# Evaluating total energy usage from just DC voltage and current

#### stube40

Joined Feb 3, 2010
42
I'm trying to calculate the total energy usage of a dynamic system that is powered by a bank of lead-acid batteries.

My method is simple - log current (using current clamp) and voltage across the battery then post process the logged data -

a) For each sample point, Instantaneous Power = V*I
b) Energy used by that sample point is Power * Time Period
c) Total Energy Used in log period is the sum of all energies calculated in (b)

This works OK, but is subject to quite a bit of error if there is any offset in the current clamp at 0A. If there is any noise on the current clamp and 0A is, say, measured as 0.1A, then this creates a false Power reading from V*I and this error accumulates across the entire measurement cycle, eventually creating quite a big error.

Is there a more clever way to calculate power usage that nullifies this current-offset error? I was thinking to try and measure the drop in charge of the battery box, but for most trials we do this drop is negligible and almost unmeasureable (at least, that is, based on my understanding of LA batteries)

#### John P

Joined Oct 14, 2008
2,013
Depending on how your circuit is set up, I wonder if the non-contact current measuring system is the best bet. I needed to measure current from a 10A supply, which sent the current out via 14 gauge wire. I was able to do it by measuring the voltage drop over about 3 feet of the wire--resistance is about 2.5 milliohms per foot. I think something like that is more foolproof than a Hall-effect sensor (but copper's resistance is quite temperature dependent, so you need to think about accuracy).

#### t-square

Joined Jan 30, 2011
11
have u try using a meter with a lower Amperage measurement that might help. and adjust it that way meter sets on the lowest dial possible