Hello my friends.
This is an example question.
I have a toy firetruck. It has a motor to move, lights and a speaker for sounds.
It takes 4.5V ( 3 AA batteries ).
I want to know how much watt hours it will use if the motor and lights and speaker are continuously running for 1 hour?
I have an OScope, Multimeter and MATLAB software and pencil and paper.

Can my friends here tell me a experiment that I can perform to know how much watt hours it will use if the motor and lights and speaker are continuously running for 1 hour?

Thanks you my friends and have a good day.

 

Power is an instantaneous measurement and is measured in Watts. It is meaningless to ask how much power it uses in 1 hour. If you mean energy, which can be measured in Watt Hours, the the energy used is the integral of the power over time from 0 to 1 hour. If the power drawn is constant it is just 1 x the power. In real life, the power will probably go down over the hour depending on how much you are taxing the batteries.

 

Not really. I think the best you can do is estimate the power consumption by running a set of batteries with a known capacity down to a discharged state. Suppose your 3 batteries have a capacity of 2500 mAh (milliamp hours) and it takes 4.75 hours for them to run down to the point that there is no motion light or sound. Then,

\( 2500 \text{ mAh }/4.75\text{ h}\;=\;526.32\text{ ma} \)

I think the alternative would be to measure the instantaneous voltage and current to compute the actual power consumption. Given the on off nature of motion light and sound this will be a challenge with the instruments that you have. Additionally since we are not dealing with AC power there are no shortcuts like trying to measure the RMS values of voltage and current.

 

Power is an instantaneous measurement and is measured in Watts. It is meaningless to ask how much power it uses in 1 hour. If you mean energy, which can be measured in Watt Hours, the the energy used is the integral of the power over time from 0 to 1 hour. If the power drawn is constant it is just 1 x the power. In real life, the power will probably go down over the hour depending on how much you are taxing the batteries.

Thank you my friend. When I put a multimeter in series to calculate the current draw, it says that it is taking 0.43 Amps. So that means Instantaneous Power = 0.43 Amps X 4.5 Volts = 1.935Watts.

So my friend, how can I calculate how much Watt Hours will be used by the system, in one hour, with the above information?

 

If, and that is a big if, the current and voltage stay the same over the hour, then it will be 1.935 Watt hours.

Let's say the current and voltage gradually decline over the hour. A ball park estimate then would be:

(V0 i0 + V1 I1) / 2

Where V0 and I0 are at the start of the hour, and V1 and I1 are at the end of the hour.

If you really want to measure it. Pick an interval (say 1 minute) during which there is no noticeable change. The multiply each voltage and current, add them all up and divide by 60 (because each data point represents only 1/60th of an hour).

 

If the voltage and current remain constant for 1 hour, then you will have consumed 1.935 Watt-hours. As batteries discharge you can expect the voltage to drop. When this happens the current to the load will increase which will accelerate the discharge rate. The observed behavior will change as the batteries discharge.