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Recently my battery has just been not holding a charge, it's the second battery to do this (albeit this battery is used (or refurbished)--cheap I know).

It's rapidly deteriorating to a point where a week ago, if I didn't drive the care within 48 hours it would be dead, now it's more like 24 hours.

I did a parasitic draw test, and upon initial contact it displays around 0.3 amps then lowers to 0.08 (I'm guessing that maybe as electrical devices in the car turn on, they transition into a power-saving mode?) the 0.08 doesn't seem to be enough to worry about, and certainly doesn't seem to be enough to drain the battery overnight.

When the car is running, the volts read 12.8~ with about 1500-2000 rpm, they go up to 13.5~ so I know the alternator is putting out power, but is that enough? Or is it reading lower because the battery is low to begin with?

I'm trying to rule out all causes before I buy a new battery. As of now I've disconnected the battery and will let it sit overnight, if it shows lower than what it was at (12.3v) prior to disconnecting, this is pretty much proof the battery is just bad rather than it being an alternator or electrical issue?

Thanks

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12.3v is actually fairly low for a coventional flooded lead-acid automotive battery expected to be at full charge. That translates to a State of Charge (SOC) around 50%.

If you don't have access to a battery load tester, the easiest thing might be to charge the battery with a regulated external charger overnight. Then disconnect the charger, turn the headlights on for two solid minutes, turn off the headlights, and check the Open Circuit Voltage (OCV) of the battery between the posts with a good voltmeter. A fully charged battery should show 12.7-12.9 volts minimum.

Much below that, I would suspect the battery. I don't know what a "refurbished" battery is, but to my knowledge there really isn't such a thing.

12.8v running idle and 13.5v at higher revs (measuring alternator voltage) output seems reasonable.

You may be able to find an auto parts store that will load test your battery for free. This test is best performed when the battery is at least 75% charged.

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  • Companies like interstate refurbish batteries. I wasn't trying to suggest 12.3v was my fully charged battery, that's just what it happened to be on when I disconnected the battery from the car. My logic being that if tomorrow I test my voltage and its 12.1v or something, I probably have a bad battery that needs replacing over an electrical issue or a bad alternator.
    – Jay
    Aug 23, 2016 at 4:46
  • @Jay this seems a reasonable assumption, but a real load test will confirm the health of the battery. OCV checks (especially after any kind of charging, external or altenator) are deceptive and hard to interpret. There's a common misconception that a 12v automotive battery is healthy if it measures 12.0v OCV. It is, in fact, dead. Probably not damaged beyond recharging, but 12.0 volts OCV is considered a "state of charge" of around 0%.
    – SteveRacer
    Aug 23, 2016 at 6:42

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