RE: 12V Gel Batteries
I use a 9 amp gel cell for my flight box and charge it with a 1 amp / 2 amp sport charger.
I try to peek charge it as often as I can. I hook up a dc volt meter to it and watch the voltage level. When it peeks out its done, generally it drops about .3 to .4 volts and that tells me its peeked. There can be a veneration less than .3 to .4 volts when charging, that's why I go .3 to .4 drop before I cut it off.
My gel cells last a long time, so I know what I am doing is good.
sticks