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Alright im at my whits end.
I have installed a NAPA battery with 850 CCA (brand new) but battery is slightly smaller than the battery housing (approx 6-7inches in length).
Now, the car will over approx 30-90 min of driving will have the volts drop (slowly) to the point where i can cycle my key a few times after a drive (without starting it) and the car will not turn over as the battery has pooched.

I have trouble shooted by charging battery and then starting car, and removing negative battery post and the car runs fine. (So altenator is doing its job)


So my question is.
Could it be the battery (its brand new) but smaller than stock.
Could it be the battery quality (as in the benz is picky and doesnt work well with the cheap napa battery)

I am lost..
 

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You need to start by testing the charging system. Then check for battery drain, lots of threads on how to test for battery drain here some just in the last couple of weeks.
 

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Hi,

Also, you should never disconnect a Battery with the Engine Running, it often does serious damage to the Alternator.

It sounds to me like the car is not Charging properly, but also, whatever, you need a decent quality Battery of the Correct Amp Rating for this Car, they are very sensitive to voltage.

HTH,

Cheers Dave
 

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If the battery drains after an hour of normal driving, it is almost certain that the charging system is not doing its job. You can do a quick'n'dirty check by:
- Carefully check condition of battery connections. Battery posts and cable terminals must be clean, bright metal. If possible, check condition of body and engine ground (earth) connections.
- Procure a decent multimeter ($15 - $30, cheaper but usable at Harbor Freight).
- Engine off, check battery voltage. Should be 12.4 - 12.7 vdc when fully charged.
- Have a helper start the engine and hold it at about 1500 rpm while you check battery voltage. Charging voltage should be 13.6 - 14.4 vdc.

If charging voltage is low, check condition of serpentine belt and tensioner. If you can find no other problem, you may need to replace or repair the alternator, but you need to check everything you can on the electrical system first, perhaps having a trusted expert do so.
 

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I'll add to this that 1 of the prefuses under the front carpet can cause it to look like the alternator is bad when it is in fact good. So if the battery does not appear to be charging check the prefuses as well.
 

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Yes, it works, at least in an '05 or later (I haven't tested it in older models). From our W220 S-Class Encyclopedia:

SIMPLE BATTERY VOLTAGE AND ALTERNATOR CHECK (later models): Before starting the engine, set the multifunction display (MFD) to show mileage (total vehicle mileage and trip miles); with the ignition key in Position 2, push the R (reset) button on the instrument cluster three times. The MFD will display "UB" and battery voltage. Leave it that way, and start the engine. After starting it will display "UB" and alternator voltage.
 
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