Measuring total energy used

Charge up a cap to a known voltage. Then power this circuit from the cap for whatever time period. When done, measure the voltage left in the cap. Change the size of the cap to whatever is appropriate for the power draw and length of the test. Here’s the formula:

$Amps = Farads * frac{(V_{start} – V_{End})}{Seconds} $

The result, amps, will be the average amps for the duration of the test. Remember that the cap is measured in Farads, so don’t use microfarads instead.

The cap voltage drop during the test should be fairly small, maybe 0.1 or 0.2 volts,Women Toms Cordones Shoes. Any larger and you get other weird effects influencing your measurements.And another note: Most caps have terrible tolerances on their capacity. Calibrate first using a 1% or 0.1% resistor load instead of your micro-controller circuit. Try to have your V_Start and V_End approximately close to what you'd have for the real test to maximize accuracy. Oh, if you can avoid it, don't leave your multimeter attached for the duration of the test, only for measuring the start and end voltages.That's the best thing I could think of for measuring very low power stuff over seconds or minutes. If properly calibrated it should also be quite accurate.I've done this with a small resistor in series with the circuit and then, like you, using the area under the curve to find the total energy used,Toms Cordones. We used an oscilloscope to capture the modes of operation and just added it all up from there.To confirm our initial oscilloscope work I then rigged up a NI DAQ and some LabVIEW software to continuously measure the energy used for 10 full periods of operation. The low power sleep of the PIC caused an issue as the 1MΩ inputs on the DAQ loaded it so I buffered the DAQ with a 200TΩ input impedance electrometer (Keithley 6514A) to stop this from happening,toms red shoes (http://www.tomssale2012.com/women-toms-cordones-shoes-c-6.html).

To be honest, although the data was accurate it was not that much different from making a couple of assumptions about the sleep mode current draw and then using the scope method.

If you want the best of both you could use the scope method to measure the operating current and then see if you can get access to a source meter for the sleep mode current draw, again there will be some assumptions but nothing to drastic.

There are various implementations: an autozero op-amp + precision capacitor + reset circuit; or a V/F converter. A V/F converter is better suited for this task; it converts voltage to frequency, and you can just keep counting the pulses ad infinitum with your favorite pulse counter (most microcontrollers have one, or you can build your own discrete counter). Battery gauges typically use V/F converters internally (see TI’s bqXXXX battery gauges) but they’re hard to find standalone.