When is a battery full?
A battery is full when the voltage is at the absorption setpoint and battery current is at a very low percentage of the battery capacity. For a 100 Amp-hour lead-acid battery at 25 C, a current less than 2 Amps while voltage is about 14.4 Volts indicates that the battery is full. NOTE: Both voltage and current are needed to determine a full battery! Old batteries may never fall this low in current.
If an engine is running just to charge the battery, it is usually a good idea to shut off the engine at a current equal to 10% of the battery capacity, because current acceptance is so slow that lots of fuel will be consumed for little gain in capacity. A longer description can be found here: http://www.amplepower.com/primer/full/index.html
How NOT to tell when a battery is full. Use an Amp-hour meter that indicates the battery is minus Amp-hours.
True story from some years back: A woman calls from the Caribbean and she's hysterical. Hurricane season is fast approaching, and they can't leave because our Next Step regulator isn't charging their batteries. It could run for hours and only replace a few Amp-hours. After finally getting her to quit screaming about the dire circumstances I asked her how she knew the batteries weren't charged.
" Because" she said, "the meter reads -536".
"How big is the battery bank?", I asked.
"It's an 8D, 230 ...", her voice trailed off for a second or two and then came back even stronger, "this meter's no good!"
At last she had something right!
There are lots of Amp-hour meters out there that fail to track Amp-hours even remotely close to reality. You can tell whether you have one because it will display Amp-hours consumed preceded with a minus sign.