I am reorganizing my garage and my MB restoration. The car stays in storage most of the time and, as a result, the battery dies often from lack of use. After replacing the battery several times over the years (much sooner than antipated from lack of use and repeated jumpstarts), I bought a "battery tender" last year (i.e., 2002) but have been reluctant to leave the car attached to it for any extended period of time. The wires are thin and get warm over a short time. Note that this is a "battery tender" not a battery charger. It is intended to stay attached to the battery in the car for extended periods of time when the car is not in use and "trickle charge" the battery so it does not die. Has anyone had any problems with these? I'm wary of leaving the warm wires, possibly starting a fire and having more to worry about than dead batteries. Any thoughts? Thanks.

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'70 280SE Cab. (111)

While I don't pretend to be an expert on batteries, I have learned a lot about how to screw them up over a couple of decades of owning sailboats.

First, car batteries are damaged by deep cycling (letting them go dead and recharging), high charge rates, and overcharging. Many of the sealed type will never recover from going completly dead. Self discharge is a problem in older batteries when salt build-up forms a conductive path (usually at the bottom of the plates) and is accelerated by improper care.

Deep cycle batteries are more flexible on charge rates and cycling but are not designed to delivery the high discharge current necessary for engine starting (especialy in diesels) This all has to do with the thickness and spacing of the plates. Automotive have many thin plates and deep cycle fewer thick plates.

To properly hold a charge, any battery should be allowed to discharge a bit then recharge. The reason is something called "electrolytic stratifcation" where the electrolyte (typically acid) actually settles in layers of specific gravity resulting in uneven plate "wear" Charging produces gas which "stir" things up.

Lots of other factors also, but to answer your question. What I would do is use a good 3 step charger that "floats" the battery at 13.6 V or so. (remember you want a high enough bulk charge to produse some gas) Hook it up with a timer that turns it on once a week for ~2-3 hours. Once a month turn the engine over with the fuel pump disconnected until you have good oil pressure, then start it and let it warm up.

If you have a good electrical system the battery should hold it's charge without being disconnected. Keep and eye on the level (if not sealed) and only use distilled water------preferably double distilled if you can find it.

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Litton
'90 420 SEL (sold)
'72 280 SEL 4.5
'98 ML320 (for sale)
'86 560SL
'05 Jeep Grand Cherokee Ltd (offroad in style)
'87 Chevy Blazer (AZ Pin Strips)

I use battery tenders and have not had a problem. Best bet is to disconnect the battery when car will sit for long periods (don't forget to find your radio code first). Clock and computer will slowly drain the battery. An inexpensive quick disconnect is available that is basically a thumb screw that is loosened to disconnect.

Some battery tenders have a "pulse" feature that supposedly desulfonates the plates - i.e., restores battery's health. I have no ideal if this is true or just a marketing gimmick.

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1992 Black 500e
1991 Black 300te 4m
1992 Black Renntech 620sl
1979 Porsche 911sc targa

I use a unit that Sears sells $30.00 -- it is a small box with two wires that attach directly to the batt. The unit comes with a bracket and a short cord attached.

I have used these on boats, cars and tractors. I never disconect the battery -- I think the "battery tender" brand products work on the same principle.

They start to charge when first pluged in and then go into a cycle mode. I find them to work very well.

I'll second Litton's comments; I use a Guest low-output three-stage charger with no problems over a four-month or so storage period, and have for years.

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Craig Bethune

'97 SL500, 40th anniversary edition

'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)


SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)