I'd let the battery rest an hour at zero amps with a disconnected power cable before attempting to check if the voltage is within calibration specs. If the battery is charging or discharging, you will get different voltages depending on where you touch the probes due to the non-zero resistance of the bus bars. There's a rule in the calibration world that in order to calibrate something, the measurement needs to be at least four times as accurate as the thing you're calibrating. Then there's the whole NIST traceable calibration and expiration dates stuff. If you have any doubt, I'd refrain from calibrating it.
