"Wayne.B" wrote in message
...
As others have mentioned a current shunt is one way to do this.

However, since you're data logger can also be voltage driven, I think
it would be cheaper and easier to build a voltage divider with a
couple of resistors in series across the compressor circuit.

Indeed: As others have mentioned a current shunt is one way to do this.
You cannot measure the current by measuring the voltage *across* the
compressor. Unles you mean that you want to deduce the current by
subtracting the voltage across the compressor from the voltage across the
battery (that's tw measurements) and divide that though resistance of the
wires. Could be done but you have a resolution problem.

Meindert