Battery Tender - Which Battery to Connent? - Mercedes-Benz Forum

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post #1 of 16 (permalink) Old 09-13-2009, 09:44 AM Thread Starter
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Battery Tender - Which Battery to Connent?

I have a 2005 SL 500 I do not drive for months at a time and would appreciate advice re if I should be connecting my Battery Tender Plus to one [Engine or Trunk?] or both batteries. The dealer said to connect to just the trunk battery, however he did not seem sure, and stated he had never been asked this question. For simplicity I would prefer to just connect one battery if this will do the job.
Thank you, Mike
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post #2 of 16 (permalink) Old 09-13-2009, 10:15 AM
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I somehow recall that the suggestion of hooking it up to the trunk battery, since the other is also tied into it.
http://www.mercedestechstore.com/pdf...2011-28-02.pdf

This describes the system

Last edited by Peter Guenther; 09-13-2009 at 10:19 AM.
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post #3 of 16 (permalink) Old 09-13-2009, 11:28 AM
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Quote:
Originally Posted by Peter Guenther View Post
I somehow recall that the suggestion of hooking it up to the trunk battery, since the other is also tied into it.
http://www.mercedestechstore.com/pdf...2011-28-02.pdf

This describes the system
The two batteries are not connected when the car is switched off.

Either use two trickle chargers or charge the rear one that has some consumers also when the car is switched off. The starter battery has no load, discharges only from internal leakage.
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post #4 of 16 (permalink) Old 09-13-2009, 05:09 PM
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I could be wrong, but I think there is a diode matrix, which bleeds from the main to the secondary. I dont think they have a SCR or relay, but I could be wrong. In my boat either engine , generator or battery charger charges the house battery, or the secondary batteries. MB cant be that stupid.
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post #5 of 16 (permalink) Old 09-13-2009, 11:06 PM
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Quote:
Originally Posted by Peter Guenther View Post
I could be wrong, but I think there is a diode matrix, which bleeds from the main to the secondary. I dont think they have a SCR or relay, but I could be wrong. In my boat either engine , generator or battery charger charges the house battery, or the secondary batteries. MB cant be that stupid.
I could be wrong but if there was a diode matrix like you probably have on your boat, that would have to be from the charger to each battery and the charger voltage would have to higher to take into account the voltage drop from the diode.

The idea with the starter battery in the car is that the consumer battery can run completely flat but the car would still start from the starter battery (and would then start charging both). From the document you posted, one can see that the SL is clever enough to use the consumer battery in case it has power but the starter battery is flat. On a W221 I believe the consumer battery is not used alone but on diesel models both can be used in parallel in cold weather if necessary and if both are good.

The question here was about a trickle charger that would be connected to one or the other battery. If there were diodes taking care of charging the other battery when the charger is connected to the first one, the diodes would have to be in the car permanently which means they would fry if the starter battery was flat and the engine was started from the consumer battery. Another option would be that the car would be clever enough to detect that a charger has been connected and would switch the diode accordingly but in that case the car could equally use the isolation relay, this would be a lot better because the voltage drop from a diode is unavoidable when the charger is connected to one battery and the other is fed with a diode from the first one.

I've never studied the details of the SL and it could work differently from the W221 which has a very similar battery arrangement. I'd be glad to learn from SL experts if I was guessing wrong.
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post #6 of 16 (permalink) Old 09-14-2009, 06:18 AM
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I also could be wrong, I briefly studied the flow chart on the link I provided. The batteries are connected thru some sort of isolation device which would not allow a back feed
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post #7 of 16 (permalink) Old 09-14-2009, 10:03 AM
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The older boats and RV had battery insulators what are diodes allowing only current flow from the alternator/chassis battery side into the house batteries.
The system not only takes 0.3 V cut passing the diodes, but very often has small leaks back as well to the ground. The better systems are battery combiners, where electronic circuit switches a relay working both ways.
Any system is prone to failure sooner or later. I lost few batteries over the years due the battery maintainers (tenders) failure, so they should be monitored as well.
The SBC backup battery is motorcycle style and those have much shorter life span, than the car batteries, therefore I would worry more about this one.
In my motorhome I am disconnecting battery clamps for 6-9 months and the coach starts after with no problem.
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post #8 of 16 (permalink) Old 09-14-2009, 11:43 AM
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Connect to same battery owners manual states to use for jump starting.

Kent Christensen
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post #9 of 16 (permalink) Old 09-14-2009, 12:17 PM
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Originally Posted by lkchris View Post
Connect to same battery owners manual states to use for jump starting.
In my opinion it isn't that simple. This is best if one wants to guarantee the car starts even if the other battery would go flat (it would go flat pretty soon, after several weeks).

But then again, the starter battery is like the one in Kajtek's motor home (even if wires are attached) and his battery was fine after several months.
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post #10 of 16 (permalink) Old 09-15-2009, 10:28 PM
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Problem is, that without disconnecting the cables there always will be small current flow.
MB likes to keep the customers in dark and not telling how the system works.
In older MB we had them sitting with alarms on for 5 weeks and they always started up after. That is pretty impressive, but low discharge is killing the battery in accelerated rate and should be avoided.
So my guess is, that the SBC battery is not powering anything on parked car till somebody triggers SBC system to start (like opening the door).
The alarm suppose is powered from the big battery and this is the one who theoretically should be charged.
Than again my experience is, that big batteries can "hold on their own" for long time, while small, motorcycle style batteries are lesser quality and require more maintenance.
Best of all would be connect both battery to battery maintainers. I wouldn't hesitate to connect them to the same maintainer, but that would require long extension cables.
Or just disconnect the clamps on both..
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