Originally Posted By: Mr_Accord
Originally Posted By: wrcsixeight
I do not think the voltages you are seeing at your temperatures are anything to worry about.
On a semi related note, trickle chargers are usually about 2 amps.
Most batteries contain at least 55 amp hours. So if the battery was drained dead, as which might happen when an alternator fails, putting it on a trickle charger overnight would not return it to full charge.
Obviously we do not know the state of charge of the battery when you hooked it to the charger, nor the size of the battery, nor how much of its original capacity it has remaining, so putting it on a charger of unknown output overnight, even for another night, is not an unwarranted response.
12.8 volts 6 hours after the charger was removed would indicate the battery is charged to its remaining maximum capacity or very nearly so.
12.8 volts 1 hour after the charger is removed means little. Takes a while for surface charge to dissipate, and it varies with battery temperature, battery construction, and health as to the rate at which it dissipates.
If this vehicle sees lots of short trip driving, do not rely on the alternator to bring and hold the battery to/at 100%.
If the battery was completely drained, and is older than a year or 2, do not be surprised if it fails in the not too distant future. Especially if your 12.8 is a surface charge measurement, and not a rested battery measurement.
If your battery has caps to check electrolyte level, then just make sure the battery is not using large amounts of water.
Refill with distilled h20. Not tap/well/spring or mineral water.
When the original alternator failed the battery voltage reading I have from my DVM was about 11 volts. The battery was not drained, and it sat on a charger for more than a day and when I removed it from the auto trickle charger the "full charge" light was on. The battery itself is only about 3 -4 months old so I don't think checking the electrolyte is warranted.
wrxsixeight is spot-on.
if you measured 11volts at the terminals during a period where no heavy loads were on it during the measurement, that battery has suffered at least one major deep discharge cycle. One alone won't harm it much, but habitual deep cycles on a starter battery will kill it.
good alternator output... 13-15.5 volts.
they regulate by voltage, not by current. the alternator/ecu does NOT know if the battery is full, moderate, weak, at 100% capacity or choked with sulfate.... it just puts a voltage out there with the current to maintain it, field coils always adjusting to compensate for loads, rpms, etc...
voltage set point is biased on (in anything past the '80s) temperature, or a timer that guesses at temperature, so that cold temps get a higher voltage.
you can sag at idle when the alt is not spinning fast enough to cover heavier loads, however some vehicles today will bump idle to raise rpm to compensate.
When testing a battery at rest, it's very difficult to get a Good measurement with just a voltmeter. 12.65 is full, 11.9 is drained. Surface charge will throw the numbers. Temperature influences capacity. Any loads will affect the decimal places, which count.
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