Dual battery diagnostic questions - PeachParts Mercedes-Benz Forum

kerry · #1 06-29-2010, 07:41 AM

When I got my FG out of storage before starting my current trip, the batteries were dead. Truck is 12 volts with 2 batteries connected together (parallel?) After quite a while on the charger, they took a charge. The difficulty of getting the batteries charged up seemed odd but I ignored it and I thought I was back in business. But while on this trip I'd noticed some oddities. I have a cigarette lighter digital voltmeter. At the end of the day it would show 12.x voltage. The next morning it would show 11.x voltage. Starter also did not sound as snappy as I thought it should. Yesterday, after stopping for fuel shortly after starting out for the day, engine would not crank. Tried jumping it with the camper battery, no luck. Got towed to a parking lot and I swapped in a spare starter I had aboard. At first it clicked, then it started. I thought I was back in business again but due to some exigent circumstance, I didn't want to take a chance on shutting off the engine since we had to drive a few hundred miles to catch a ferry. After driving for a couple of hours, I pulled over to fuel up and opened up the box door to check the batteries. I immediately smelled acid. Right battery was bulging, boiling acid fumes were coming out the fill holes and it was too hot to touch. Obviously it was overcharging. Since I didn't want to shut down the engine in case it wouldn't start again, I decided to disconnect the hot battery and continue. Probably not a wise thing to do. Working on an object bulging with pressure from boiling acid fumes is not that safe. Nonetheless, I disconnected the negative post. Voltage jumped up to 14.5 from 13.8 after disconnecting. When we got to a campground with electricity where I could plug in my battery charger, I shut off the engine. Left battery showed only 9 volts and would not start. Obviously the left battery was bad. I switched the batteries over, connecting the previously hot right battery and disconnecting the low voltage left battery. Voltage showed 12.4 Truck started. I concluded left battery has been bad, drawing down the right battery and calling for constant voltage/amperage to charge up. This constant high amperage had overcharged the right battery. Put the right battery on a 2 amp charge overnight. This morning, voltage showed 11.x on right battery but truck started snappily. But driving to the ferry, I began to smell acid again. Right battery is hot again. No visible discharge of acid fumes but it smells. Perhaps I boiled off enough of the battery liquid that the remaining is getting hot on a normal charge.
But the lesson of the story (for me) is that diagnosing electrical/battery problems with a constantly connected dual battery set up is not simple. When my starter failed to work, I should have separated out the batteries and checked the condition of each separately. I probably could have jumped the starter if I had disconnected the low voltage bad battery. The voltage at the cigarette lighter is giving some kind of average of both batteries and is not a true indication of the condition of each battery.
It also points to the problem of lack of full instrumentation on the FG. My $5 cigarette lighter voltage meter was very helpful in figuring out the situation but an ammeter would have been even more helpful since it would have showed the high charge amperage as a result of the bad battery. I may install an ammeter near the batteries.
I still don't know why the batteries went dead initially. Was something drawing them down or did one (pretty new) battery just go bad and suck down the other one.
But in any case, for a guy like me, used to working on single battery cars or dual battery sailboats with a selector switch, diagnosing electrical problems on a constantly joined dual battery set up is more complicated than normal.
Right now, I'm on the ferry back to Nova Scotia. I disconnected the one functional battery for the ferry trip in case there is a draw on the system. I planned to continue the rest of the trip with the one remaining battery. Perhaps this will be possible by refilling the battery with fluid. Perhaps I will pick up a spare battery in case I irreparably damaged the good right battery by severely overcharging it.
Any advice/comment is appreciated.

BobK · #2 06-29-2010, 08:16 AM

I have had dual batteries on several rigs now. First rule is to replace batteries in pairs. When one goes bad, the other seems to fail soon after. They need to be identical. Charging issues are another matter. Charging system must be able to put out 13.8-14.2 volts to do any decent charging. Also must be able to supply full rated output current. Alt. on current truck went haywire a couple of years back and tried to go OVERVOLTAGE!. First time I had ever seen that happen. I ran every device in the truck to keep the voltage low enough to keep from frying all the electronics. Another issue is of course bad battery connections. Around here, you have to redo the connections about twice a year as the large temperature swings during a 24 hour period will play heck with the connections. Next thing you know, you aren't really charging the battery very well. This may have been part of your problem. If one battery had more resistance in the terminals, than the other, the lower resistance battery gets most of the charge and does most of the work. Then you have one worn out battery and one that is dead from not being charged for the past 6 months.

kerry · #3 06-29-2010, 08:44 AM

This set was replaced as a pair a little over a year ago. Bad connections could be an issue. I'm wondering if it's better to switch the batteries like a sailboat so either or both can be used and the volt meter measures either or both.

