Generally speaking, you need a converter or charger that will hold Vabs (absorption stage voltage) at a suitable level... usually in the 14.4v - 14.8v range. Most converters drop to ~13.6v for abs charging, which results in about a 70% drop in current (amps to battery). This results in MUCH longer charging times/ generator run times.
Ideally, you want to charge as fast as possible, and in most cases, it will be your generator that will limit this, as BFL pointed out, above. For example, my 430Ah bank of four 6v's could easily handle 100+ charging amps, but my Honda eu2000i (1600w/13.3a) can only handle a 60a charger (75a, if power factor corrected); staying well within its "continuous" rating.
To give you an idea of charge times, a 70-100% charge on my bank, using a 45a charger, takes about 8 1/2 hours; most of which is taken up doing the top 10%. (the 70-90% portion takes about 2 hours). A 75a charger would get the 70-90% done in about 1 1/4 hours... saving about 45 minutes of generator run time... and the top 10% would be the same as using a 45a charger. That 6 1/2 hours of getting the top 10% done is best achieved using solar, thus saving enormous amounts of generator run time.
