Wierd 12-volt battery behavior

[Richard Lorenz]

I have a 12-volt car battery that keeps on charging when I connect it to a 12-volt, 750 milli-amp Battery Tender. It is charging now and the battery reads 13.1 volts. It has been at this super-high voltage for about 6 hours. When I connect this Tender to any other 12-volt battery, it stops charging at a battery voltage of about 12.6 like it should.

What is there about this battery that the Tender does not know that it should stop charging. This battery will start my modern Subaru, so it is strong in that respect. Any ideas?

[René W]

13-14 volts is normal,no worries mate

[Paul Mike]

IIRC a fully charged 12 volt battery should read 14.4 to 14.6 volts

[Bill G]

What’s magic about 12.6? I agree with others that a typical charging voltage would be 13 to 14 or so. Maybe after it reaches a fully charged state and then turns off the battery on its own could be 12.6, but while it is actively being charged, definitely you are in good territory.

[Patrick L.]

How long has the charger been running ? It takes a long time sometimes, a day or more.

[Crankster]

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Originally Posted by Richard Lorenz

I have a 12-volt car battery that keeps on charging when I connect it to a 12-volt, 750 milli-amp Battery Tender. It is charging now and the battery reads 13.1 volts.

It's done charging when the steady green LED is illuminated. 13.1 volts is a "float" charge. Juuuuust high enough to maintain an already, fully charged battery.

A sealed maintenance free battery has a resting open circuit voltage of 12.80 volts @ 77° F

It has been at this super-high voltage for about 6 hours. When I connect this Tender to any other 12-volt battery, it stops charging at a battery voltage of about 12.6 like it should.

I'm not sure where you come up with this? It takes about two (2) volts above and beyond the resting voltage to fully charge a battery. This is about 14.5 volts.
There is also a temperature correction factor involved. Consequently in very cold weather charging voltages in excess of 15 volts is completely normal and expected.

What is there about this battery that the Tender does not know that it should stop charging. This battery will start my modern Subaru, so it is strong in that respect. Any ideas?

I use a Battery Tender, it is a great product. But it is far too small to efficiently fully re-charge a heavily discharged or sulfated battery. It would take a week, it contains protection circuitry to prevent excessive boiling of the electrolyte. It will automatically switch to a float charge after about 6 hours.