2004 X5 3.0D
My battery had to be changed after 4 years.
The voltage can be checked in the instrument cluster by keeping the "Check Control" pressed until you see "Test 01".
Then go to "Test 09" and press the odometer.
To unlock all the tests you'll have to add up the numbers of your vehicle's "serial number" which in found in test 01 and activate that number by repeatedly pressing the odometer until the correct number is displayed and then press the "Check Control".
A search on this will probably give you a better explanation of how to do it...
My old battery showed 10.something, the new one stays at 12.0 with lights on, engine off.