# measure power for battery calculation

Discussion in 'General Electronics Chat' started by aamirali, Aug 12, 2014.

1. ### aamirali Thread Starter Member

Feb 2, 2012
415
1
I have a battery operated circuit.

1. Circuit wake up for 100ms. Do its work & go to sleep for next 1sec.

2. In 100ms, it does many different task. one after another.
So current keeps on varying in that 100ms also.

I have to measure power of circuit so that I can calculate the battery requirement.

1. I tried to put simple ammeter in between but it couldn't measure high frequency spikes.

2. Then I put a 5 ohm resistor in power path, & check the voltage across it with oscilloscope. It shows repetitive readings in the circuit.

At any instant I can get current by dividing the voltage noted from CRO by 5ohm.

By how to consolidate the total power across one cycle.

3. Is there any low cost solution available which can measure the power drawn.

2. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,436
1,625
Since you have a scope you have all you need to do the computation.

Find the "area under the curve" for the voltage. That is just X volts times t time. Feel free to break the wave into as many segments as you see fit. Just "eyeballing" an average voltage over the time is fine too as this is going to be compared to the average energy contant of a battery.

Divide this by the sense resistor (5 ohms) to get current.

Now look at what the period of the current draw is. As you stated, it is .1S in a 1.1S interval, so the current is drawn for .1/1.1 = .090909. Call it .1

Multiply the current by the fraction and that is your average current draw. Compare that average to the battery capacity to estimate the run time.

For example, say your device draws .1 A when on. The average current is .1A * .1 = .01 A.

Given a battery with a 1AH (amp-hour) rating the life time is 1AH / .01A = 100 hours.

Last edited: Aug 12, 2014
3. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,968
1,135
You could apply the voltage to an integrator to get the average voltage, then do the maths. What magnitude voltages are you getting across the 5 Ohms?