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Question about battery load test results


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6 replies to this topic

#1 porcupine73

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Posted 18 September 2010 - 05:24 PM

So I did a load test on the '94 Legacy battery today. It was fully charged at the time. The tester is supposed to be 100 amps. It says to load it for 10 seconds, which I did. The voltage at the end was around 9.6V. The tester gauge said this is 'weak', not even 200 CCA.

Should I replace this battery before winter then? It was in the car when I got it 3-1/2 years ago. I don't know how old it is. Thanks. :)

The tester is the harbor freight special. It gets as hot as a toaster after that 10 seconds let me tell you. But at 100amps at 12V that's 1200 watts so she's an egg cooker alright.

Old pic of tester (that's not the battery I'm testing though)
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#2 OB99W

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Posted 18 September 2010 - 06:22 PM

You might be able to determine the actual age from a date code that's usually stamped into the case.

Lead-acid batteries get sulfated as they age, especially if they've ever been fully discharged or left in a state of partial charge for any significant length of time. That can impact their capacity quite a bit. Certain charging techniques can reverse the sulfation to an extent, and recover some of the capacity.

However, if there's a substantial difference in output from one cell to the next, it's unlikely that the battery can be sufficiently rehabilitated so that it will be reliable on some frigid mid-winter morning in Buffalo. Assuming that it's not a sealed type, and you can get your hands on a battery hydrometer, compare the cells and see what the balance is. If it's not to bad, let us know and I'll post links to charging info that might improve things...

...or just buy a new one (or take your chances :)). I nursed a battery through a couple of rough winters, but it "got" me one day when I really needed it. I had enough, charged it one last time, and got a replacement.

#3 OB99W

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Posted 18 September 2010 - 06:42 PM

By the way, the real test for CCA is done at 0 deg F, and measures how much current the battery can deliver for 30 seconds while still being able to output (for a 12 volt battery) a minimum of 7.2 volts. That corresponds to 1.2 volts per cell for the typical six-cell battery.

To draw 100 amps from a "12 volt" source would imply a resistance of 0.12 ohms. With a fixed resistance, as soon as the battery voltage drops, the curent draw does as well. At 9.6 volts, a 0.12 ohm load is only going to draw 80 amps. So, of course, the tester gives an estimate of CCA, which in itself may or may not be very accurate.

#4 porcupine73

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Posted 18 September 2010 - 07:45 PM

Hi thanks for the info. It has the removable caps on the battery; I forgot to even check those. I think I have a battery hydrogemeter somewhere I'll have to look.

I might just replace the battery. I've used this load tester this way on other Subaru size batteries and they've faired quite well, like in the 400-600 range, so it doesn't make me feel good that this battery dropped so low. It's probably cheap insurance to replace it at this age.

On a side note the previous battery I think cracked, maybe when the PO smashed up the front. It leaked acid all over the place inside, which ate the ATF cooler lines last year. I neutralized it all with baking soda water but it took a lot to flush it all out.

Then the PO put the leaking battery in the cargo area. I could tell because the carpeting was all eaten up back there, then he put a plastic shoe tray over it. Just last week I finally took the cargo carpet out and hosed it down with baking soda water. It foamed pretty good for a while. Much of the carpet is eaten away in that area. Plus now it smells like burned plastic.

#5 GeneralDisorder

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Posted 18 September 2010 - 09:02 PM

I think the easiest way to use that chinese tester is to *assume* it's not very accurate but get some baseline readings from new batteries of similar size to what you typically test with it. That way you can get an idea of what the tester indicates for a good battery, and then from there decide if you can live with whatever percentage of "perfect" you read from subsequent battery tests. If you normally read 600 Amps and this one is 200..... I would probably consider a replacement...... We are talking about something that costs about as much as a cheap axle but can cause enormous amounts of frustration.

Personally I have one of those rechargeable jump start boxes - the Harbor Frieght one that's $50. It has saved the day on many, many occasions and was well worth the price. Frankly one of the better products I've bought from there - it would be nice if it were more powerful but it pretty much does the job asked of it in the mild climate of the PNW. Probably wouldn't handle midwest and east coast winter jumps though..... I have a set of 4 AWG jumper cables my pop bought in the 70's for that type of work :grin:

GD

Edited by GeneralDisorder, 18 September 2010 - 09:04 PM.


#6 Allpar Mod

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Posted 19 September 2010 - 07:24 AM

I have the same kind of load tester and it is accurate enough to give you a reasonable idea if the battery's heading south or not, at least from my experience.

Having a hydrometer is a good thing, but it will not tell you if the load capacity of the battery is impared or not.

#7 OB99W

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Posted 19 September 2010 - 07:57 AM

Having a hydrometer is a good thing, but it will not tell you if the load capacity of the battery is impared or not.

True, but hydrometer readings can tell you whether there's a chance that the battery can be salvaged by proper charging, or if one or more cells are likely beyond salvaging. Using both a load tester and hydrometer provides more insight than either alone.




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