Resistor power ratings

[peterpops]

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!

 

[FvM]

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.

 

[albbg]

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

 

[D.A.(Tony)Stewart]

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?

 

[peterpops]

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.

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?

 

[D.A.(Tony)Stewart]

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.

 

[peterpops]

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.

Thank you!