RE: Best Way to charge gel 7a 12v on way to field?
I just measued by own 7 Ah gel cell. This is what I measured using a Hanger 9 charger (nominally 12V/600mA):
battery voltage no load: 13.4 V (it is fully charged)
charge voltage when the Hanger 9 charger is attached: 15.2 V
charge current: 300 mA
(p.s those number don't make complete sense to me...they say that batteries internal resistance is 6 ohms. higher than I would have guessed?)
So... If I connected this to 13.8 volts the current would be lower.
Repeating the excercise with a discharged battery would be more informative. (But I don't have any easy way to discharge my battery at the moment.).
If you do try the direct connect to your car, and the fuse blows, a resistor in series would limit the current. But if you're uncomfortable with it, don't sweat it. Just get the inverter.