Car batteries and charger

I'm quickly succumbing to Analysis Paralysis.

What voltage should a 12 volt car battery read when it's "fully charged"?

Very close to 12 volts? or close to 13 volts? or 12.5? or ... ?

I'm trying to see if my battery maintainer is actually doing its job. If it goes from charging mode to maintainer mode when the battery voltage is 12.5 volts, is that "correct"?

On a related point, does a car battery charger have an internal standard that tells it to stop charging when the battery is at a certain voltage? (And if that's the case, then wouldn't different companies' chargers have different voltage cutoffs?)

Interesting and somewhat contradictory articles:

https://www.autobatteries.com/en-us/battery-testing-and-maintenance/car-battery-voltage-and-testing

https://www.batterystuff.com/kb/fre...ies-faq/12-volt-battery-reading-13-volts.html

Thanks.
Tom M.

No, 12.5 is not high enough. Nominal voltage for a fully charged car battery at 70F is 12.6. However, a "smart" charger should hold it a bit higher than that, even after finishing the charge cycle.

And normally, you'll measure a bit higher as well right after charging it. I don't know the chemistry behind it, but the effect is called "surface charge" and goes away quickly once you start to draw from the battery. If you are trying to judge state of charge from voltage, you have to apply a load briefly to bleed off the surface charge.

Battery chargers vary widely, many if not most have no voltage regulation at all. And even "smart" chargers vary widely in what algorithms they use for charging. Some, like the cheap "maintainers" from HF have just a single voltage limit. Others have 3 or 4 different stages in the charge cycle, with different parameters at each stage. I don't have my documents handy, but it can be something like charge at full rate until the voltage reaches some level; then charge at a constant low current until the voltage rises a bit more, then hold that voltage until current drops below some level. Then stop charging until the voltage drops to a much lower value and start all over again.

All of the above voltages are dependent on battery temperature and should be adjusted if warmer or colder than 20C (or whatever).

Then there are the chargers with "desulfation" cycles.

Thanks Randall. It's another of those things we often just "assume" - but don't check the details.

I've just found many "one star" ratings for many popular chargers and maintainers. We plug 'em in and walk away, not knowing what's actually happening to the battery.

Onward through the fog.
Tom M.

Also, while it's being charged, the voltage will be higher than when you remove the charger and the battery is just sitting there. Modern chargers and modern auto electrical systems raise the voltage to about 14.5V, plus or minus a bit (for one thing, it depends on temperature) while charging. These are high-current chargers, not maintainers, which might be a bit lower; I'm not sure. Older cars (like ours) are set up for about 13.5V. There is no specific value; it's a trade-off. Higher voltage carries the risk of overcharging, but minimizes the chance of sulfation.

I have a smart charger, which charges the battery to 14.5V, but it is, as its name implies, smart enough to cut off when the battery is charged. If the car hasn't been used for a week or two, I hook it up and recharge the battery. Keeping the battery well charged will help to maximize its lifetime.

Thanks Steve. Interesting about the "smart charger" which goes up to 14.5. Wouldn't that cause a problem with the older cars that are set up for about 13.5?

Tom M.

No, nothing on a car is that picky, besides the battery. Your fan might turn a bit faster, or the coil get a bit hotter, but not enough to hurt.

Besides which, the TR3 regulator is set to almost 16 volts open circuit.

16 volts. Yikes. Never knew.

Thanks.
TM