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06'E350 wgn,05'E320 CDI,04'E320 wgn,85'300CD,67'230SL
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Discussion Starter · #1 · (Edited)
I leave a W211 vehicle in our garage for 3-4 months while we vacation.

I lock the car before leaving to minimize parasitic drain. I use a good quality battery maintainer that charges to full voltage, then pulse cycles the main battery to de-sulfite the battery.

However, apparently the auxiliary battery doesn't get recharged from the maintainer on main battery? We were away longer than expected this year due to Covid travel restrictions and the 1 year old aux battery was totally discharged. Main battery was 100%

I've added another 5amp fused pig-tail (for easy charger connection) to the auxiliary battery and plan to use two battery maintainers next time away.

Does this strategy make sense??
 

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If you are using CTEK charger set it for small battery to limit charging current.
 

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06'E350 wgn,05'E320 CDI,04'E320 wgn,85'300CD,67'230SL
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Discussion Starter · #3 · (Edited)
If you are using CTEK charger set it for small battery to limit charging current.
If you are using CTEK charger set it for small battery to limit charging current.
Thanks Witek for your reply.
I'm not using a CTEK brand per-se, but yes, I have several battery maintainers that stop charging when voltage level is reached and 'float' on-off to maintain that level (I edited original post to be clear: MAINTAINER).
Good suggestion to use a smaller maintainer for the AUX battery.

I have either a smaller 1.5 AMP Schumacher maintainer, or a CEN-Tech .5 Amp float maintainer designed for smaller motorcycle batteries I could use for Aux battery.
I have a larger 3Amp Schumacher 'Battery Extender' maintainer I've been using for Main Battery.

My major question is...Does it make sense to use TWO maintainers?? Should I assume with car locked, there's no interface between the two batteries??
 

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Thanks Witek for your reply.
I'm not using a CTEK brand per-se, but yes, I have several battery maintainers that stop charging when voltage level is reached and 'float' on-off to maintain that level (I edited original post to be clear: MAINTAINER).
Good suggestion to use a smaller maintainer for the AUX battery.

I have either a smaller 1.5 AMP Schumacher maintainer, or a CEN-Tech .5 Amp float maintainer designed for smaller motorcycle batteries I could use for Aux battery.
I have a larger 3Amp Schumacher 'Battery Extender' maintainer I've been using for Main Battery.

My major question is...Does it make sense to use TWO maintainers?? Should I assume with car locked, there's no interface between the two batteries??
Aux battery is isolated from the rest of the vehicle by relay K57/2.
 

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06'E350 wgn,05'E320 CDI,04'E320 wgn,85'300CD,67'230SL
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Discussion Starter · #5 ·
Aux battery is isolated from the rest of the vehicle by relay K57/2.
Thanks Witek,
If the relay isolates the Aux battery, I guess that's why during a longer-term storage, a maintainer on just main battery; won't maintain Aux battery too?
Just trying to understand how the circuit operates. And IF my 'two maintainer' strategy is going to work to maintain both batteries independently during long-term storage.

When does the K57/2 relay 'connect' or 'disconnect' the AUX battery? Maybe the action of locking the car?
Am I wrong in assuming the AUX battery is there to keep enough voltage to the memory of various electronic modules in the car?
 

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Thanks Witek,
If the relay isolates the Aux battery, I guess that's why during a longer-term storage, a maintainer on just main battery; won't maintain Aux battery too?
Just trying to understand how the circuit operates. And IF my 'two maintainer' strategy is going to work to maintain both batteries independently during long-term storage.

When does the K57/2 relay 'connect' or 'disconnect' the AUX battery? Maybe the action of locking the car?
Am I wrong in assuming the AUX battery is there to keep enough voltage to the memory of various electronic modules in the car?
Auxiliary battery is backup for brakes. SBC.


 

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W212 350CDI -09 W219 320CDI -07 Ex:W220 320CDI -01 Ex:W210 270CDI -01 EX: W211 320CDI -04
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Ecu detects if aux circuit voltage goes low, and then controls the relay short perior right after engine start. So car sleeping, none controls the relay.

And as above, it is emergency volatge source if main circuit fails, mainly safety reasons to feed SBC
 

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Discussion Starter · #8 ·
Ecu detects if aux circuit voltage goes low, and then controls the relay short perior right after engine start. So car sleeping, none controls the relay.

And as above, it is emergency volatge source if main circuit fails, mainly safety reasons to feed SBC
Ahh, Thanks Witek and Mersum, Really Helpful info. That gives me a better understanding. Aux is for SBC.

But I would still think there's 'some' electronic controls being fed from AUX battery, even if only having to do with SBC. And electronics don't think or remember very well on low or no voltage. :(

So, unless you guys suggest otherwise......
For long-term storage I'm going to add a small battery maintainer (<1.5amp) for just the AUX battery. A slightly larger one (3amp max) for the Main Battery. In-line fused pigtails to each battery makes connection a snap. Worst case, both batteries will be fully charged and ready to roll upon my return from a long Holiday. Batteries may last longer that way as well, and I suspect maintaining adequate voltage throughout the vehicle will help electronics (the Achilles Heel of these wonderful vehicles) last longer and stay healthier as well. ;)
 

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I was wondering the exact same thing because my car will sit for 6 months exactly every winter. Sounds like I will need to buy two maintainers.
 

