Power (W) vs. current (I) - please explain the relations

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ArminVanBuuren

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My vacuum cleaner states "1600 W". What does it mean? Is this figure related to current (amperage) it draws from the socket? Is there an equation describing relations between the wattage and current?

Thanks
 

So the device comsumes 7 A? :-O (Voltage in the socket in the Czech Republic is 230 V).

For electricity we pay in kW hours ... does this mean that if this device runs for an hour, the price for this will be 1,6 (1600 Watts) times the price for kWh unit?

Thanks
 

Yes, or maybe less depending on how much power the vacuum is actually consuming. Maybe it only consumes 900W when running on hard wood floors, maybe it draws the full 1600 when on a thick carpet.
 

P = V*I, so you are correct, 1600W equals 7A from a 230Vac mains. Note that cos(phi) issues are ignored.

kWh = time(hours)*power(kW)

So if you run your 1.6 kW cleaner for 2 hours, it consumes 3.2 kWh. So if 1 kWh costs 0.15 Euro, you pay 0.48 Euro.

As j33pn mentioned, 1600W is the maximum power, it will be less in real world.
 

It makes sense, thanks.

One more thing. I want to buy this battery charger. If I want to calculate the total number of kW hours required for full charge of 4x 3400 mAh accumulators, do I take the output values, or input? I want to know, how much would I pay for the 4x 18650 cells full charge.

Thank you.
 

The Intellicharger does not have North American CSA and UL certification.
It does not have an input power spec so you cannot calculate its electricity cost. You also do not know how deeply the battery was discharged.
 

I don't know the losses in the charger and how it charges the batteries. Both affect the overall efficiency

I assume 50% overall efficiency. so 2 J input results in 1 J stored energy in the batteries.

Total required energy 2*4*3.4Ah*1.2V = 32.64 Wh = 0.033 kWh.

The factor 2 is for the guessed efficiency.
 

If power is written in VA units than P=VI is Ok but if in watts than it is actual power and should be calculated as P=VICos(phi).
 

You are right.

Instantaneous power = V(t)*I(t).

Average power = (1/T)*integral(V(t)*I(t)*dt)

For pure sinusoidal waveforms this will be P = Veff*Ieff*cos(phi) [W]
Pblind (or Preactive) = Veff*Ieff*sin(phi) [VAr]
Papparant = Veff*Ieff [VA]

When phi = 90 (think of a pure inductor or capacitor), energy is delivered back and forth, but the average over a time period is zero, hence the power is zero, but there is AC current flowing. For sizing of conductors (or fuses), the VA rating of a device is of importance.

For non-sinusoidal current (such as in a PC supply without power factor correction), the situation becomes different as harmonics will increase the RMS value, but without carrying real power. You meas search for "crest factor" and "power factor".
 

In Canada everything electrical must have CSA and in United States everything electrical must have UL certification to prove a product is safe.
This charger does not have the certifications so it might not be safe.
 

Audioguru said:
In Canada everything electrical must have CSA and in United States everything electrical must have UL certification to prove a product is safe.
This charger does not have the certifications so it might not be safe.
Click to expand...

OK, I didnt know that. I am not going to buy that one, then. Would you recommend something similar? I really liked the opportunity of charging 4x 18650 accus simultaneously; then stopping when full (you can easily do this through night during your sleep)...

Thanks
 

Lithium is a dangerous VERY active metal. The parcel post and some couriers refuse to carry them. All the Lithium batteries I have seen have a warning label, "Never leave charging batteries unattended". So do not sleep when they are charging.
 


You can buy a kind of electricity meter, you plug it in the socket and in that meter you plug in your charger.
Now you can read how much your charger consumes. Also, in some of these 'meters' you can insert your electricity cost so it can calculate it for you.
 

Audioguru said:
Lithium is a dangerous VERY active metal. The parcel post and some couriers refuse to carry them. All the Lithium batteries I have seen have a warning label, "Never leave charging batteries unattended". So do not sleep when they are charging.
Click to expand...

I would worry about this with some cheap Chinese batteries. With HQ ones, I would not worry about leaving them unattended (while sleeping). What do you think can happen? They can catch fire or sth? I doubt this has ever happened.


How precise the device is? Is it only approximate, or really precise? Also, how much it consumes itself, is not it too expensive to measure the current?

Thanks
 

The accuracy of consumer market energy/power meters at very low power levels varies. So you have to check the accuracy at least with known resistive and reactive loads.

Why do you want to measure/know the exact energy consumption?

Regarding charging of batteries. Make sure you charge them at a place where fire can't spread to a size you can't control. I know from limited experience that batteries may burst/leak, etc
 


I would like to know how much 4x 18650 full charge of 3400 mAh costs, in order not to potentially waste too much. If I know that 1 'shot' costs eg. 1 €, then I'm going to save a lot. If it's for only 0.01 €, then I can use the light very often.

Do you know how the process of 'starting fire' from charging batteries starts? Because the time during my sleep is almost the only one that I can at least be in the room for the continuous hours of the entire charging time. If I use an extension cable and but the charger very near to my head, do you think it is safe? In case of some problems, first a small fire would appear? Or it would blast enormously?

Thanks
 

Some Lithium batteries are fakes and quickly become over-charged which causes an explosion or fire.
The protection circuit inside some batteries can also be fakes.
Some cheap batteries catch on fire anyway.
The charger parts might be cheap then fail causing a fire.
 


overheating lithium batteries will make them more prone to rupture as it may causes the electrolyte to leak out, which is very flammable. if the film that separates the anode and cathode fails you'll end up with a battery that heats up quickly causing the cell to rupture, which when it catches fire will cause any cell next to that one to also go up in flames.

I would be most worried about stopping the fire quickly. http://avstop.com/news/lithium_batteries.htm based on this recommendation I would keep a bucket of water handy between you and the batteries. I would also probably want to charge it on something like a pizza stone. Hmmm, maybe I should move some of my chargers off my wood desk. :-S

Regards
 

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