MB Sprinter, battery drained, can't even turn ignition

[Hollander79]

Originally Posted by Hatteras Jim

Hi There:

Thanks for all your information and troubleshooting over the past 3 years and for sharing the information with us. I will agree that having two batteries permanently connected is not ideal; but, I don't think you solved your parasitic draw problem. I say that, because I have a 2013 Sprinter that needs charged or disconnected when not being used to prevent discharge, and I only have a single battery under the floor. See what happens this winter. Hope your problem is solved, mine goes on. Cheers.

You may want to have the battery load tested. Which likely you have, but they need to pass 2 consecutive load tests, many garages only do a single test, and they have enough to pass, but fail with a secondary test. This is a sign of a battery with a poor cell usually.

The other small thing I found was a couple of the wires for the wiring in the back were on the wrong side of the 12 v disconnect solonoid. So even when the vehicle was off, parked, off of shore power and 12v disconnect pushed... these systems still had power, and shouldn't have. This is something you could investigate.

Also, just for shits and giggles, disconnect the positive battery cable then:
Check for voltage with your voltage meter from ground to the battery cable. This may show voltage and help you identify more specifically what you are looking for... may be that your house batteries are connected to your front battery continuously.... possibly inadvertantly through a secondary system.

Then check both voltage and amperage across between the positive cable and the positive post of the battery. This will give you an idea of how big your draw is, and may also help to narrow down your search.

Good luck!

[avanti]

Originally Posted by Hollander79

If one battey develops a bad cell, that bad cell will continually draw down the entire battery trying to equalize the voltage across all cells attached. If that bad cell has a short, that draw will be larger and continuous and essentially becomes a draw that will drain the batteries to nothing or almost nothing.

Again, I do not believe that this claim is quite correct.

If a cell in a battery shorts, you end up with a battery with effectively a lower voltage. For example, if you have a 12V battery with six cells, and one of those cells shorts, you will end up with a perfectly normal battery, but it would be a 10V battery, instead of 12V.

So, the question becomes what happens when you connect two batteries of different voltages in parallel. The exact answer depends on the internal resistance of the two batteries, but generally speaking, the higher-voltage battery tries to charge the lower voltage battery. This may lead to an over-charging of the remaining cells of the damaged battery, or other bad things. But you will always fairly rapidly end up with some kind of equilibrium. The good cells of the "bad" battery would soon stop accepting current. It is not possible for there to be a long-term "parasitic load" that continues to consume power. Where would the energy go?

There is another situation that may occur, however. In the steady-state, you often end up with a net voltage of somewhere between the two parallel batteries. So, for example, the battery bank may end up delivering, say, 11V. This will confuse external charging (such as a solar charger), and cause it to continuously charge in an effort to reach 12V. This would be bad, but it is not a "parasitic load".

I am not disagreeing with your basic point: a good battery in parallel with a bad battery is not a good thing. But as I understand it, no such arrangement will lead to a continuous drain on a battery bank that is not being charged (which is how I would define "parasitic load"). You will always reach an equilibrium (ignoring, of course, the normal tiny "self-discharge" that every battery exhibits).

As I said, I could be wrong. But you would have to explain the physics to me.

[GeorgeRa]

Originally Posted by Hollander79

Ok, so I have been chasing this bastard for about 3 weeks (after hours), have rewired the entire electrical area, traced circuits, checked diagrams, spent HOURS killing myself on this stupid thing.... and what did it end up being? A cheap shortcut by the manufacturer!! If you have a sprinter converted to an RV 90% chance this is also your issue. The Mercedes Serenity is considered a luxury unit (apparently), so to find such a cheap short cut move was disgusting. ……………………..

Originally Posted by Hollander79

…………………..Just for clarification, I didn't say that MB made the shortcut, they did not, MB did their due diligence in adding the solonoid to account for battery equalization loss and for someone running the one battery down using interal lights/power while the motor/ignition is off etc. It's quite clear in their literature.……………………...

I concluded from you first post that short cut was on the Mercedes side, my mistake.

All 6V or 12V have multiple cells, 3 for 6V and 6 for 12V so even single batteries could have an issue of a bad cell. Mixing different batteries will make this problem worse.

[avanti]

Originally Posted by GeorgeRa

All 6V or 12V have multiple cells, 3 for 6V and 6 for 12V so even single batteries could have an issue of a bad cell.

This is absolutely true. In addition, all high-capacity battery banks are always realized by wiring multiple batteries together.

Note, however, that the cells in 6V or 12V batteries are wired in series, not in parallel. A failed cell will cause problems in either case, but the symptoms are entirely different for series vs parallel batteries.

[GeorgeRa]

Originally Posted by avanti

This is absolutely true. In addition, all high-capacity battery banks are always realized by wiring multiple batteries together.

Note, however, that the cells in 6V or 12V batteries are wired in series, not in parallel. A failed cell will cause problems in either case, but the symptoms are entirely different for series vs parallel batteries.

Good point.

[booster]

If everybody starts to get worried about using multiple batteries connected in parallel, none of us would have enough power to do much of anything. Obsessing over a potential single internal cell short causing a parasitic loss will drive you nuts. Sure, when batteries get old they will fail, that is why we check them and their condition periodically. Trying to build a system with only one battery and no parallel batteries these days is not going to be easy to do.


If you have other parasitic you can usually get rid of them unless they are bred into the computers someplace, like the 2008 Chevies had.