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Devon battery isolating switch

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Post by Bob Evans Mon Apr 12, 2021 3:53 pm

I have a Devon (2010).  For years I have wrestled with the issue of the engine battery totally discharging in about 2 weeks when parked up. The present battery is almost new. The Autosleeper handbook says on page 10-4 "Mercedes recommend that if the vehicle is to be parked up for a long period that the vehicle electrical system is to be switched off at the battery isolating switch" They then refer the reader to the Mercedes handbook. The Mercedes handbook on page 94 says the same and shows a picture of the switch to the right of the accelerator pedal.  BUT it is not there.  I have also looked in the passenger footwell as the manual seems to serve both L and RH drives. The Mercedes Garage is also puzzled. Has anyone come across this and found a solution please? I can't leave it on mains where I store it, and I'm really struggling removing the battery every time.  I think it ought to be possible to remove certain fuses when leaving it, surly that would achieve the same aim, but which ones?
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Post by bolero boy Mon Apr 12, 2021 4:08 pm

im sure a Merc owner will be along shortly but, rather than removing the battery, couldnt you just disconnect it and leave it in situ?
do you have solar? if so, should it be charging the vahicle battery as well as the leisure battery?

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Post by PLOUGHLIN Mon Apr 12, 2021 5:00 pm

The detachable negative shown in the MB manual is an option, unfortunately not one that AS ordered, mine doesn't have it. Other than detaching the negative at the battery (under the passenger side floor) it can't be isolated.

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Post by gemdeco Mon Apr 12, 2021 5:10 pm

Hi 
     Peter beat me to it The battery is located under a floor on the passenger side, you may have a master switch on the battery terminal but I doubt it. I always jump started or trickle charged mine from the connections under the bonnet
Regards
Alan
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Post by willy eckerslike Mon Apr 12, 2021 8:13 pm

I have a 2008 Devon Bob. I have never found an isolator switch for the vehicle battery, which is not easily accessible, being under the cab rubber mats. Do you have an alarm fitted, my battery holds it’s charge well even when stood on the drive. I haven’t seen it below 12.4v and it started the engine ok from that.

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Post by Bob Evans Tue Apr 13, 2021 9:30 am

Thanks Willy - I do have an alarm fitted but never set it to avoid battery discharge. I am amazed you don't have this issue, I lose 0.3 volts per day! Even having a solar panel attached in the windscreen plugged into the 12 volt permanent socket makes no noticeable difference. Is yours a Merc?
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Post by HairyFool Tue Apr 13, 2021 9:46 am

The smallest solar panels are barely enough to compensate for self discharge, you need to go up a bit to account for accessory drain on modern vehicles.
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Post by PLOUGHLIN Tue Apr 13, 2021 9:51 am

Bob Evans wrote:Thanks Willy - I do have an alarm fitted but never set it to avoid battery discharge. I am amazed you don't have this issue, I lose 0.3 volts per day! Even having a solar panel attached in the windscreen plugged into the 12 volt permanent socket makes no noticeable difference. Is yours a Merc?

Do you let the radio and interior cab light extinguish on the ignition, if so that may be your drain. Switch them off at the radio and light.

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Post by willy eckerslike Tue Apr 13, 2021 11:44 am

Bob Evans wrote:Thanks Willy - I do have an alarm fitted but never set it to avoid battery discharge. I am amazed you don't have this issue, I lose 0.3 volts per day! Even having a solar panel attached in the windscreen plugged into the 12 volt permanent socket makes no noticeable difference. Is yours a Merc?
Yes it is a Merc, two years older but I don't think they changed much in two years.

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Post by bolero boy Tue Apr 13, 2021 12:33 pm

Bob Evans wrote:Thanks Willy - I do have an alarm fitted but never set it to avoid battery discharge. I am amazed you don't have this issue, I lose 0.3 volts per day! Even having a solar panel attached in the windscreen plugged into the 12 volt permanent socket makes no noticeable difference. Is yours a Merc?
0.3v over24 hrs is an ongoing drain of 125 mv, have you established this is actually the current drain (using, say, a clamp meter or is this just the measured drop in battery voltage?
if the latter, and the battery was goosed then it would lose charge even if there wasnt a large drain.
have you charged the battery fully and left it disconnected to ensure that it maintains charge in its own?
if it does, then it is the drain thats the problem which might need investigating.
if it doesnt then its the battery itself...
you can do this the reverse way by checking the current drain first if you can.

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Post by Bob Evans Tue Apr 13, 2021 12:55 pm

The battery holds charge when unconnected for months - it held charge right through winter at home out of the vehicle. Mercedes trucks have confirmed that battery drain is running at between 1.4 and 2 amps. They say this is due tot he complexity of the 'computers' that make up their systems and don't seem phased by it. They suggested the isolating switch but it doesn't exist. They've enquired if one can be fitted retrospectively but it isn't available. I've asked them if there are fuses that could be removed and they are to speak to Mercedes tech support to answer this. Nothing seems to resolve this other than disconnecting the battery earth each time which is a pain but its better then taking the battery out I suppose. I am appreciating your thoughts - thanks
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Post by Roopert Tue Apr 13, 2021 1:29 pm

Bob Evans wrote:Mercedes trucks have confirmed that battery drain is running at between 1.4 and 2 amps.

Do you mean that they have measured the current, and this is the long-term measurement?

I don't see how they could possibly not be phased by it. Even the lower of those two figures will completely drain a typical 80Ah starter battery in two days or less. Any car electrician will tell you that a current drain of that magnitude is not in any way "normal".
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Post by HairyFool Tue Apr 13, 2021 4:06 pm

Normal quiescent drain should be no more than a few 10s of milliamps so they should very much be "fazed" by it (phased is something completely different smile! )

Any competent auto electrician should be able to break the circuits down to try and find the cause of the discharge. The obvious 1st check for a Merc dealer is to isolate the habitation circuits to prove it is their issue or an AS problem.

The only complication is that low drain state only kicks in after about 25 minutes from turning off the ignition. Even so the higher "standby" state should only be a few hundred mA, not your 1.4A. It can be a problem if each test takes 25 or so minutes to wait for full shutdown.

It is a relatively easy job to fit a battery isolator but only if there is a single lead going to the battery, competition vehicles have to have them as standard. The implementation of junction blocks, relays and fuse holders directly attached to the battery terminals make it much harder. The picture showing the earth post in the foot well raises the question of what precisely is earthed through that point.
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