F350 7.3 charging issue. Twin batteries, one alternator.

[Dirtman]

Asking for a buddy. He bought a 2001 f350 with a blown up 7.3. We swapped in a new "used" motor and got it running great (had to replace the IDM) but now it's got a charging issue.

It started and drove fine, checked voltage at the batteries at startup and got 14.2 which is ok. But this is a twin battery single alternator truck. Out of my league. After a while the alternator apparently fried and now is only putting out 8 volts.

Swapped alternators, problem solved... for another 50 miles or so. Fried another alternator.

Now here's my understanding, with twin batteries it obviously must have a isolator so the alternator doesn't charge both batteries at once or let the batteries back feed each other correct? With 2 known good batteries and a known good alternator is it possible the isolator is bad and allowing the alternator to dump into both batteries at once and overloading it?

Thoughts? I'm not familiar with twin battery setups.

 

[adsm08]

I don't see an isolator, The batteries share a positive cable, and ground independently.

 

[Uncle Gump]

It's been awhile since I worked on one of those but I maintained 4 of those in a snow plow fleet. I don't remember an isolator. Batteries in parallel full time. I seem to recall a strange issue when the owner only replaced one battery of a set. But I can't remember exactly what it was now.

 

[Curious Hound]

Shouldn't need an isolator. 2 batteries in parallel just act like one larger battery. No problem. Both batteries should be same size age and condition. If you replace one, you replace both. If using the stock size alternator, or larger, it should handle everything fine.

 

[adsm08]

It's been awhile since I worked on one of those but I maintained 4 of those in a snow plow fleet. I don't remember an isolator. Batteries in parallel full time. I seem to recall a strange issue when the owner only replaced one battery of a set. But I can't remember exactly what it was now.

If you replace only one battery it tries to charge based on the average of the two, which leaves the older battery under charged and ruins the new one by over-charging it.

 

[Uncle Gump]

If you replace only one battery it tries to charge based on the average of the two, which leaves the older battery under charged and ruins the new one by over-charging it.

Ultimately that was the cause... and why I mentioned it. I think it would turn on the battery light at times. What I do remember was putting another new battery in it cured whatever it was.

 

[Dirtman]

Guess im more use to boat setups.....

The batteries hes using are 2 random ones from the shop but both tested good. He doesn't want to buy new batts untill he's sure of the issue so he doesn't fry them.

 

[Biggfoot44]

Never say never or always , but :

On Diesel powered ( consumer sized) trucks , dual batteries are wired together to give serious cranking power . The alternative used on early diesel pickup trucks like my early Dodge Cummins was a really huge ( & expensive) single battery , that was kinda marginal .

On gas powered RV, tow vehicles, etc typically one battery runs vehicular needs as if it were the only battery , and the second battery is dedicated to the RV , or whatever specialized equipment powered while parked . With an isolator between the batteries .

 

[Curious Hound]

Never say never or always , but :

On Diesel powered ( consumer sized) trucks , dual batteries are wired together to give serious cranking power . The alternative used on early diesel pickup trucks like my early Dodge Cummins was a really huge ( & expensive) single battery , that was kinda marginal .

On gas powered RV, tow vehicles, etc typically one battery runs vehicular needs as if it were the only battery , and the second battery is dedicated to the RV , or whatever specialized equipment powered while parked . With an isolator between the batteries .

That makes perfect sense. On a boat or RV the need to keep one isolated and available for starting is important. I wonder if a diesel RV would be the same. Or would it have 2 starting batteries plus a third, isolated battery for auxiliaries.

 

[adsm08]

Guess im more use to boat setups.....

The batteries hes using are 2 random ones from the shop but both tested good. He doesn't want to buy new batts untill he's sure of the issue so he doesn't fry them.

This may well be the issue. When you are using a single generator to charge a pair of parallel batteries having the batteries be unbalanced can place an incredible and unusual load on the generator.

 

[Dirtman]

Thanks Adsm, I sent him off to get a new matching pair.

 

[Dirtman]

That did it. Got 2 new matching batteries and its charging normally now. Thanks everyone. Now just gotta setup a third "house" battery since this is a camper rig. I'm afraid to mess with actually trying it into the trucks electrical even with an isolator switch so I think I'm just gonna do solar panels on the roof of the camper to keep it charged.

 

[Dsetz]

My 06 gas F250 has an(I assume aftermarket) isolator to separate the winch battery from the engine side battery. I'd never used the winch until the other day. It blew the isolator fuse and discharged my new Interstate, died as I limped up to the shop.
I'm hard pressed to believe that a diesel doesn't still have an isolator to avoid batrey issues, its still possible to charge both batteries and crank in paralell depending on how its setup.
Mine is essentially an aluminum heatsink mounted on the passenger sidewall behind the engine battery. My fuse was easy to find as the wiring was seperate.
On a diesel an isolator should be stock. Check the fuse map for it.

 

[Curious Hound]

My 06 gas F250 has an(I assume aftermarket) isolator to separate the winch battery from the engine side battery. I'd never used the winch until the other day. It blew the isolator fuse and discharged my new Interstate, died as I limped up to the shop.
I'm hard pressed to believe that a diesel doesn't still have an isolator to avoid batrey issues, its still possible to charge both batteries and crank in paralell depending on how its setup.
Mine is essentially an aluminum heatsink mounted on the passenger sidewall behind the engine battery. My fuse was easy to find as the wiring was seperate.
On a diesel an isolator should be stock. Check the fuse map for it.

There's no need for it. No reason to separate the batteries. RV's, boats, vehicles with a winch or stupid huge stereo have a reason to separate the batteries.