SL55/63/65/R230 AMG: Rear battery maintainer - hidden and inexpensive
Thread Starter
Senior Member
Joined: Jul 2013
Posts: 306
Likes: 19
From: West Chester, PA - USA
2003 SL55 AMG
Rear battery maintainer - hidden and inexpensive
I installed a hard wired Harbor Freight battery maintainer in the trunk area of my '03 SL55. Plug it in for about 24 hours once every 5-7 days to keep the battery maintained. I've used this (or the predecessor from HF) for about 10 years on various batteries, and had great success with keeping them 'alive'. I'll post some pictures to show the maintainer, and where it is located (rear of trunk, above battery, 'pushed' inside a cavity under the luggage cover latch at the upper right edge).
First pix shows the maintainer - whose original wires I extended and put lugs on the end to attach to the battery. The second shows maintainer partially pushed into the area where it is kept, and the third shows the maintainer fully hidden with the AC connector attached. When the 110V power is not connected, the only thing that shows in the trunk is the male connector from the maintainer. The maintainer brings the battery to 13.7 volts after a few hours, and maintains it there as long as it is connected to 110V. Battery quickly drops to about 12.6 V after disconnecting, then over 5-6 days with an occasional door being opened, etc. it is down to about 12.4 V.
Cost of the maintainer is less than $20 at HF, and they frequently have 20-30% discount coupons for one item.
Gary Knox
Attached Thumbnails
First pix shows the maintainer - whose original wires I extended and put lugs on the end to attach to the battery. The second shows maintainer partially pushed into the area where it is kept, and the third shows the maintainer fully hidden with the AC connector attached. When the 110V power is not connected, the only thing that shows in the trunk is the male connector from the maintainer. The maintainer brings the battery to 13.7 volts after a few hours, and maintains it there as long as it is connected to 110V. Battery quickly drops to about 12.6 V after disconnecting, then over 5-6 days with an occasional door being opened, etc. it is down to about 12.4 V.
Cost of the maintainer is less than $20 at HF, and they frequently have 20-30% discount coupons for one item.
Gary Knox
Attached Thumbnails
Last edited by kittyandgary; Mar 6, 2014 at 01:07 PM.
Senior Member
Joined: Aug 2013
Posts: 372
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From: CT
CTS-V
Nice. I have been keeping my rear battery on a Battery Tender all Winter. Haven't had anything on the front battery though. Seems to start fine every time I started it this Winter. I'm guessing the starter works off the rear battery too...
Thread Starter
Senior Member
Joined: Jul 2013
Posts: 306
Likes: 19
From: West Chester, PA - USA
2003 SL55 AMG
ttboost,
I know both the front and rear battery are charged by the alternator (with car idling, both show about 14V as I recall). But, I do not think there is any connection between them when the switch is turned off. Like you, my front battery maintains a full charge for a month or more - no loss of voltage. The ONLY battery in my 'old' MB 560 SEC maintains its charge without being driven for 6-8 weeks.
Enough parasitic draw on the rear rear one that they seem to discharge in a few weeks if the car is not driven or the battery not maintained. My '94 Porsche 928 GTS has that same loss of voltage over time. All those darn computers and stuff suck it out!
Gary-
I know both the front and rear battery are charged by the alternator (with car idling, both show about 14V as I recall). But, I do not think there is any connection between them when the switch is turned off. Like you, my front battery maintains a full charge for a month or more - no loss of voltage. The ONLY battery in my 'old' MB 560 SEC maintains its charge without being driven for 6-8 weeks.
Enough parasitic draw on the rear rear one that they seem to discharge in a few weeks if the car is not driven or the battery not maintained. My '94 Porsche 928 GTS has that same loss of voltage over time. All those darn computers and stuff suck it out!
Gary-
Last edited by kittyandgary; Mar 7, 2014 at 04:17 PM.
