Remember, the BMW OE issue battery is not a maintenance-free battery!
Now is also a good time to check the battery's voltage which will give you a good indication of it's condition. It's not quite as accurate as a load demand test but will still let you know if your battery is well conditioned or borderline. If you have a digital voltmeter you're all set.
Before checking the battery's fluid level hook up your digital voltmeter to the battery terminals and read the voltage. This test should be done an hour or so after the car was last driven so the battery's voltage will drop to it's "conditioned" level rather than it's "just charged-up" level. (Testing the battery too soon after charging will indicate a higher voltage than will be maintained and will give a false indication of the battery's true condition.)
100% (12.6 Volts)
75% (12.4 Volts)
50% (12.2 Volts)
25% (12.0 Volts)
0% (11.7 Volts or less)
The battery in my '99 M3 measured 12.49 volts indicating a strong, well-conditioned battery.