24V System - One 12V Battery Failing - Damage to Good Battery?

[loonwheeler]

My Unimog is an ex-miltary truck and runs as 24V system. Currently I have 2X 12V Group 24M hybrid deep cell marine batteries wired in series for the starting duties. Since my truck is also a camper - I also have a dual battery setup that connects the starting batteries with the house batteries when the ignition is turned on.

I noticed late this fall that the truck was getting harder to start with only the starting batteries - just before the solenoid switched to connect the house batteries. I have checked the voltage of the starting batteries and combined they are normally close to 24.0 or so at rest which made me first suspect something was wrong. Once the house batteries are connected (thankfully) I haven't had any issues starting even since the cold weather has arrived this winter. As another backup - I also have a Honda EU3000is generator on board and a jumper pack in case emergency charging or jumping is needed.

As the title suggests, I recently started to look a bit deeper by disconnecting the two batteries and measuring the resting voltage by themselves. As it would turn out - one battery sits at 13V while the other is around 10.3V. Obviously this points to one of the batteries being completely dead and the root cause of my low voltage issues.

My question is this. Will the one dead starting battery have any negative affects on the other batteries in the system? Should it be replaced ASAP? The other alternative is to take a few months and budget to purchase a new one before this spring's adventures begin.

The failing battery is a MagnaCharge 24M1000 that is less than 1 year old. I would love to warranty the battery - but too bad I don't live in Canada. Anyone know if this can warrantied here in the US?

If I were to replace the battery - get another brand that is similar in specs or best to replace both batteries at the same time to be on the safe side?

 

[762X39]

I have replaced the batteries as pairs in my Unimog twice (every 7 years). It is cheap insurance (I think I paid about $350 per pair both times) .I made sure that each time the batteries were from the same lot #/date of manufacture. To make maintenance easy, I have a pair of 50 amp Anderson connectors on the exterior of the battery box fused with 50 amp fuses at the battery posts. This allows me to connect my smart charger to each battery separately for routine maintenance testing and charging while the truck sits.

 

[Tennmogger]

If one battery is "a MagnaCharge 24M1000 that is less than 1 year old", is the other battery identical? Wording sort of implies it's not. If you have dissimilar batteries in series, well you now know what happens when you have two dissimilar batteries in series LOL.

If the two batteries are identical, there's still a chance the 'bad one' isn't. Why not take a few days and charge each 12v battery separately while disconnected from the truck. Do you have any connections to the midpoint of the two batteries, that is, a 12 volt 'tap'?

The only way to evaluate a battery is measuring voltage under load. "Resting" voltage does not mean much. You should also measure the charge voltage with your truck running too. You should see over 28 volts.

 

[Haf-E]

Yes - it isn't always obvious which battery is "bad" when you have two in series for 24vdc. Best to pull them, charge them individually and then do a load test on each individually. Depending on the failure - the "good" battery might be the one with the low voltage if the "bad" battery is keeping it from getting recharged... it can be tricky to tell when they are in the vehicle still.

You don't have any 12vdc loads connected to the mid-point of the two batteries do you? That is a very common reason for failure on the lower battery. Use a step down converter for all 12vdc loads.

There are balancing devices which will keep the two batteries equal all the time - both when charging and discharging - one common one was made by a company called "vanner" and another one is offered by a company called "solar converters". I am sure there are others. The big advantage of these is that they allow use of two batteries which don't exactly match - i.e different brand, age, etc. I would still try to match the capacity and type (sealed, vented, lead-calcium, lead-antimony, etc.) of the two batteries. These devices also allow a weal battery to continue to be used for a long time - saving money in the long term.

Post up the results of the testing...

 

[loonwheeler]

Sorry - I should have mentioned in the first post that the two batteries are exactly the same model and were purchased by the PO at the same time.

Thanks for the advice. I figured that further independent charging & testing might be the next step so thanks for confirming that.

Would you suggest a starting load test as a fair judge of the battery's condition? Since my truck is a 24V system - a starting load test would be difficult to complete unless I am missing something. Or maybe another kind of test?

 

[Haf-E]

You should disconnect / remove the two batteries and charge them independently using the same charger for both of them (but one at a time). Then take them to a battery shop and have them tested with a load tester - or - go buy one (Harbor Freight has them cheap for example). Alternately, put the back in the truck and use the starter to test them - they challenge there is you really should have two volt meters to do it well so you can see what each does. If you only have one then just do the test twice with it connected to each battery one at a time.

