I tell people with solar and gas chargers to use the gas for a short time (1/2 hour) if the batteries are quite low, then let solar finish the charging.
Chargers will maintain the battery voltage. If the charger voltage is set to say 14.3V, the charger will only supply the amps needed to maintain the batteries at 14.3V. If that happens to be 1 amp, then its going to charge at 1 amp.
Let the gas do the 1/2 hour bulk charge. 1st thing in the morning is most logical, after a nights use and when the surface charge has balanced out. Then the solar will top them off.
If the system is 12 or 24V, it could be worth the money to use a lawn mower engine and automotive style alternator. Common device around here. Cheap. Uses a heck of a lot less gas! Quieter too.
A small alternator, 35A?, will start charging easier.
Large alternators, 105A?, tend to stall the motor a few times before the battery volts are high enough to limit the amps, so the motor can keep going.
After 1 or 2 minutes, the charge rate is the same anyway, and 35A alternators are cheaper.
An outfit on the web sells metal plates cut to match mower engines and alternators. The site name escapes me at present. Very interesting site, although I believe much of the tech info is flawed.
I've seen it done with angle iron, door hinges and 2x6's.
Should get a few hours of charging at the same amp rate for a gallon. I've heard ~4 hours with 5/8 of a gallon. At 4 hours per day, that would save about $11 a day in gas, here.
G-
