Don’t know why I didn’t see this thread before, but let me try to shed a bit of light on this subject.
A fully charged battery produces 2.12V per cell or 12.72 volts for a 12V battery.
It is commonly accepted that 12.6 or above is fully charged. Excluding any surface charge.
When charging a battery, there is some resistance in the battery, so a battery charger has to produce more that 12.72 volts to fully charge a battery,
This resistance is about 0.2V per cell or 1.2V for a 12V battery.
Doing the math that means to fully charge a 12V battery a charger needs to put out between 13.8 and 13.92 Volts (charging systems sometime go a bit more to accommodate voltage drops in the wiring)
Does this clear things up?
What is this surface charge you speak of? I’m not familiar.
surface charge is some extra voltage that hangs around on the battery for a bit after it has been removed from a charging source. (How’s that for an in depth technical explanation :))
Let me give you an example. let’s say you drive your car for a bit, turn it off, then open the hood and measure the voltage at the battery. We will assume that everything is OK with your car, so the battery should be fully charged and should have 12.72 V. It will probably measure 13-13.2V
WTF? I can see the post now: Rick you ignorant slut, you said 12.72 if fully charged, but my battery puts out 13.2, a full 1/2 volt over what you said, you are full of it.
To which I would reply Ah, grasshopper, you have discovered surface charge. Go turn on the headlights for 30 seconds, then turn them off, and wait a couple of seconds and remeasure, or let the battery sit for >4 hours and you will be able to read the true open circuit voltage.
With a fully charged battery, the surface charge is not too big a deal, but let’s say your battery is only about 50% charged 12.2 V. If you measure the surface charge, you might read somewhere between 12.6 and 12.72 V and therefore think the battery is fully charged.
So battery testing protocols always call for removal of surface charge before testing open circuit voltage. Also some battery testers, will detect surface charge and tell you to turn on the headlights for 30 seconds or so before testing will commence.