Some hints for those wanting to check these. It is easily done inside the car at the cigarette lighter.

I tried OBC test #9 before but still prefer to measure the voltage the old sure way because my OBC for some reason measures a voltage 0.2V lower than a calibrated DMM. (=digital multi meter)

Find a digital multimeter and stick the probe into the cigarette lighter socket in the center console.

The following are the detail figures I use for my own evaluation:
With key out of ignition switch, should see 12.4V or above. 12.0 V is getting low.
With key inserted and turned to position 2 (don't start car), should see 12.0V or more. 11.6V would be too low.
Crank car, catch the lowest voltage reading during that couple of seconds. Should see 10.0V or above. 9.5V would be marginal.
The above just consider the health of battery itself.
Car running at 1500 RPM or more, should see 13.7 to 14.3V, 13.6V or below means the charging system is weak.

These are the data on my 750iL with an M70 engine, battery is 2 years old. Temperature when measured at abt. 5-10 degree Celsius.