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Battery Testing - Load Test vs Maintain Voltage ?

Biggfoot44

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Disclaimer - This literally involves Daughter's '06 VW TDI Beatle , but the question involves general principles . ( And she previously had Ford SUV , and claims 50% driving privileges on the Ranger .)

With increasing frequency , it won't start because of low battery voltage . Battery installed 6/19 .

Since I work all the time , directed Wife to take it to Advanced Auto Parts for their free battery testing . The battery does well with the load testing , and alternator puts out plently of juice . BUT the problem persists .

This afternoon I had a window inbetween my Music Thing , and pulling out for evening shift , so I dabbled with some experementing .

Tested battery voltage before turning the key - 12.5 V . Started up , drove 15mi in 3rd , to keep rpms 3- 3.5k to maximive alternator output .

Parked and turned off , 12.65 V .

Now 7 hrs later , turned off , nothing left on, no cellphone chargers plugged in . 12.37 V .( @ 12.35 V will just barely turn over fast enough to start .

Is it Possible for a battery to be good for cranking load , but simultaneously be " Bad " for spontaneous voltage loss ?
 


adsm08

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The car has a parasitic draw.
 

RonD

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+1 ^^^ or the new battery has internal short

Another test
After battery has sat for 2 hours or so after driving, test voltage(write it down or take picture of it, then unhook either battery cable, let it sit for 2 to 8 hours and test again
If its lower then it has a shorted Cell, so self draining, it can happen to a 1 day old battery or 6 year old battery

If voltage is the same then yes you have a drain coming from the vehicle when key is off and doors are closed


You don't mention voltage engine running but it shouldn't be any lower than 13.5volts or any higher than 14.8volts
And needs to be under 14volts after 5-8 minutes of driving before shutting it off, if voltage regulator sticks/breaks, battery will get 14+ volts all the time, this "cooks" the fluid out of the battery, shortening its life
 

Biggfoot44

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Sort of charged it to 12.77 V , disconnected , then 5 hrs later is 12.57 V . Is this sign of internal battery problems needing replacement ?
 

RonD

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No, not necessarily, you have to wait a few hours after charging to get "at rest" voltage, THEN you can test it again after 4-8 hours to see if it dropped, if so it has a shorted cell inside

Battery will show higher voltage for an hour or so after charging, but it can't hold it, so it will drop

This is why you can start the engine through out the day driving errands, but after it sits overnight, battery can't start the engine, can't hold charge long enough
 

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