# a simple question about power measurement

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#### winglj

##### Full Member level 4 Hi, every one. I have a simple question about the power measurement of a circuit which is powered by battery. Can I just disconnect the positive wire from the battery to circuit, and use a current meter to read the current while the circuit is operation?

And should I measure the DC current or AC current? If the AC current is measured, can I use the formula P=0.5*V*I to calculate the power?

#### teteamigo In respect to mean power...

You have to sure that currente is in phase with voltage your formula. If not, you have to use the formula 0,5*V*I*cos(angle out of phase I/V).

### winglj

Points: 2

#### epp

##### Full Member level 5 winglj said:
Hi, every one. I have a simple question about the power measurement of a circuit which is powered by battery. Can I just disconnect the positive wire from the battery to circuit, and use a current meter to read the current while the circuit is operation?

And should I measure the DC current or AC current? If the AC current is measured, can I use the formula P=0.5*V*I to calculate the power?

I am working with battery powered circuits for GSM/UMTS sites. You sholud always use current clamp meter (DC range) in order to measure current while circuit is in operation. Other way is to put shunt in series and then measure voltage drop on it. AC component is usually small and is neglible but I do not know the current's values in your circuit. I am working with currents in order of 1-600 A DC.
Regards.

### winglj

Points: 2

#### winglj

##### Full Member level 4 Hi, thanks for the replys. But I am wondering if the current is distorted and not sinusoidal any more, the average current will not the same as the DC current. I am wondering whether the current meter deal with the non-sinusoidal current? Is it just take the average current or anything else? Thanks

#### teteamigo the average current will not the same as the DC current

Average of anything is always DC.

mean power:

P= 1/T ∫ {T/2,-T/2} ( voltage(t)*current(t) ) dt

You have to measure voltage & current during time e.g ampmeter & voltmeter.
this method is brute force (valid for all systems).

In other case you have to measure peak values of voltage and current ( just for linear systems) and phase displacement between voltage and current.

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