Starting battery drains; not charging from shore power

[dvo_michka]

Hello SMB community --

I own a 2015 MB Sprinter SMB conversion. I don't drive it very regularly this time of year, but have it hooked up to 110V shore power most of the time. Problem is, even though my house batts stay fully charged, the voltage on my engine battery goes down and down and down... today I checked it after charging it fully only 1 week ago, hooked up to shore power the whole time, and it was down to 11.7V.

I have a battery isolator that allows me to jump the engine battery w/ a "manual override/reset" button, which I understand puts all batts in parallel. I want the engine battery to share the charge from the house batts, but they do not stay in this configuration for long -- it gets a jump but then stops accepting charge.

I had a parasitic drain on the MB side addressed by the dealer only last summer, with a new battery installed. I can't understand why it is dying again. I thought the isolator was supposed to favor the engine battery when the RV is hooked up to shore power.

Help appreciated!

[doublevan2]

No expert here - but possibly another parasitic charge for the chassis batt as well as something wrong with isolator ? Bad work from MB dealer ? Good luck !

[kbeefy]

If you charged it a week ago and it's down to 11.7 there is a problem with your battery or a pretty significant draw.


I would deterime if and what that issue is, then after addressing that possibly modify your shore power charging system to compensate for normal draw.


I am not familiar with what isolator you have or how it is wired, but typically they are hooked up to the starting battery and vehicle alternator, and only combine/charge both banks when the engine is running. The shore power 'converter' or DC charger only manages the house batteries.


I did a recent build where I used a 'combiner' instead of an isolator. It connected the start and house batteries any time the voltage on either battery was above 13v or so. That way it did charge the start battery on shore power, but did not combine batteries while cranking. You should only combine batteries while cranking if your house battery cables are up to the current imposed by starting an engine. Mine was not so it worked perfect in my system.


Typically I like to keep the start battery isolated unless the engine is running, all my vehicles will start reliably after sitting at least a month, sometimes more. Newer vehicles are becoming less capable of going that long, too many computers.



Normal draw should leave enough voltage to start (12.3-12.4v) after sitting for at least 2 weeks.


One way to test would be to disconnect your start battery from the house system and fully charge it, then see if the voltage drops as you have been experiencing. If it still does your vehicle has excessive draw or your battery is failing.


I don't like waiting that long so I check the parasitic draw current on the start battery after all vehicle systems have gone to 'sleep'. It can sometimes take 30 minutes. IIRC the sprinter has its battery under the drivers seat, if you want to do that test you need to trick the vehicle into thinking the door is closed, the door switch will wake up the computer and mess up your test.


I like to use a 'fuse buddy' as a dedicated tester, it's cheap, robust, and keeps my multimeter free for other tasks.
This is the one I use, Do Not get the HD version that only reads out to 0.1a, it is not sensitive enough to read small parasitic draws.

[Scalf77]

If you identify the make of the "isolator" we will be able to assist on understanding why it is not combining when you are plugged into shore power. I believe that SMB was using Blue Sea ACR's at that time. They are all bidirectional, it possible you have a Surepower, I have seen some SMB's with unidirectional model installed, instead of the bidirectional. Or of course it could be different. A snap-shot would be great.

This of course would only mask your parasitic draw issue.

[dvo_michka]

I have a "Battery Doctor" 100amp Isolator, pictured below. Is this the same thing as a solenoid?

When I press the "reset" button, I hear a click and get a boost on the engine battery voltage as the batteries are connected. Could this open connection actually drain the engine battery even as the house batteries are float charging on the shore power if I keep the van sitting? I.e., doing the exact opposite of what I intended?

I agree that a drop to 11.7 volts in a week is bad. I don't really have the electrical know-how to track down small drains; that's why I paid MB $2,000 to do it, and fix it... They found a fault with a glass break alarm system due to water leakage somewhere, and replaced it...

Provided I don't actually have a parasitic draw, do I need something like this to achieve what I want?

https://www.magnum-dimensions.com/sm...ttery-combiner

[Scalf77]

Ok, Thanks. The Battery Doctor would be considered unidirectional. It is designed to only automatically charge the house battery when the start battery goes above 13.4 volts. This would be an indication that the alternator is running. As you have seen, when the start battery goes below 12.8 Volts for 60 sec it will disconnect.

The override switch looks like it will force you to combine the two battery banks to jumpstart a weak start battery (As long as both batteries are above 7.00 Volt) This of course as you have found out only last 3 mins.

The BlueSea 7622 or 7620 is pretty much then gold standard for automatic charging relays.

I really haven't looked at the magnum unit in a while, but I don't remember anything that would change my recommendation.

[dvo_michka]

Ok I see -- and that BlueSea charging relay would essentially act as a combiner? It would replace my Battery Doctor but also allow charging of both banks from shore power -- and from my alternator?

[Fitz]

dvo_michka

Suggest you look at the posts in my build thread (for a 2016 Sprinter) on the topic of changing out the original Separator for a BlueSea 7622 Automatic Charging Relay (ACR). I received lots of help from members here on the Forum when the change was made.

https://www.sportsmobileforum.com/fo...tml#post251479

Also, where are you located? Adding a location to your name or signature can often be very helpful. For example, if you are anywhere near the San Francisco Bay Area you are more than welcome to see the ACR installed in my van.

[KenInRockwall]

Seems you have gotten lots of good advice. I will just add that, in my case, I use a charger plugged into house power then fed into the always on 12vdc outlet in the center console of my 2017 MB sprinter. Otherwise as far as I can tell the shore power does not charge my battery.

[jsenge]

Our 2014 SMB Sprinter had the same type of problem and we seem to have resolved it by installing an AMP-L-START Starting Battery Charger/Maintainer. Since installing this device a couple years ago, our MB Sprinter battery stays fully charged when the van is parked for several weeks. It is easy to install following the directions provided by the manufacturer. It has solved our battery discharge problem so I can safely recommend it.

AMP-L-Start Starting Battery Charger/Maintainer
LSLProducts.net
P.O. Box 681072
San Antonio, TX 78240
Support@lslproducts.net