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Auto battery load test violates Ohms Law!

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Mosaic

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Ok guys, I am confused about the values that my BT100 CCA tester displays.

http://www.amazon.com/Schumacher-BT...BOI0/ref=sr_1_2?ie=UTF8&qid=1338663317&sr=8-2

As is typical of this direct load tester it applies a fixed .13 Ohm high wattage load (about 100A) @ a batt V of 13V and then correlates a CCA value based on the voltage drop.


Well here's the data from the voltmeter gauge markings on it:

1000CCA @ >=10.6v after 10sec load.
800CCA @ >=10.4V
600CCA @ >=10.2V
400CCA @ >=10.0V
200CCA @>= 9.6V

using V=I*R.

Now @ a 100A load, to get the 800CCA display value we have a drop of 13-10.4 = 2.6V => a battery IR of about .026 ohm. Now this might actually mean a higher Int.R as the terminal voltage of 10.4 will work out to an actual current draw of 10.4/.13 = 80Amps.
So we actually have 2.6/80 = .0325 ohm in resistance.

I cannot see how this can correlate to 800 CCA! Even based on the standard that permits the largest voltage drop to 7.2V , we have a drop of 13-7.2 = 5.8V, @ 800CCA => 5.8/800 = .00725 ohms Int Resistance MAXIMUM!.

An Int Resistance of .0325 ohm is going to give 800 x .0325 = 26V drop at 800CCA...which is impossible.:confused:

I cite this site as reference:
http://www.smartgauge.co.uk/nosurge2.html

Can anyone clear this up for me please? I was planning to make an electronic version of the load test but this is too confusing.:(
 
I don't see any reference to the bulk temperature of the battery at any point during these tests.. Without temperature being taken into account CCA can't be calculated; if those tests were done in series the cell heated each time which will change it's current sourcing ability.

A true cold cranking test for something like an automotive battery would require the ENTIRE battery to be fully thermally equalized to 0C to have any meaning. Any kind of series testing would invalidate the results.

Basically the meter based on assumption that are not taken into account during testing.

I fail to see how the topic of this thread has any relevance, this is an electro-chemical battery, not a a fixed metallic conductor.
 
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M:
You need to be looking at ΔV/ΔI at t=0 and t=n for a fixed discharge Resistance. It should be long enough not to increase the temperature of the battery much.

I have a battery tester/charger too and I believe the instructions have you manipulate a meter pointer to do the test. I haven't used it in quite some time.
 
Some more digging reveals some useful info.
First it appears that the BT 100 is giving correct info.

See here:
https://www.youtube.com/watch?v=tVHlG9qgr8k

Second, what is actually going on is the 800CCA derives an int . res. of .00275 Ohm given a 2.2 V drop (12.6V nominal) which IS in the ballpark. I guess this is all about ball park as opposed to precision, seeing that 20A +/- isn't going to change much as far as SLI CCA goes.

This site has good comparisons of int res vs CCA vs RC etc. batts with higher RCs and lower initial Int. res. end up doing better on CCA ratings. Presumably their IR does not drop off much during the CCA load testing.
**broken link removed**

Although i don't know how a 100A test can replicate an 800CCA result when thinking in terms of genuine battery capacity.
 
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I generally don't trust any battery tester that doesn't have at least 0 ga cables, weighs less than 50 pounds, and doesn't get very hot very fast while doing the testing. ;)

I have seen too many of those fancy hand held digital multimeter units with two 3 ft long 14 ga leads say a battery was good but watched first hand as they failed to start the machine they were in. :mad:

Being able to pull 100 amps for .1 seconds doesn't tell me anything useful about a battery where as pulling 500 - 1000 amps for five to ten seconds tells me all I need to know and more! :D
 
I guess the best test is to crank the engine... if it fires there is nothing wrong.
However, when it doesn't, that's when some testgear is useful ... well it implies it will be useful - that's why you bought it.
However, testing a car-battery is tricky due to the low internal resistance (although others have touched on this being an electrochem process...we take Int Res" in an attempt to simplify things. If this internal res is low, even very low, it doesn't mean that battery can crank a car - it may have low charge.

Testers are useful, within limits and I guess OP has now discovered this.
 
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