Resistor power ratings

Hi!

I have a question about power ratings.

This text is from www.allaboutcircuits.com

"For checking large batteries such as an automobile (12 volt nominal) lead-acid battery, this may mean a resistor with a power rating of several hundred watts."

The chapter is about battery ratings and how to test a battery.

Question:
Why do you need a resistor with a power rating of several hundred watts? If I have a 100Ohm resistor the current through it is 12/100 = 0.12A

P = I * U = 1.44W

So why does it say I need several hundred watts?

My intuition tells me I can't load a battery that big with a tiny regular through hole resistor.. but it has been wrong before..

Thanks in advance!

The requirements for a battery load check are discussed one page before. The keyword is substantial current, I think. 0.12 A won't be considered substantial for a starter battery that has to source several 100 A.

The best check for a battery's condition is a voltage measurement under load, while the battery is supplying a substantial current through a circuit. Otherwise, a simple voltmeter check across the terminals may falsely indicate a healthy battery (adequate voltage) even though the internal resistance has increased considerably. What constitutes a "substantial current" is determined by the battery's design parameters.

Click to expand...

Yes, but in your case the load seen by the battery is negligible. You can't understand its status with a mere 120 mA load. You need tens of amps that means hundred of watts. The example shown in "www.allaboutcircuits.com" is referred to a less "powerful" battery. The internal resistance of a car battery is much less

The CCA rating of a battery essentially determines the maximum current for a drop from full charge at 12.5 resting to 7.5V at a cold temp of 0'C.

Another method of estimating CCA is by measuring ESR which requires 5 to 10% of the rated CCA current.

Considering car batteries are from 500 to 1000 Amp (CCA)

Take 10% of 500A or 50A and what do you get? (50x12=600W)

Make sense now?

FvM said:

The requirements for a battery load check are discussed one page before. The keyword is substantial current, I think. 0.12 A won't be considered substantial for a starter battery that has to source several 100 A.

Click to expand...

Ahh okay, so a more proper load is for example 0.5Ω? And this small resistance would need to have a power rating of more than 288W?

That is correct. 300W is reasonable

Often wire is oil-filled for short term power handling in a canister inside battery tester or has forced air cooling MOSFETs.

the current needs to surpass the non-linear chemistry effects to get to the linear resistance bearing currents from the plates and electrolyte. specific gravity test is also important.

SunnySkyguy said:

That is correct. 300W is reasonable

Often wire is oil-filled for short term power handling in a canister inside battery tester or has forced air cooling MOSFETs.

the current needs to surpass the non-linear chemistry effects to get to the linear resistance bearing currents from the plates and electrolyte. specific gravity test is also important.

Click to expand...

Thank you!