Does anyone know how to measure the battery current battery voltage of a battery while it is charging? I am working on making a battery cart that automatically tells you which battery has the most charge automatically.
Patrick
With a meter? 
There is no such thing as “current of a battery” but you can measure the voltage of the battery while charging, which will give you a rough (somewhat inaccurate) estimation of the state of charge. Perhaps what you want are little LED 7-segment display panel-mount volt meters.
Hi Patrick,
How are you going to use measurements of voltage and current taken while the battery is charging to determine “which battery has the most charge”?
If you explain your idea clearly, there are folks here who have studied batteries who might be able (and willing) to assess the validity of your idea.
Here’s a thread discussing batteries that might interest you:
http://www.chiefdelphi.com/forums/showthread.php?t=98400
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
Patrick,
You might want to read this manual, it gives a lot of useful information about battery testing, and may answer your question:
http://www.crosstheroadelectronics.com/Beak%20User%20Manual_1_0.pdf
The short answer is that the influence of the charger - it is supplying a voltage in excess of the battery voltage - complicates getting meaningful data from such a measurement.
Battery chargers generally use current, which eventually drops to near zero, as their monitored parameter to determine state of charge. More specifically, the change of the charging rate is monitored (the derivative (dx/dt) of current) and when it ‘flattens’ to a specific value the battery has reached a certain charge state.
What we have done in the past is to have a momentary switch that cuts power to the chargers so that we can get an accurate reading (each battery is connected to a voltmeter and LCD display to give us voltage, but while the chargers are on you aren’t just reading the battery).
Of course, this year between the new charger on sale from AndyMark and the Cross The Road Electronics Battery Beak, we will probably just use one or both of those.
Patrick,
There are a lot of variables here that make that kind of measurement meaningless. There are a lot of charge methods that are employed that use pulsed voltage or simply unfiltered DC voltage to charge the battery. In lead acid cells, most often, chargers will supply a voltage that is above the standard terminal voltage of the battery. This higher voltage is what forces charge current into the battery as Don suggested above. Simple car chargers actually will have pulses that 3-4 volts above the maximum voltage of the battery. In your suggested method, as the charge current varies over the charge cycle, the voltage drop in the wiring will change. The greater the charge current the greater the voltage drop in the wiring. In the last part of the charge cycle, the charge current is relatively small and there will be little voltage drop in the wiring. Unfortunately, many smart chargers also switch off the charge current to measure the battery as part of the charge cycle. If your measurements were to take a reading at this point, they likely would read lower than normal voltages.
As a general rule of thumb that all teams should remember, voltage is not an indication of charge status on our batteries. A nearly dead battery with no load will measure 12 volts. The Batterybeak mentioned above will supply brief loads to make calculations for the internal resistance of the battery. Internal resistance is a good indication of battery charge.