it will be more than offset by the amount of fuel required to charge them.

far better off to use them down to 50 percent, charge back up to ~80-85%
and repeat the cycle for a week to 10 days, then on about the 10th day fully charge them as needed, which will require several hours run time.

the 50/80 regime is the most economical regime for a flood lead acid battery
you can step up to a higher charge rate of approx 20-25% of capacity
or around 200 amps. this allows you to get the charge cycle done in about 2 hours per day as opposed to going the top 10 or 20 every day which will likely take over 4 hours to finish.

hope that makes sense?

check Balmar, Ample power and just about any marine site and study up on this charge regime. it is the most cost effective all things considered if you have to charge with an engine driven generator.

you might also consider losing one layer of conversion, that being a gas driven gen
providing power for a charger to charge batteries

if you must use an engine, use a high capacity large frame alternator, and a balmar
mc612 or mc624 or equivalent. that will provide 3 step charge control, the ability to tailor the alternator to the available hp of the engine driving it, soft start and a bunch of other stuff.

no sense doing any more conversions than necessary in my opinion.

Bob g's advice is on the mark.
The 80/50 cycle is good, don't freak out if you discharge occasionally goes below 50%.
Deep cycle batteries are made to be discharged, 70% for a short time should not hurt anything. Now discharges near 80% or below will damage the battery and is a good time to freak out!

Lead acid battery charging efficiency is 91% when SOC is below 80%, above 80% SOC the charge efficiency begins to drop off linearly to zero at 100% SOC. The 80/50 makes good use of the efficiency of the battery for day to day operations, a full charge 90-95% SOC should be done along the lines of Bob's recommendation. Two to four times a year an equalizing charge should be done, 100% SOC. Not good to do more than 6 equalizing charges a year. Of course follow manufactures recommendations.

Sulfation is about the time spent discharged and depth of discharge.
Better to charge ASAP after battery discharged.
Therefore charge at end of day, let it sit overnight charged!
Goal, set up the charge / discharge to achieve the minimum time discharged.

Thanks! This is just the kind of practical advice I was hoping for! I'll use the charge regime y'all describe.

Dinx, the batts will be used for both 12V loads and to run inverters. Is it ok to have an inverter and a desulfator runnning on/off the batts at the same time? I'm going to be off-grid for the rest of my life if all goes well. I am going to start adding PV's as finances allow, but ATM all I have are 2*60 watt panels. On the plus side, I have as much sun at the spot as you can get in my zone (coastal, fog).

I'm going to look into converting an old genny I have to run an alternator as you suggest. I think my bro in law has one with a dead genset but good motor. If not, I have a decent honda lawnmower I might rip the engine from. I have an old truck with a
pretty fat alternator if IRC.

 Just some thoughts, running your entire system off gas is probably the most expensive way to generate electricity there is. Diesel would be a bit more efficient, propane a bit more so. Also the least amount of conversions lessens the losses in each step.

 I'm sure you've thought of a lot of ways to conserve power and energy use. Wood for heat, for example, solar for heating water. Doing a totally new setup in a new environment takes a LOT of work, so try to keep each thing as efficient as possible, that way you'll be less likely to have to change a system in the future.

I have no choice but to use the gas 5kW genny for starters. Economy. That link for making a genny from a lawnmower engine is great! He shows using motors to get 120VAC, but I'll go with an automotive alternator. Is it best to use one with an external VREG? I can see easily making an electronic VREG to substitute for the old school mechanical style.

Luckily for me I live in a little cabin in the woods now, with an outhouse and poor power, so the shift won't be to radical a change. I converted my hot tub to solar/wood heat last year, so I have some panels and practical water heating experience. I have a few tanks too. I LOVE the idea of routing the genny exhaust through a water heater tank. That should be pretty easy to do.

not sure about european trucks maybe they are 12/24 or straight 24 volt systems

the leece neville/prestolite 110-555jho alternator is a large frame 160amp unit
12 volt nominal, it uses a piggy back regulator that is easy to remove and fit a
proper 3 stage controller such as the balmar, xantrex, amplepower, hart, etc
and there are at least 2 other controllers made in england that i am aware of
that do the same job.

the 110-555jho uses large ball brgs front and rear, has brushes that are user accessable without removal of the alternator or a complete teardown, the unit is rated at 160 amps at 70 degree's but figure on ~120 continuous output hot.

they use a standard 7/8" bore pulley so you can get any size pulley you want to fit
them easy enough.

they are about 54% efficient at charging 12volts nominal, which is about as good as any alternator, but
should you decide to go with a 24 volt bank, i can get you well into the low to mid
70's efficiency with a bit of modification. i have a prototype system that is doing quite well at very good efficiency generally not seen with automotive alternators.

the beauty of an belt driven engine/alternator setup is you can dial back the rpm
when you are topping off the battery and reduce noise and fuel consumption as well as increase the longevity of the engine in most cases.

also with a bit of work, one can fab up and fit a heat exchanger to the exhaust and make domestic hot water as well, which is a very good use of waste heat that would normally go up the stack anyway,, everyone needs domestic hot water for laundry, washing dishes and a hot shower is something that is nice as well.

the last thing i would want or recommend would be a 3600rpm AC genset, screaming for hours to provide power for a charger, to charge batteries so that you can then
power inverters to power loads later. too many conversion steps and your efficiency will be dreadful even with the best of equipment.

eliminating conversion steps enable you to use less costly equipment and still get better efficiency overall.

don't get sold on "single" point efficiency, such as "our charger is 98% efficient"
(which it likely only is 98% efficient on a cold day, at a specific load, at sea level, at a specific baro pressure, if you hold your mouth right.. etc, and be well off that level of efficiency in the real world anyway)

what you should focus on is "total" system efficiency, learn what it takes to improve the total efficiency. small things sometimes can make significant improvements in total system efficiencies.

it is the total system that consumes the fuel to produce kwatts that you will consume.

and remember each watt conserved is worth a lot more than each watt generated
as ben franklin said, "a penny saved is a penny earned" in this game it likely
is "a watt conserved is worth 3 watts generated" maybe more, but likly not much less.

as for desulfators, forget them!

you got new batteries, so use them properly, charge them properly, and it is unlikely that you will ever have a need for a desulfator. There is ample evidence that they will not improve the lifespan of new batteries and some evidence that they might even be detrimental.

if you are set on useing one, use it only during your equalization charging, the rest of the time just do the standard 3 step charging, keep a log, and learn to use your temp compensated hydrometer.

good luck

The units sip fuel when in idle mode and are 3cyl units.
New the units normally start at 6Gs and go up from there. Used are had even around here less than half.

I have procured a 5.5hp Honda lawnmower engine and have ordered a bracket to mount it and an alternator from a place called Epicenter. I'm going to start with a regular alternator and use it for bulk charging. I'll add a smarter regulator as finances permit.