Measuring Voltage of a Charging Battery?

[baronep]

My idea was to have a battery rack which would be able to measure the voltage of batteries and have an LED which would light up next to each battery when the battery was fully charged and then have the LED blink next to the battery with the highest voltage. I read somewhere on the web that to measure the voltage of a battery while charging (because while charging you can't just measure between the battery terminals because you would be measuring the charging voltage) you subtract the voltage drop between the positive terminal and ground And between the negative terminal and ground.

Batt Voltage = (V Drop:[Pos & Ground]) - (V Drop:[Neg & Ground])

Is this in any way correct?

-Patrick