The BATTERY SCIENCE rules for charging a depleted 12VDC battery in a short three hours time is to hit it with 14.4VDC for two hours and then 13.6VDC for an additional hour. If your battery charge source has 20AMPS of DC CURRENT available the battery will demand most of the 20AMPS for around 15 minutes and then start tapering back as the battery starts taking on the charge.
If you have four 12VDC batteries to charge with a portable smart mode 40AMP charger like the VEC1093DBD 40AMP charger you can only re-charge two of these at time in a quick three hour run time with your generator. Then you will have to duplicate the same event for the other two batteries requiring a total of six hours or so of running the small generator.
You can re-charge all four of these at the same time but it will not get to the 90% charge state in a quick three hour time frame. Having never done this I have no idea how long it will take to get to the 90% charge state for all four 12VDC batteries being charged with only a 40AMP source of charge current...
Just something to think about...
Roy Ken
