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Old 10-05-2009, 01:38 AM
Strife's Avatar
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Join Date: Apr 2004
Location: KY USA
Posts: 2,238
How Long Should a Battery Last in the Trunk?

With no serious heat exposure, and tremendous capacity, the colossal battery in a US 107 should last a long time. But "how long"? Mine are at least four and five years old, and still seem OK...any experience?
86 560SL
With homebrew first gear start!
85 380SL
Daily Driver Project
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Old 10-05-2009, 08:24 AM
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Location: Ocean Isle Beach, NC
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The Mercedes battery in mine lasted nine years.

'59 220S Cabriolet-SOLD and living happily in Malta
'83 240D 351,500 miles original owner-SOLD
'88 560SL 41,000 miles - being parted out
'99 E300 turbo 219,500 miles
'03 SLK320 40,000 miles - gave to my daughter
'14 Smart electric coupe 19,000 miles
'14 Smart electric cabriolet 13,500 miles
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Old 10-05-2009, 06:18 PM
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Join Date: Jun 2009
Posts: 111
Here is a good indicator of how long your battery will last in the trunk. Take a multimeter and set it to DC. Test the Alternator. Test the battery.
Lets say the alternator reads 14.5
The battery reads 14.5 No voltage drop between the two points.
If there is voltage drop then there is a problem and your battery and alternator could be at risk. Your alternator because it has to work harder because the voltage regulator is commanding it to work harder.
If there is a voltage drop then there is more than likely corrosion.

The upside is no heat in the summertime.
Battery in the trunk makes some or no difference and could last longer because there is less heat to deal with because no engine compartment.
If you live in Minnissota the electricity has to travel alonger distance to reach the starter at 20 below zero.

Nine years is a pretty good life for a battery. What kills off batteries? Not using them. An example is storing the car for long periods of time without trickle charging, (sulfation). A two amp trickle charger will maintain the battery just fine for storage. Corrosion at the terminals, Heavy wires, Alternator.
If your battery happens to die because you were shining bugs with the engine off. Trickle charge the battery right back up when you get back home even if you jump started to get home.
There is a myth that once the battery is fully discharged the life of the battery is shortened. I disagree to a large extent if you know what to do.
Socking a battery with frequent jump starts will shorten the life. Shoving heavy amperage into lead plates causes small particles to flake off.
Good maintainence involves baking soda, water and a toothbrush. Clean them off until there is no reaction left.
Because there is a battery in the trunk place a copper penny between the terminals. Replace the penny as often as needed. The copper penny will take the beating before your terminals will.
Inspect your heavy wiring where they meet at the terminals. Don't let the corrosion start crawling along and inside the insulation.
Napa has battery cleaner in the can and corrosion guard in a can. Two wonderful preventers.
On another note. The older your battery gets the quicker the terminals will corrode, much quicker. It is a fair indicator that you are on borrowed time.
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Old 10-05-2009, 07:07 PM
Wodnek's Avatar
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Location: Southeast Wisconsin
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The penny trick works well, but you have to find a penny minted before 1982. After that they have very little copper, like less than 1%.
1959 Gravely LI, 1963 Gravely L8, 1973 Gravely C12
1982 380SL
1978 450 SEL 6.9 euro restoration at 63% and climbing
1987 300 D
2005 CDI European Delivery
2006 CDI Handed down to daughter
2007 GL CDI. Wifes

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Old 10-06-2009, 07:07 AM
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Its the anode principle. In the marine world (water wise) there are zinc anodes that take the beating instead of the batteries and the wiring. Simply placing a copper penny between the terminals is well within the electrical field of the two terminals.
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Old 10-07-2009, 02:09 PM
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Just replaced mine this year. The previous battery lasted nine years.
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Old 10-07-2009, 03:47 PM
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Location: Southold, NY
Posts: 257
The original battery in my '85 380SL lasted for 15 years. The replacement battery is going on 9 years and it was stripped out of a wrecked M-B that my FD was using for Hurst Tool practice. 15 was almost a record for me as I got 16 years out of a Sears 36 battery in another vehicle. Boat batteries are not pushed as hard - I replace the older one of the pair every three years thus never having a battery older than six years in service, since I do not want to chance a battery failure on the water.
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Old 10-07-2009, 07:27 PM
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I thought I was doing good with eight years on a battery. Fifteen years? Holy smoke!!
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Old 10-08-2009, 01:20 AM
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Join Date: Oct 2008
Location: Phoenix, AZ
Posts: 349
Heat is the most serious aggravating condition to shorten battery life. It speeds up the chemical reaction that puts an insulating lead sulfate coating on the plates. Disuse is neck-in-neck for troublemaking, letting a "slime" coating form on the plates.

Here in Phoenix batteries are lucky to last 3 to 5 years. I usually get a 60 month battery and end up having a replacement prorated out around 40 to 50 months.

I did have one episode of great comedy with a Benz battery though...A parts monkey tried to pick up the old battery when I exchanged for a new one. He wasn't expecting it to be so heavy. His upper body moved, The Benz battery didn't. He darn near kissed the thing.

Remember, it's not just a car. It's an Adventure.
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Old 10-09-2009, 09:33 AM
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Join Date: Jun 2009
Posts: 111
True to form last night after working on my Mercedes R107 I left the keys in and the ignition on.
A little fact finding for you. Never believe the what the charger settings say. Never believe what the DC amperage output is until you hook up a an inductive DC ammeter to the charger cable.
the end result is that:
This charger is only putting out 10 amps and for a heavy duty battery like this one, its going to be a while to take a full charge.
DC Voltage is a great indicator of how well your battery is doing but the load is measured in amperage.
Amperage is like a savings account. What you take out is what you have to put back in.
Voltage is pressure and Amperage is capacity. The number of electrons that travel along a wire.
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