 

[762X39]

As stated in my previous post, I use Anderson connectors to access the batteries for charging/testing without having to do any heavy lifting.

 

[loonwheeler]

As stated in my previous post, I use Anderson connectors to access the batteries for charging/testing without having to any heavy lifting.

Good idea - I like the flexibility that this affords.

 

[loonwheeler]

You can perform a starting load test with a voltmeter, Preferably a meter with minimum-maximum memory function.

Assuming battery is fully charged. 12.6v or higher.
1,Switch off the injector so the motor wont fire up.
2,Measuring voltage whilst cranking, should not fall below 9.5v. (assumes battery at 70º F) Dont crank longer than 10 sec or so.

... Then take them to a battery shop and have them tested with a load tester -or - go buy one (Harbor Freight has them cheap for example). Alternately, put the back in the truck and use the starter to test them - they challenge there is you really should have two volt meters to do it well so you can see what each does. If you only have one then just do the test twice with it connected to each battery one at a time.

My truck has a 24V starter - how will this affect the test if only using 1 battery? This method is more simple than testing with both batteries with one meter. Then again - just buying a load tester is also pretty easy to do...

 

[Haf-E]

When you put the recently individually charged batteries back in the truck in the original 24vdc configuration, connect a voltmeter to one of the batteries so it reads that battery's 12vdc voltage. Then crank the engine and see how much it drops. Move the volt meter to the other 12vdc battery and repeat the process. The two voltages should be similar if the batteries are in good condition.

So were there any 12vdc loads connected to the middle "tap" of the two batteries?

 

[loonwheeler]

Got it! That's pretty simple to do if I can recruit a helper. Maybe even my wife! :sombrero:

No 12V loads on the middle tap of the two batteries. They are just connected in series to each other with the house batteries attached as parallel circuit on the (+) terminal in the bottom right corner of the picture (top cable in-line with the wing nut). This same battery on the right is the one measuring 10.3V at resting voltage before individual charging.

 

[Tennmogger]

Are you saying the 'house' batteries are not permanently connected in parallel with your truck batteries? I know you said "only when the truck is running", but the photo shows a POS to POS hard connection? What gives??

You should disconnect the house batteries from the truck batteries, charge all the 12 v batteries individually (no need to take them out of the truck), then NOT hook the house batteries to the truck batteries unless through a disconnect that only connects when the truck is charging.

Looks like a 404 Unimog, right? Still have the original generator? The batteries will only get charged if you are putting a lot of miles on the truck at good engine speed. These generators (not alternators) put out very little at low speed (but you no doubt know that). A battery charger is a permanent accessory for my 404's because the batteries always get low between occasional uses.

Do you still have the ground disconnect 'master' switch? How did you manage the house batteries with respect to the ground disconnect?

I, for one, would enjoy seeing pictures of your truck.

Bob

 

[loonwheeler]

Hey Bob - thanks for chiming in!

Actually the truck is a '86 U1300L. The house batteries are only connected to the starting batteries by a solenoid (out of view in the upper right of the photo) energized by the ignition switch. While the engine is running, the alternator (with Balmar smart charger) charges both battery banks. Since the truck cruises at ~2400 RPM - the amperage output of the alternator should be more than enough to charge the batteries when driving.

Yes- I do have the disconnect switch in the cab that came standard with ex BW trucks. I disconnect the batteries via this switch and flip the breaker for the speedo (parasidic drain) anytime is is parked to avoid draining the starting batteries and in the hopes of more difficulty if someone were to break in and try to steal the vehicle.

 

[loonwheeler]

Just doing a little cross referencing about the alternator "generator" and it's specification. Rated voltage at 28V. Starts charging at 1180 RPM with a 1:3.268 pulley which means that even at idle (~500 RPM) the alternator pulley is doing somewhere over 1500 RPM which is high enough to start charging according to the specification. Rated at 55 amp output is enough to get juice into the batteries while driving about 1 time per week (25 min each way plus idle "warm up" time.

Honestly since one of the starting batteries seems to have no problem while the other is having issues points to one of the cells has gone bad. At a resting voltage of 10.3V / 5 cells gives very near 2.1V which makes a lot of sense. But to know for sure - individual charging and testing is the way to go!

 

[loonwheeler]

Yes my truck does have a tachograph without a disc. I have measured the current draw at about 18 mA and shouldn't be too much draw if driving once a week or so. Since I started to notice these issues last fall, I just flipped the breaker off for the tachograph so it has been out of the equation.