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Premium Member 2002 SLK230K
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Discussion Starter #1
I have a problem keeping a charge in my battery. A little history; over the winter i took the battery out of the car and placed it on a trickle charger all winter. This past month i took the car out of storage for the season, ran the car at idle for an hour then took it for a short ride. One hour later the battery was dead, i took the car to a local parts store and had the charging system tested, all was good. I had the store charge the battery, went home and it was dead. i checked all cables and all looks good. Either the battery is dead (2 years old) and the test was done wrong or there is somthing draining the battery fast. Any ideas??
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Is the battery case cracked anywhere? Can you check the water levels in the cells? If so are they low?


If you fully charge it on your trickle charger while disconnected from your car, and then let it sit off the charger overnight (and not connected to your car either), then reconnect it to the car and check to see if it is dead then or not. That should isolate whether your car is draining it fast or if the battery won't hold the charge.
 

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Founding Member #2 2008 SLK55 AMG
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Wasn't there a post on how to find out if some on-board device is drawing too much from the battery?

IIRC (really fuzzy memory), it was something simple: a DC light bulb placed between the negative post of battery to the negative ground/chassis. Or use an amp meter. If there is an usual power draw, then pull fuse one at a time to isolate the device that's drawing too much power. I cannot recall the milli-amp allowed when all devices are asleep. (Please correct me or post a link to that post, when found)
 
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Premium Member 2002 SLK230K
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17,144 Posts
Discussion Starter #4
Is the battery case cracked anywhere? Can you check the water levels in the cells? If so are they low?


If you fully charge it on your trickle charger while disconnected from your car, and then let it sit off the charger overnight (and not connected to your car either), then reconnect it to the car and check to see if it is dead then or not. That should isolate whether your car is draining it fast or if the battery won't hold the charge.
No cracks in the case and the water level is good. I will try charging the battery and let it sit overnight. When they checked the battery they set the machine for 1200 cold cranking amps, that sounds wrong 1200 should be for a deep cycle battery.
 

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Premium Member 2002 SLK230K
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17,144 Posts
Discussion Starter #5
Here's a thought, what if i jump the car then pull the battery terminals while its running? if it stays running then i know the alt. is working, if it dies then the alt is bad. Do you think that is a valid test?
 

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Don - Founding Member #4
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Here's a thought, what if i jump the car then pull the battery terminals while its running? if it stays running then i know the alt. is working, if it dies then the alt is bad. Do you think that is a valid test?
:td:Wouldn't recommend this.
 

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Sadly Woolly has passed away
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Here's a thought, what if i jump the car then pull the battery terminals while its running? if it stays running then i know the alt. is working, if it dies then the alt is bad. Do you think that is a valid test?
If the alternator is working fine, then disconnecting the battery whilst the engine is running will almost certainly ensure the alternator will not work :eek:

Wasn't there a post on how to find out if some on-board device is drawing too much from the battery?

IIRC (really fuzzy memory), it was something simple: a DC light bulb placed between the negative post of battery to the negative ground/chassis. Or use an amp meter. If there is an usual power draw, then pull fuse one at a time to isolate the device that's drawing too much power. I cannot recall the milli-amp allowed when all devices are asleep. (Please correct me or post a link to that post, when found)
Get hold of a cheapy multimeter. Set the range to 20VDC. put the probes across the battery - when the engine is off, it should read about 12.7V - slightly less than that means your battery is flat, a lot less means your battery is toast. Turn the engine on and re-measure the voltage - it should be around 14.5V - this means the alternator is working, get someone to rev the engine whilst you are measuring. If it stays about the same, it means the voltage regulator is working.

To check current draw whilst the car is turned off, do it in two stages because cheapy multimeters cannot handle high current.

Firstly, with the key out, disconnect the POSITIVE terminal (+ or red) and connect a 12v bulb(not an LED bulb) between the +ve terminal of the battery and the+ve lead (that you have just disconnected). If it glows brightly, and everything is turned off, then something is seriously wrong, and you'll need to see your dealer. If the bulb doesn't glow, set your multimeter on 'current' or amps and take a reading and report back.
 
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