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Ahh, Thanks Witek and Mersum, Really Helpful info. That gives me a better understanding. Aux is for SBC.

But I would still think there's 'some' electronic controls being fed from AUX battery, even if only having to do with SBC. And electronics don't think or remember very well on low or no voltage. :(

So, unless you guys suggest otherwise......
For long-term storage I'm going to add a small battery maintainer (<1.5amp) for just the AUX battery. A slightly larger one (3amp max) for the Main Battery. In-line fused pigtails to each battery makes connection a snap. Worst case, both batteries will be fully charged and ready to roll upon my return from a long Holiday. Batteries may last longer that way as well, and I suspect maintaining adequate voltage throughout the vehicle will help electronics (the Achilles Heel of these wonderful vehicles) last longer and stay healthier as well. ;)

How do you handle closing the trunk?
 

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Discussion Starter · #11 ·
How do you handle closing the trunk?
Carefully! lol
I keep the 120v side and maintainer block outside of trunk. I feed the 12v 'battery maintainer' wire just above the lower right trunk corner where there's a rubber gasket and close it gently. Pigtail connects inside trunk near battery and has small (I think 7.5amp) in-line fuse for protection. That way, I can lock the car to turn off as many parasitic draws as possible. I'll rig something similar at the Aux battery now.
 

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I ordered Noko Genius 1 for the front battery and Genius 2 for the rear. I'm picking the car up tomorrow from Myrtle Beach and driving back home to Northern IL. It was grandparent's car and only has 24k on it.at benz dealer now getting new battery. My plan is to park for the 6 month winter so battery maintainer sounds 100% necessary.
 

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my experience annual "sleep" periods of not more than 3 1/2 months was ok w only 1 charger. i didn't even know there was a second one, which i recently replaced after 15 yrs , even though it was still good/.. What a sport, heh ?
 

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Discussion Starter · #15 · (Edited)
I assembled a Defa charger Onboard battery chargers • Mounted quality chargers for your car • DEFA on trunk of my W211, and pulled the wire under rear bumber. There is a ready hole for parking sensor wires, where I could wiggle the defa plug through....
+1 ^^^^^^^^^ Great idea. Having an on-board set-up makes it a snap to hook-up.

I have a car that's in a storage unit that doesn't have electricity. However, I got them to allow me to install this Solar Battery Tender on the exterior wall that just uses my 12v 'pig-tail' fused connection. Car (89 300CE) is left almost 12 months at a time near the airport so we can fly and drive. It's worked perfectly now for several years. Also have one on my boat. :)
P.S. It's $159.99 thru Amazon Prime. 35 watts at 12-13v = to approx 2.5amps daytime.

 

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Thanks Witek for your reply.
I'm not using a CTEK brand per-se, but yes, I have several battery maintainers that stop charging when voltage level is reached and 'float' on-off to maintain that level (I edited original post to be clear: MAINTAINER).
Good suggestion to use a smaller maintainer for the AUX battery.

I have either a smaller 1.5 AMP Schumacher maintainer, or a CEN-Tech .5 Amp float maintainer designed for smaller motorcycle batteries I could use for Aux battery.
I have a larger 3Amp Schumacher 'Battery Extender' maintainer I've been using for Main Battery.

My major question is...Does it make sense to use TWO maintainers?? Should I assume with car locked, there's no interface between the two batteries??
No it doesn't. The 2 batteries are wired in parallel. Charging one will charge the other.
 

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Discussion Starter · #17 · (Edited)
No it doesn't. The 2 batteries are wired in parallel. Charging one will charge the other.
Doesn't relay K57/2 (controlled by the battery control module) separate the two batteries with ignition switched off and/or car locked? I guess you could verify by disconnecting the terminals of main battery and seeing if there's still 13v across coming from the AUX battery? Maybe thet's not a good idea.
I'm adding the pigtails to each battery, so it will be easy to measure the voltage at each battery independently at the pigtails. If both batteries are normally connected, voltage when charging the main battery should be identical at the AUX. I suspect they're NOT connected unless the BCM detects low voltage and closes the connection via relay? I'm just trying to insure that BOTH batteries remain fully charged during long-term storage.
You guys are more familiar with how the BCM operates, and if it still operates when engine(alternator) is off and car is locked?
 

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Doesn't relay K57/2 (controlled by the battery control module) separate the two batteries with ignition switched off and/or car locked? I guess you could verify by disconnecting the terminals of main battery and seeing if there's still 13v across coming from the AUX battery? Maybe thet's not a good idea.
I'm adding the pigtails to each battery, so it will be easy to measure the voltage at each battery independently at the pigtails. If both batteries are normally connected, voltage when charging the main battery should be identical at the AUX. I suspect they're NOT connected unless the BCM detects low voltage and closes the connection via relay? I'm just trying to insure that BOTH batteries remain fully charged during long-term storage.
You guys are more familiar with how the BCM operates, and if it still operates when engine(alternator) is off and car is locked?
I added a charging plug (same as pic in #11) for my harbor freight 1.5a charger at the jumper terminal in the engine bay. It kept both batteries charged.
 
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