I'm not following your math where you state that 10 minutes will give the batteries a decent charge. Assuming one is getting 100 amps (amp hours) to the batteries via a decent alternator, it seems that less than 17 amps are actually getting to the battery bank in ten minutes. Not know what size this hypothetical battery bank is, it's hard for me to believe that putting so little power in achieves much of anything.
Ok, maybe I should take that back. I was responding to "floridajakssailor". I agree getting a good charge takes a lot longer, and it all depends on how many batteries you have and what kind of alternator you have. Based on my limited knowledge of batteries it is the beginning of the charging cycle when batteries accept a charge quite rapidly, and that should be enough to keep you going for a little while, with controlled usage. I did also imply that batteries should be maintained with as full a charge as possible. When I'm living on the boat I usually charge my 5 batteries every day or every 2nd day. I try not to let them go below 12V, because as I understand when they go below 12V you are actually hurting them. On the generator my charging system goes thru 3 stages, a full charge cycle takes about 2 and 1/2 to 3 hours. I don't know the math, but I would venture to guess that on an engine alternator it would take just as long if not longer.