I have an on-board battery charger that charges both my cranking battery and my trolling motor battery and I'm sure there are tons of different models out there, but my question is this:

How do you know when your batteries are fully charged? There are two lights on the charger...a red one toward the top of the unit...and a green one underneath it. Both of them come on when you plug the charger in....and stay on the entire time they're charging. It's my understanding the charger can't "overcharge" the batteries if it's left plugged in overnight or something...but shouldn't the lights go off when they've received a full charge? I've just been kind of "guessing" how long to leave it plugged in.

Thanks in Advance

what  brand is it? amps per bank? how long are you chargeing? one of those lights should go out when it charged, well on mine it does, but you can leave you charger plugged in 24/7/365  and it will not hurt the batteries, infact it will keep them at tip top shape. both of my boats have them and stay plugged in 24/7/365 . if its a small charger and your batteries are low and you don't leave it plugged in you might not be charging the batteries all the way.

It's a Charge Pro...10 amp charger.  I guess the longest I have had the charger on is probably 4 hours or something like that...I'm assuming it's a smaller size charger than a lot, but not sure.

You can see the operator's manual online at http://www.marinco.com/docs/inst/727852.pdf

When the battery approaches a full charge, the green light comes on and the charger reduces the charge to as low as 0.01 amps.