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I've owned my car since mid-2005 and have not serviced the battery because the owner's manual told me it was maintenance free. Now I've noticed that the electrolyte level is less than halfway up. Should I just add distilled water to bring the level up & buy a cheap hydrometer to check the specific gravity. Or do I need to check the specific gravity before adding any water? And what should the s.g. be? Is it possible the battery is damaged from low electrolyte level. The battery does good except on one single digit temperature day it seemed weak on first start. How do I determine the manufacture date of the battery. It is a MB Hoppecke battery & has 0322 stamped on it. Would this be 22nd week of 2003? The service history does not mention anything about the battery being replaced? Thanks for suggestions!
 

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Double check if it is really maintenance free. In such a case the battery is sealed and adding distillate water is not possible.
Never maintained any battery on Mercedes. One in ML will be 10 years old this summer.
 

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If the battery has caps that permit water to be added, then top off each cell, to the mark with distilled water. Clean the top first to ensure crud doesn't enter the battery. Ifin the plates have been exposed, then the battery will have lost capacity due to the surface changing from air exposure.

It may be that the level is low due to excessive charging. After the engine has run for a while, with the still engine running, the battery voltage should be about 13.8 VDC, for a fully charged battery.

The BEST way to quantify all the important battery characteristics is with a Midtronics battery tester.
 
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