!

Diesel trucks have had dual batteries for a long time. My 87 chevy diesel has to batteries. Not only do diesel starter motors need a lot more amperage to turn the motor over, but glow plugs draw a lot of current too. In short, Diesel engines and their systems put a lot higher demand on batteries than gas engines.Both of your batteries should be replaced, or your new battery will suffer an early demise.

Most diesels have two batteries. They have much higher compression, and are much harder to turn over. So the two batteries team up to tackle the task. As for replacement of only one, I've heard that's a no-no. If there is a difference between the capacity of the two, the more used up one will drag the other down. Someone with more knowledge on this subject will chime in soon.

Your truck has two batteries to run glow plugs and crank a big heavy diesel engine. Both are connected to your electrical system so they don't have independent purposes. 5 years is getting old for a battery and I'd replace both of them if I were you. I don't generally try to get more than 4 years out of a battery, but I live in a location that sees temperatures over 100 and below 0 every year which is pretty demanding on them.

I replaced the passenger side battery on mine a couple of months ago. But I think I'll do the other one ASAP

Agree with others both batteries should be replaced at the same time.

Two batteries in 2012 F-350? | Good Sam Community

2003silverado
Explorer II
Jul 09, 2017
Diesel trucks have had dual batteries for a long time. My 87 chevy diesel has to batteries. Not only do diesel starter motors need a lot more amperage to turn the motor over, but glow plugs draw a lot of current too. In short, Diesel engines and their systems put a lot higher demand on batteries than gas engines.

Both of your batteries should be replaced, or your new battery will suffer an early demise.
RSD559
Explorer
Jul 09, 2017
Most diesels have two batteries. They have much higher compression, and are much harder to turn over. So the two batteries team up to tackle the task. As for replacement of only one, I've heard that's a no-no. If there is a difference between the capacity of the two, the more used up one will drag the other down. Someone with more knowledge on this subject will chime in soon.
IdaD
Explorer
Jul 09, 2017
Your truck has two batteries to run glow plugs and crank a big heavy diesel engine. Both are connected to your electrical system so they don't have independent purposes. 5 years is getting old for a battery and I'd replace both of them if I were you. I don't generally try to get more than 4 years out of a battery, but I live in a location that sees temperatures over 100 and below 0 every year which is pretty demanding on them.
agesilaus
Explorer III
Jul 09, 2017
I replaced the passenger side battery on mine a couple of months ago. But I think I'll do the other one ASAP
colliehauler
Explorer III
Jul 09, 2017
Agree with others both batteries should be replaced at the same time.
BB_TX
Nomad
Jul 09, 2017
I agree with replacing both. I did on my last F350 and getting ready to on the current one. Likely will save you from going thru the same fail to start scenario in the near future.
Farmerjon
Explorer
Jul 09, 2017
I agree with replacing both batteries and they should be identical to each other.
5 or 6 years is not uncommon especially if it's not used very often, although the battery tender should have helped.
FishOnOne
Nomad
Jul 09, 2017
I agree with everyone's recommendation that both batteries should be replaced on a 5 year old truck. Having said that this scenario shows that there are dealers who don't know how to service these trucks.
milsuperdoc
Explorer
Jul 09, 2017
I specifically asked the Ford Dealer to check both batteries, mentioning that they were both original factory batteries. I informed the dealer that given the age of the batteries, I am thinking of replacing both. But the dealer said their "diagnostic test" showed only one really needed to be replaced. I should have followed my gut instinct and insisted on replacing both. I'm heading back to the dealer for a new driver's side battery tomorrow morning.

Thanks, everyone!
Lwiddis
Explorer II
Jul 09, 2017
A Ford dealer that says only replace one of two batteries on a Ford diesel...hmmm. Dealer may be wrong but he/she was trying to save customer some money.

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Two batteries in 2012 F-350?

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