Automatic battery charger wiring - Mercedes-Benz Forum
 
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#1 (permalink) Old 11-06-2009, 07:32 AM
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Automatic battery charger wiring

As we get closer to winter here in the Northeast, decided to use one of those battery tender plus charging devices on my 86 560sl, so that the battery will stay fully charged over the cold winter months.
Since the battery is in the tightly fitting box in the trunk, I couldn't figure any easy way to hook up the charger by running the wires under the car and up into the battery box from underneath. I ended up running the wires by just opening the trunk, opening the panel that covers the battery, and taking the battery box lid off ( and leaving it loose on the top of the batttery after wiring the charger to the terminals ). I then realized I couldn't leave the trunk slightly open because the trunk light comes on and would cause a battery drain over time. So what I did is find a spot on the trunk rubber seal where i could lay the wire without damaging it, and close the trunk lid. This worked, and the unit is now working as planned.
The only slight hassle now is that when I want to run the car, I'll need to go into the trunk and reverse everything, which will take a little time. These charging units are designed with a short piece of cable that can be left attached to the battery, which can be removed from the charger wiring by a simple plug in .
Has anyone who uses these chargers figured out a better/simpler way of running the cable to the battery ?
Steve
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#2 (permalink) Old 11-06-2009, 09:10 AM
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To keep my SL's battery healthy over the long Michigan winter, I bought the Schumacher 3 Amp SPEEDCHARGE, Model 300A. It has an electronic maintenance cycle you can leave connected indefinitely. Included as quick-release accessories are a cigar lighter adapter and a more permanent pigtail cable, along with the usual spring clamps for direct charging.

The cigar lighter adapter wouldn't work unless I rewired that circuit to be hot all the time. But the other accessory, a pigtail with two ring terminals, worked great.

I routed the red cable through the battery box cable grommet to the positive battery cable accessory terminal. I wired the negative, black cable to the nut and stud that holds the vacuum unit in place (not visible in picture).

I still have to run a cable out of the trunk to plug in the charger, but this saves me from having to open and close the battery box to connect and disconnect the maintainer.

Good road,
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#3 (permalink) Old 11-06-2009, 11:50 AM
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Good idea- that at least would save the step of dealing with the battery box lid every time. Do you think it makes a difference in using the negative post of the battery versus attaching to some frame member to get grounded ? I had always assumed that the suggestion to ground to the frame was made in case the battery might not be properly grounded. I'm sure my battery is properly grounded, so I am using the alligator clips on both terminals. Is there another reason that the negative battery post should not be used ?

Sounds like you are also closing the trunk lid on the wires to tie in to the charger. I'll be curious if anyone else has figured a creative way around that.
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#4 (permalink) Old 11-06-2009, 12:09 PM
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Quote:
Originally Posted by steve 86sl
Is there another reason that the negative battery post should not be used ?

Whether attaching charger clips or using a battery to jump start another car, grounding to a remote location eliminates the possibility of sparking in the proximity of any battery fumes, and battery explosion.
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#5 (permalink) Old 11-06-2009, 12:34 PM
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you have a trunk light ? well i must have one also, time to get a new bulb
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#6 (permalink) Old 11-06-2009, 12:44 PM
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In my case the Schumacher-supplied pigtail was too short to reach both battery terminals and exit the battery box via the grommet the way I wanted it. And I didn't want a cable spanning the top of the battery and joining the battery cables that would make it awkward to pull the very heavy battery out of the box if needed.

The nut and stud securing the vacuum unit next to the battery box was clean (and conductive), close enough, and a direct ground to the chassis, so it was the best logical choice for me. Getting clear access to the vacuum unit to attach the pigtail's negative cable required me to temporarily remove the battery box, though, so the job was a little more involved.

As far as closing the trunk on the charger's power cord, it's really not harmed the weatherstrip noticeably, but I exit the cord from the center rear of the trunk just to be sure.

Being able to plug in the charger/maintainer without opening the battery box each time saves wear-and-tear on the brittle battery box (you can no doubt see mine's cracked a bit)...and if there ARE any hydrogen fumes lingering in that box--as isthisdave notes--hopefully saves me from reliving the Hindenburg spectacle in my own garage.

Good road,
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#7 (permalink) Old 11-07-2009, 01:16 AM
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I just disconnected the battery, and will put it on a charger in the spring when it's closer to time to bring her out for some fun.

I'm not a real dickhead...but I acted like one in high school.
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#8 (permalink) Old 11-07-2009, 06:14 AM
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A very timely post. I went out to the 560 yesterday and the battery was dead. Guess I need to get a float charger. Thanks for the wiring ideas.
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