Sparks when reconnecting Aux battery ground

asimba2

ourkaravan.com
I have a 2016 NCV3 with the factory under-hood Auxiliary battery. If I have to disconnect the ground on the Aux battery for any reason, I get quite a display of sparks upon reconnection.

I have several things connected to the Aux including the Amp Powerstep, Espar D2 and my Alpine audio amplifier (none of which are operating when I reconnect). I have never had a battery drain issue.

Has anyone else experienced this?
 

220629

Well-known member
If any of your connected components have memory function they will always be pulling some current. That can cause sparking when the negative is reconnected.

:cheers: vic
 

4wheeldog

2018 144" Tall Revel
Sparks indicate flow of electrons, so there is some sort of draw. As AP said, memory functions could be enough to do it.
 

asimba2

ourkaravan.com
Another oddity is 100% of the time I reconnect the battery the Amp step will not function until I pull and reinstall the fuse. Not a big deal but this combined with the sparks show has me wondering.
 

autostaretx

Erratic Member
The sliding step is a PSM (parametric special module) function, so the disconnect of the Aux battery may be telling the PSM that something's hiccuped (which it may take as a shorted motor or damaged wire) ... so it may be engaging a safety fall-back of "don't try to move things until a known state returns" ... which the fuse-pull probably does.

added: reading the option book for the T57 electric step ... next time this happens, try opening and closing the sliding door *instead* of pulling the fuse ... if that "fixes" it, then it was the obstacle-detector going into "safe mode" that was the cause.

more added: according to that book, the only things MB hangs on the Aux battery are:
* Code E46, Power socket in cab
* Code EE3, 115 V Socket
* Code ES2, 12 V Plug socket in trunk / load compartment
* Code ES5, Charging package, dashboard
* Code V85, Smoker package
* Code E1I, Smartphone cradle
* Code E1U, USB-C Socket, 5 V
The ES5 and E1U do draw a little power, even when there's no load.

even more: if the isolation/cut-off relay were to fail in a "closed" (i.e. connected) position, then the Aux battery would be permanently attached to the starter battery, and reconnecting the Aux negative cable would have the two batteries instantly try to equalize their voltages. You'd get sparks similar to those when attaching a jumper cable (between two good batteries).

You can test for that last situation by using a voltmeter between the Aux battery's positive post and the Sprinter's metal frame (or any "starter-side" ground connection (usually brown wires)). With the Aux batt's negative cable disconnected, the Aux batt's positive post should not show a voltage (it should be "floating" (no loads at all) or downright zero). When the negative cable is reconnected, that voltage would hop back up to the battery's level.
If the isolation relay *had* failed in a "connected" state, then the initial voltage test would read the starter battery's voltage (via the failed relay).

--dick
 
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4wheeldog

2018 144" Tall Revel
Another oddity is 100% of the time I reconnect the battery the Amp step will not function until I pull and reinstall the fuse. Not a big deal but this combined with the sparks show has me wondering.
Maybe try pulling the fuse before you reconnect the ground. If it doesn't spark, at least you know where the draw is coming from.
 

asimba2

ourkaravan.com
even more: if the isolation/cut-off relay were to fail in a "closed" (i.e. connected) position, then the Aux battery would be permanently attached to the starter battery, and reconnecting the Aux negative cable would have the two batteries instantly try to equalize their voltages. You'd get sparks similar to those when attaching a jumper cable (between two good batteries).

You can test for that last situation by using a voltmeter between the Aux battery's positive post and the Sprinter's metal frame (or any "starter-side" ground connection (usually brown wires)). With the Aux batt's negative cable disconnected, the Aux batt's positive post should not show a voltage (it should be "floating" (no loads at all) or downright zero). When the negative cable is reconnected, that voltage would hop back up to the battery's level.
If the isolation relay *had* failed in a "connected" state, then the initial voltage test would read the starter battery's voltage (via the failed relay).

--dick
Good point. It makes no difference whether I reconnect the Aux or the starter battery first, it still sparks. But that said it did not occur to me that it's possible the OEM battery combiner could fall-back to closed (batteries combined) when no power is present. It's certainly acting that way.

Next time I reconnect I'll first try opening the breakers to my aux battery loads as 4wheeldog suggested, as well as the test you suggested.
 

autostaretx

Erratic Member
But that said it did not occur to me that it's possible the OEM battery combiner could fall-back to closed (batteries combined) when no power is present. It's certainly acting that way.
You can quickly test for that right now (no disconnections required) with a voltmeter.
With the Sprinter off (so combiner should be "open" (not joined)), measure the starter battery voltage (at the Jump post under the hood, for example) and then measure the Aux battery (where-ever, positive post is fine). In both cases the meter's black lead is to the frame (any exposed screw head would suffice).

If they're joined/combined, they'll be the same. If they're not joined, you'll probably see a difference ...
If you *don't* see a difference, turn on the headlights ... that should drop the starter battery a bit, and leave the Aux battery untouched.

"Difference" means that they're not combined. (which is good, with the engine off).

"The same (even with the lights on)" suggests a problem.

I'd suggest using the 12v sockets as test points, but i'm not sure which ones are on the starter circuit and which ones are on the Aux circuit.

--dick
 

asimba2

ourkaravan.com
The batteries are definitely not combined when the van is off, however I'll have to conduct the test and see if there's continuity across the combiner relay when no power is present (i.e. both the starter and Aux battery disconnected). Thanks for giving me a place to get started.
 

mikecol

Active member
The other possible cause of the "sparks" when reconnecting is the bulk capacitance in the connected equipment. Removing and reconnecting the ground quickly will usually leave the caps charged and not "spark" if this is the case. This
might be a possibility if you didn't have any drain issues. Caps in power amp would be my guess.
 

calbiker

Well-known member
It's highly likely the spark is caused by capacitors located in your various electronic units. I just installed a 2500W inverter to a LiFePo4 bank. The first time I connected them together there was a major spark that removed some metal from the battery connection.
 

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