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My local car parts dealer presented me his inventory of car batteries. One 12V battery had a 'cold crank amperage' of 600amps. The other 12V battery had a 'cold crank amperage' of 585amps.

Questions:
[1] If the resistance, my truck, is constant for both batteries, how can the amperage ratings differ?
[2] What is does a battery's 'cold crank amperage' really mean?

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The amount of current that a battery produces is determined by the load, not by the label on the battery. When there's no load, there's no current. The label only tells you the maximum current that the battery can produce. –  endolith Aug 15 '11 at 21:48

3 Answers 3

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[1] If the resistance, my truck, is constant for both batteries, how can the amperage ratings differ?

Because the total resistance includes the "equivalent internal resistance" of the battery. All practical (realizable) power sources have a non-zero internal resistance.

[2] What is does a battery's 'cold crank amperage' really mean?

"Cold Cranking Amps" means the amperage that the battery can deliver at a given cold temperature. It is specified this way because car motors are harder to start at cold temperature (they require more amperage), and lead acid batteries produce less current at cold temperatures. (This is because they are using a chemical reaction to produce current.) Thus you have a combination of the two worst cases at cold temperatures. If a battery will start a car in the middle of winter in South Dakota, you can be absolutely sure it will do great in summer in Arizona.

So, a lead-acid battery at cold temperatures APPEARS, at the terminals, to have a higher internal resistance. But it is not a higher resistance that is holding back the current flow: it is the inability of the battery to produce current at the demanded rate, because of the slower chemical reaction at lower temperatures. Manufacturers compensate by designing batteries with more reaction surface area, etc. The measure of how well they did this is reflected in the CCA rating.

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It is due to non zero internal resistances being different. If you make a short circuit, the current won't be infinity but of a finite value. It is the internal resistance who limits high currents.

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You have to be careful trying to apply idealised circuit theory to real world problems. $V=IR$ is all well and good for theory but practice is a bit different.

Essentially I think the manufacturers are stating different capabilities of each battery to provide the required current when the vehicle is starting (If I have interpreted 'cold crank' correctly). The actual chemistry/engineering inside the battery is more complicated than just Ohm's law and hence two batteries may have different abilities to 'instantly' provide the required current.

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