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Thread: Car lead acid battery advice

  1. #1 Car lead acid battery advice 
    New Member
    Join Date
    Mar 2012
    Hi, not sure where to post this but i thought chemistry would be the place for these batteries. My car battery is 4 years old and have been checking it out. Firstly it's charged as much as it can be , the car does long runs 600 miles a week . Lights etc are never left on so it's never run down. The terminal voltage at rest won't go over approx 12.45v. It's not reaching the 12.6 it's supposed to. I put one of them conductance testers on it that measure CCA's. The battery is rated at 570 and it reads just over 400 with the red fail light. A different tester said battery ok but recharge ( but is charged)..

    I checked with a hydrometer( green for good, white for fair and red for rercharge), the cell on the end - side reads the lowest into the white at 1225 SG, the rest are are just into the green section i think around 1250 ish.

    Does this battery need replacing? The funny thing is though i did a crank test on it one morning at around 32 degrees F. The car was lying up all night. I disconnected a relay to prevent it starting and i cranked for around 15 seconds. It cranked really good and never started to lose momentum so to me the battery seems ok. I'm just confused by the readings i'm getting with voltage etc., anyone know about these batteries on here,

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  3. #2  
    Forum Ph.D.
    Join Date
    Jan 2009
    use it till it dies, also check out

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  4. #3  
    Comet Dust Collector Moderator
    Join Date
    Mar 2011
    New Jersey, USA
    Yeah, sound like you have one cell dying. You can keep using it till it croaks, but of course, following Murphy's law, it will fail the one morning when you need it to start the most
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  5. #4  
    Forum Professor jrmonroe's Avatar
    Join Date
    Mar 2010
    Ask a friendly mechanic to test it for you (for free?). S/he will use a "battery load tester" to test it's voltage at CCA and at CCA loads. For you, we're talking 142.5 amps and 285 amps, respectively. To draw these currents, the load resistors are about 1/12 Ω and 1/24 Ω, respectively, rated at about 2,000 W and 4,000 W, respectively. It should be able to maintain a certain voltage for several seconds at each setting. A dud battery will usually fail the CCA test without any need to go onto the CCA test. A questionable battery might pass the CCA test, but fail the CCA test.

    Or you could test it yourself, with a battery load tester that you buy or borrow, the kinds that test into the 100's of amps cost much more than a new battery. More accurately, these things are battery "power" testers as explained here, which makes sense.

    Or you can get a really rough estimate by using a simple voltmeter with an analog (needle and scale) display while cranking your car as you described (best tested under cold temperatures). The suggestion to use an analog display is because the voltage will bounce around, and analogs are easier to read and estimate than digital meters, some models of which can be impossible to read because the numbers change too quickly.
    Last edited by jrmonroe; March 24th, 2012 at 07:26 AM.
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