While sometimes you need a real serious ability to charge batteries, that's not the norm. (But see this link). Usually the sun is giving you at least a little something, one is careful not to run the batteries very far down in the first place without taking some action, and so forth. Bigger generators, like for instance my Hobart welder/generator really eat the gasoline, and in that case, aren't very efficient either. My first efficient generator, a Honda 6.5hp driving the DC generator geared so the engine is running at the torque peak, is also on the large side, producing around 100 amps @ 30v when running in efficient mode -- at the torque peak and throttle wide open for best net compression ratio and cleanest burn, least pollution due to unburned charge getting through the engine during valve overlap (in fact, it's over 100% volumetric efficient in this mode due to manifold tuning). But 100 amps is still a lot, when my campus needs say 20 or so when we're running computers and watching TV (with the other loads like the freezer etc running as required). So our active-house average is in the 500w range (it's one heck of a computer, this one, and on all day every day). I need those big generators sometimes when things have gotten real low, after all, the batteries are in the 1500 amp hour range, so a full charge from down takes awhile even with a big generator.
But that's way not the norm. More often is a situation where we just didn't quite make gain on the day, and tomorrow is going to be cloudy too, and we want to enter the next day AM with plenty in the batteries (microwave breakfast etc and just never have to worry about it). Thus, a smaller generator that just keeps up, or makes a little gain at night was needed, and of course, I want that efficient as well.
It turns out that Honda has made some advances in this field. What they have done is to take some of the new tech in IC engines and apply it to a tiny one (95 cc). This broadens the envelope over which its very efficient quite a bit. Better fuel control, spark control -- no longer the fixed timing magneto, better all around, and now, computer controlled. To compliment this, they use a DC generator, followed by an inverter, so the engine doesn't have to run 3600 rpm all the time to make 60hz AC, and the computer can simply throttle it up and down depending on load requirements. A 95 cc engine running in this mode uses one heck of a lot less gasoline per output than say, your average efficient 3.5kw generator at the same load, spinning full speed.
So that was a start on this project. I bought one of these Hondas (way high priced, but that's the early adopter fee) and took it to the testing lab of real life. For starters, I just used it like I did the 3.5kw AC generator - using a Trace (now Xantrex) inverter as the charger, which lets me adjust the load. However, the fancy charging scheme in the Trace, which varies its power requirements all over the map doesn't work well with the Honda in "eco-throttle mode" -- you get battling computers and a kind of divergent operation, and it's kind of stressful on the Honda to have the load going from max to zero to inbetween every few seconds.
So, (and BillF found this) I ripped apart an old industrial forklift battery charger we got surplus, because its computer stuff had failed. It had a really nice ferro-resonant transformer good for 24v battery charging with about 30 amps max output when the output voltage is low. I swiped the transformer out of it, and replaced its original diodes with some nice 200 amp shottkys, and am now testing that. It seems made to order for this job so far, quiet (when under load, which it's hardwired to be), efficient, predictable load on the input, nice taper charge on the batteries, the current goes down with rising voltage a lot more than it would with an unregulated charger. The Honda nicely throttles down when driving this, and is nearly silent, while using almost no gasoline (something like 12 hours per gallon) and everything runs nicely cool.
I'll get some more data on this as time goes on. Another nice thing about this new Honda -- it's light and can be carried with one hand, even by my wimpy self. This should make a perfect adjunct to various things I do around here that would like portable electricity -- I have an electric chain saw I like a lot better than nearly all gasoline ones (and I have a ton of experience there, FWIW -- the torque on the electric, quiet, light -- eat your heart out Stihl or Husky, no comparison). I have an electric water pump that with my truck, gets water from anyplace there is water to get into a big tank for later use, an electric string trimmer and so forth. Most of the time, I can use all those things off either the truck inverter or a long extension cord, but the saw, not so much, and you have to at least cut the wood into sizes that can be moved, even if you're going to make it firewood and split it back at the home station. So there's a need (or desire) for power in the deep woods, and this does it great.