BobK · #4 06-29-2010, 08:52 AM

You can, but you lose the advantage of twice the current capacity for cranking. Faster a diesel cranks, better it starts. Plus, if yu have glow plugs, they put a big load on a battery. Connections aren't hard to keep clean. We just have to remember to do it. And I forget all the time...until the first car won't start..then the rest get cleaned up!

kerry · #5 06-29-2010, 09:10 AM

The switch on my sailboat allows me to hook both batteries together for maximum power.

mpolli · #6 06-29-2010, 01:33 PM

When I had a 2 battery system I just had what they called an "isolator" which was this aluminum thing which basically had 2 diodes in it, so you could charge both at the same time, but they would not be connected together, so one cannot kill the other. I wonder if your charge voltage is too high? To get boiling and heating like that I would think you would have to be over 16V at least.

G-Benz · #7 06-29-2010, 01:38 PM

Do any of these systems use a "battery isolator"? It's function is to sense the charging load on the batteries and switch the charging to the weaker battery when the other one is fully charged. Takes a TON of load off the alternator as far as trying to maintaing a good charging amperage to keep multiple batteries juiced.

I went with a dual battery setup in my VW (ran the stereo independently on one of the batteries, while the rest of the car electronics ran off the other). The stock 55-amp Motorola was ill-suited for maintaining a charge to two batteries. The isolator allowed the alternator to maintain the charging load, and I wasn't faced with replacing an alternator every year!

aklim · #8 06-29-2010, 02:09 PM

check the alternator. Quite often the alternator doesn't charge the battery properly and causes it to die out

4x4_Welder · #9 06-29-2010, 02:25 PM

On a multiple battery system, if you have one battery with a high charge resistance, then the other one will receive the charging current. The alternator will sense the low voltage and continue charging, just burning up the "good" battery. I have seen this several times in heavy trucks, they use three or four batteries connected with 2/0 cables, no isolators.
Replace both batteries with a matched pair, you may have to go to a truck shop to get the proper batteries. Make sure all the cable connections are clean and tight, no visible corrosion on the terminals or cables, and spray thoroughly with a terminal protector after assembling. Keep an eye on the volt meter, and take a quick check of the battery temps after the first run cycle.
FWIW, unless you have a switch like the sailboats use, you can't isolate dual cranking batteries, that defeats the purpose of having dual batteries. The automatic isolators are designed for charging at a maximum of about 100a.

kerry · #10 06-29-2010, 04:48 PM

I think 4x4 described what happened. What I've decided to do is as follows: Since I bought these two batteries at NAPA in the US a little over a year ago I should be able to get them replaced. So I bought one single 850CCA battery at Canadian Tire to use until I got home. I disconnected the dual battery setup and hooked the cables just to the new single battery. We'll see if that works.
I don't think I was getting high voltage to the batteries from the alternator since I never had a voltage reading higher than 14.5.
I don't see a huge advantage to the current dual battery system I have given the possibility of just this problem arising and having to replace two batteries at once. The engine is only a 3.9L diesel, there's no real reason to have two batteries apart from the fact that Fuso used to run these trucks on 24 volts and never reverted to a single battery when they changed to 12 volts.

aklim · #11 06-29-2010, 04:51 PM

Quote:

Originally Posted by kerry

I think 4x4 described what happened. What I've decided to do is as follows: Since I bought these two batteries at NAPA in the US a little over a year ago I should be able to get them replaced. So I bought one single 850CCA battery at Canadian Tire to use until I got home. I disconnected the dual battery setup and hooked the cables just to the new single battery. We'll see if that works.
I don't think I was getting high voltage to the batteries from the alternator since I never had a voltage reading higher than 14.5.
I don't see a huge advantage to the current dual battery system I have given the possibility of just this problem arising and having to replace two batteries at once. The engine is only a 3.9L diesel, there's no real reason to have two batteries apart from the fact that Fuso used to run these trucks on 24 volts and never reverted to a single battery when they changed to 12 volts.

It will work. When it gets cold, it might have an issue. Diesel is harder to start in the cold and you have a huge draw when you glow the plugs. My MB runs on 1 battery. If your lights are left on or there is some other drain when the battery is fresh, probably you will be ok. If the battery gets older, maybe not.

Angel · #12 06-30-2010, 06:34 AM

I have never had much luck ' reviving' a battery that had gone below 10v.

If one of my car/motorcycle batteries ever reads below 10 (or even 11v) when just sitting there - it has been discharged enough that it is probably permanently damaged. I replace it or plan on it failing.

I know that its physically possible to rejuvenate lead-acid batteries that have discharged too far, but I've never seen it result in a 'reliable' battery afterward.

-John