The battery in the BMW Z3 is NOT a maintenance free battery, it needs occasional servicing. The leading cause of battery failure in the Z3 is from a lower water level, fortunately it's pretty easy to check the level and add more water.
The first step is getting access to the battery, for all but very early 1996 models the battery is in the trunk. You will have to raise the trunk liner and remove the plastic cover over the battery but then you will looking at the top of the battery.
Along the top of the battery are six plugs that unscrew from the battery. Remove the plugs but take care since you've just exposed corrosive liquid. You can look down into the battery but don't get to close (the water vapor can't be very good for your eyes).
When you look down into the battery, you will see a little metal tab in each hole. The water level in the battery should be just over this metal tab. In the picture above the battery needs water.
BMW Techs have a special water can that makes filling the battery easy and accurate, but there really isn't anything fancy about this procedure. A steady hand with a standard water pitcher or funnel would also work. Just gradually add water and rechecking the water level until it covers the metal tabs. It's best to use distilled water so no mineral deposits will be left from the evaporation of ordinary tap water.
When the battery is properly filled the tab will still be visible but obviously under water. Regular checking of the water level should greatly extended the life of the battery, especially during the hot summer months when the water will evaporate at a faster rate.
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