Energy consumed(E)= W.t, (Power)(Time)=V.I.t, (voltage)(load current)(time), as power(W) is voltage multiply by load current, at no load, power and hence energy consumption would be zero, the current called no load current and power consumed, the no load loss..

What kind of load does your system supply? What is percentage change in the battery voltage at the end of the mentioned duration i.e. one second or less?!

There are DC energy meters. For example, search Alibaba, then select a suitable meter based on the current and voltage ratings of your DC load?

Dear Mohamed EL-Shimy

Thank you for the answer. It is a DC motor runs momentarily for few seconds every time when it is turned on. Since the current consumption is really low (few mA) and it has inrush current as well as no load consumption by PCB.

We have some space constraints so that we cannot simply go for a larger battery. It is necessary to find out the overall energy consumption for the entire cycle and limit the size of the battery.

Does the energy meter sensitive to such a small current? Could you recommend some standard Energy Meter for it?

We will be using Li-Po battery and its voltage profile is slopy.

Thanks in advance.

It is known that Energy [J]=Power [W] x time [sec]. Since you want to measure the consumed energy by the DC motor (whenever it is operating) then first you need to decide the dt or the quantized time. This is the sampling time for acquiring measurements (smaller the better). Then you acquire the voltage applied on the motor and the current passing through the motor. These numbers will give you the Power (Power [W] = Voltage [V] x Current [A]). Having these information you may calculate the instant energy consumption (dE= P x dt). Integrating the Energy for a time window then you get E= P*dt+Eo (this is the accumulated energy consumption of the system). The Eo is the initial energy consumption (when you start your system is 0) or the previous energy consumption from the last time window (when it is in operation). For example, the DC motor operates for 100msec. A good (this is personal opinion) sampling period is 10msec (10 times smaller; but this is something that depends on other constraints you might have from requirements). So dt=0.01 sec (or a sampling frequency of 100Hz). Acquire the current consumption (i.e. by using a shunt resistor or a coil) and applied voltage on the motor (you need to use a high input resistance amplifier like an op-amp). Then multiply the Voltage with the current and the sampling time as mentioned before (dE=V x I x dt). Add it to the previous Energy calculation E=dE+Eo (the first measurement will have Eo=0). The other ones will include the previous Energy calculations. Repeat this every sampling period (10msec for this example). I assume that you digitize the voltage and current measurements and do the calculations in a microcontroller or other digital system displaying the result on a screen or other visual module. In addition, if the load, hence the motor operation, changes then what is the fastest change that can happen during the operation? Therefore, in this case the sampling frequency must be much higher, in order to be able to detect the fast changes and include them in the calculations. If for example the fastest load change is 1msec, then the sampling time must be less that 500usec (or as a personal opinion 10 times smaller, which means 100usec). The meter I proposed can be built cheap and easily and can have small volume. In case, you want to buy one, then look in the datasheet the time response or the sampling frequency or sampling rate of the meter. The ones I know, have sampling rates of hundreds of msec (I don't know if they are adequate for your application). Hope it was helpful. Any feedback is welcome.

Well, you need an energy meter for pulse load energy measurements. Such meters are used in HV testing systems. Please search Google using measuring the energy of a pulse load', and you will find some commercial devices and techniques.

Nick Panagiotopoulos Thank for such a detailed answer.

Energy is an integration of power (W) with respect to duraion(time) as you have rightly point out as V *I *t. Thus you must acquire current series with a smallest possible sampling rate for the whole duration its in operation.

The square- sum of the current series with respect to the sampling rate with which u have acquired the signal would give you a measure of energy directly as ((I*I)* R *t) in which R and duration both are known quantities while I will be a time series array acquired by data logging.

Energy consumption of DC motor can be measured direct measurement using 3 ph energy meter.VFD power supply DC BUS connected with BATTERY or DC power source range from 250VDC to 750VDC from which the square pulse conversion using sine filter. then sine wave power supply rectified to run the dc motor. From VFD's 3 Ph supply you can connect standard 3 ph energy meter to observe the energy consumptions in month , in year so on. You can dedect 1-2% losses for VFD.