Auto batteries, part two: cold cranking amps and internal resistance
The tutor continues to explore auto batteries (aka car batteries). While this topic is rare during tutoring, it’s probably relevant to virtually everyone at some point….
I recall, decades ago, a friend of mine bought a new battery for his car. “It’s got 600 cold cranking amps,” he smiled. “No more doubts about starting.”
I soon learned the meaning of 600 cold cranking amps: his car started perfectly from then on. At the same time, he was impressed by the number because of its technical meaning; being a mechanic, it spoke to him in a more precise way.
Years went by: I finished my degree, then went back for (of all things) some mechanics training. Today, when events from the past pop in my head, I try to answer questions that I let go at the time.
So it is with the issue of “cold cranking amps”. The other day I suddenly realized: if a 12-volt battery is pushing 600 amps, its internal resistance must be less than 12/600=0.02Ω. The obvious question: do car batteries really have internal resistance that low?
I started searching the net. The answer was harder to find than I’d expected, but here are some numbers:
car battery resistance | link |
0.003Ω | chiefdelphi.com |
0.01Ω | tap.iop.org |
0.001Ω | furryelephant.com |
So I guess it’s true: 600 cold cranking amps – or even more – is possible, based on internal resistance alone.
I have yet to define “cold cranking amps”; I will do so in a coming post. Cheers:)
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.
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