Calculating that power supply won't burn my laptop

These are two laptop power supplies:

(1) Output 20V 4.25A max. (2) Output 16.5V 3.65A max.

Assuming my laptop safely works with (1), I want to know if (2) would be safe to use.

The laptop input impedance is 4.70 Ohms, which means it will draw less than 3.65A from the power supply (2). So I conclude nothing will burn.

I do this by using Ohm's Law.

R=V/I - > 20/4.25 = 4.70 Ohms for power supply (1) V(2)/R(1) = 16.5 / 4.70 > 3.65A

Because the amperage is less than power supply (2), the laptop will consume less than 3.65A from the power supply, so it won't burned. The only drawback which does not constitute a hazard, would be that the laptop would get charged slower.

Could a real engineer (unlike me) confirm or correct me? Thanks!

Adelita

Note: both power supplies are from the same maker for the same line of laptops, only laptops are different models. So other variability should not be a problem.

Answer 1

[Fangjun Jiang]

Modern power supplier/charger are all "smart" devices. The power/volt/max current numbers are not derived directly from the simple Ohm's law. Your logic here is most likely invalid. You could try. The second power supplier may not even be able to charge your laptop at all. 16.5V is much less than the required 20V. It may not be able to "open the gate".

[The Finnish Rein Deer]

Could you expand on your answer? I am as a matter of fact using power supply (2) with the laptop (whose original power supply is 1). Sofar so good. I don't know how to asses what is a significantly lower voltage for a 20V & 4.25A max. laptop. Could it be that lower voltage with adequate amperage is only going to only slow down the charging process (as opposed to burning the machine)?

[Fangjun Jiang]

A better approximation is Current=(Vsupply-Vbattery)/(Rsupply+Rbattery).

Take a look at your laptop battery, the lable might read 19.5V. The actual voltage might be between 16V to 19V, depending on the state of charge.

The open circult output voltage of a power supply is usually higher than the label. Your 16.5V power supply might output 18V.

Charing your battery with a lower voltage power supply usually won't cause burn or over heat. The concern is more about whether the power supply has higher enought voltage to charge the battery. I bet you can't charge your battery with a 12V power supply. I doubt your battery will be fully charged. It's not good for the battery either. Just try to find a matching power supply.

[The Finnish Rein Deer]

Thanks for commenting. Would it be possible that the laptop may request more power than supplied by the power supply (2), and this "trying to draw" more power is damaging the laptop architechture itself? Imagine I am running Matlab on my laptop.

[Fangjun Jiang]

Extreamly unlikely in your case. Not good for your laptop battery though.