Battery Questions

[wheelspinner]

Well I am stumped. The two batteries in the spots closest to the frame continue to go dead. The alternator is charging correctly (14.4, 27.7) on both ports, and the batteries take a charge off of a charger perfect. But they go dead. When reading voltages on the batteries while engine is running, I see 27.7 across the 24V side, but only 13 on the 12V side. I don't completely understand the set up in these trucks as far as the hybrid 12/24 so possibly I'm reading things wrong?

I verified the battery connections per the diagram on the battery cover (A0, no shut off relay) but it still goes dead every night. Not sure if its just bad luck and I have 2 bad batteries, or?????

Thoughts?

 

[Dock Rocker]

Well I am stumped. The two batteries in the spots closest to the frame continue to go dead. The alternator is charging correctly (14.4, 27.7) on both ports, and the batteries take a charge off of a charger perfect. But they go dead. When reading voltages on the batteries while engine is running, I see 27.7 across the 24V side, but only 13 on the 12V side. I don't completely understand the set up in these trucks as far as the hybrid 12/24 so possibly I'm reading things wrong?

I verified the battery connections per the diagram on the battery cover (A0, no shut off relay) but it still goes dead every night. Not sure if its just bad luck and I have 2 bad batteries, or?????

Thoughts?

I would put the batteries on a charger and then have them checked with a battery tester to make sure you don’t have a dead cell or something like that before I did much troubleshooting on the truck.


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[Suprman]

One bad batt killing the other or some type of drain. Take the 2 good batts. Do left side at the frame and right front spot tape up the extra connections. I run trucks like this all the time. See how it goes. And you can charge and test the other two while out.

 

[wheelspinner]

OK, just to close it off: Had two dead batteries, tried slow and fast charging both, they are now on the scrap pile. I changed the truck to two batteries, and have had zero issues. Weird that both went bad together

 

[tennmogger]

I have found that same situation on two LMTVs, the frame side batteries associated with 24v were bad. The other two batteries, associated with 12vdc, were still good.

Here's what I think is happening: The FMTVs run a 12v system and a 24v system, both referenced to ground. The dual alternator and voltage regulator aim for a good charge voltage for "their" batteries. However, the 24v batteries (on the frame side of the box) are not ground referenced, they are physically connected to the positive of the 12v batteries, yet the 24v charging is voltage limited to an overall voltage referenced back to ground. For that reason the voltage across the "24 volt pair" varies with the charge state (and therefore voltage) of the 12v pair and over time this lack of voltage control allows the 24v batteries to be poorly maintained. Haven't I seen that the 12v side of the alternator and the 24v side have different current ratings? That would cause different battery charge rates too, if heavily discharged then heavily recharged.

Ever wondered why the CUCVs have two alternators and each alternator is across one 12 volt battery? I don't know either but perhaps that is why the CUCV is designed that way. The CUCV system avoids the battery imbalance problem the FMTVs have.

Best to rotate your batteries occasionally, the outside pair rotated to the inside, etc.

 

[rosco]

Charging problem

I know nothing about the LMTV, but your battery issue sounds like my old days driving my 211's. 24 V system, Two 12 V batteries, & pulling 12V from the battery next to ground. Basically it was not possible for the regulator to charge the system adequately, with out over-charging one, or low charging the other.

Is your load balanced between the 12V & 24V systems? Are all the battery connections clean & greased? Especially to ground? I once had a heavy truck with a 12/24V starting system that was driving me nuts. I was probing with my multimeter with it set on Volts & probed a relatively new ground cable to the frame... one prob on the frame & the other on the terminal & found 2 Volts measured, hence incomplete ground! "I saw the light" that day!

Good Luck

 

[Suprman]

Some of the mrap use battery equalizers in their dual volt systems.