Maximum battery charging using small generator idea

Better not abusing your generator. A 700W+ expensive variac will not raise Watts; only Volts, at the expense of current and with the variac losses insertion added.

A battery charger capable of 700 Watts would be the maximum improvement to shorten time.

 

The output will also still be AC.....

Best bet would be to use a small motor/generator set that has a simple car alternator on the driven side. Many DIYers have made such setups using old mower motors (I have one!) that can deliver 120 amps at 12V - I use two alternators.

 

And a lead acid battery cannot absorb dozens of Amperes by itself on recharging. It will 'boil' and get destroyed. Only many of them in parallel may.
Lead acid batteries are to be charged on constant voltage near 14V., not dumping countless Amperes into them. At 14V, they will take/accept what they can, not what you want.
A 120A capable alternator (1700W) does not mean a lead acid battery will absorb such current, nor a 700W capable engine would be able of turning it to full rating.

 

A car alternator is the ideal solution - built-in regulation (on modern alternators anyway...) and plenty of current when needed.
Of course, modern chargers will make your batteries last longer...

 

A car alternator is the simple solution, but not necessarily the most efficient one. More efficient would likely be to determine the peak voltage for all batteries connected in series, and the voltage off the generator he has, then use a switching buck regulator capable of the I/O voltages, and a cutoff circuit when current @ 14V reaches some low threshold value.

Of course you will also need to factor in a current limit as Externet mentioned. Consult your battery datasheets or contact the manufacturer for the peak charge current capability. You can experimentally adjust the switching regulator output voltage down to achieve this if you don't have active current sensing, but it will take disproportionately longer to charge, the lower the batteries are discharged.

Essentially you can't have it all, cheap, easy, and fast. What if you just keep the batteries charged off mains AC or solar panels instead? You do realize that 700W, isn't much power, that you can run an extension cord and put a barrier up to mitigate the noise level? You can also fit a quieter muffler on it if you accept a little power loss.

 

Efficiency is a relative concept in this situation - you could be talking about saving a coffee cup of fuel in a 12 hour charging period.... worth the effort expended?
The KISS principle applies unless you have the time, money and inclination to go the full 'hog' and nitpick the solution.
And 700W is still nearly 60A at 12V (losses not withstanding of course) so is more than enough to charge most LA batteries as quick as is safely possible.
Many deep discharge batteries will accept a 6hr charge (1/6th the total Ahr capacity) and with the average battery of 120Ahr this means only 20A but I've yet to meet anyone that discharges ANY lead-acid battery to such an extent as requires such 'hard' recharging.

 

The difference in fuel would be much more than that. The generator has variable fuel consumption based upon load. I don't see the exact model pictured on HF but a similar model claims 5 hours at 50% load and this with a 1.1 gallon tank.

A mower/etc engine, would have the throttle set high enough for the full alternator load, then stay at that throttle when the alternator regulator stops output, plus it will consume more fuel to provide the same output the whole time even if the load were the same, which it's not since the alternator is less efficient.

Whether it's worth the effort expended is hard to guess about, not knowing the application. It could be that no solution is worth the effort... there are a lot of things that one can think "wouldn't it be nice to have that" but when it gets to the point of investing the time, labor, money, it never comes to fruition.

Plus, you might not be realizing the effort. Sourcing a reliable used mower to dedicate to it, significant money or refurbishment work for a used one, modifying it, adding an alternator, determining pulley ratio experimentally along with throttle, adding a larger tank, all quite a bit of effort and expense, perhaps more than using a generator one already has and adding a switching regulator circuit. After all this is an electronics forum, no?

 

It is really painful to just be clear and elaborate on exactly what you're trying to do? This topic makes no sense if it's application based instead of only theoretical, so a solution can be found in an efficient manner.

You wrote efficiency initially but it seems that what you really meant was fast. You can't charge lead acid batteries as fast as you seem to want to, as Externet already mentioned.

 

Assuming a 3000W motor then...... (and 100% efficiency)...

@12V this equates to 250A. To pull an anchor up for, what?, 3 minutes??? A 'total' of 12.5Ahr...... (note that the 250A load won't be continuous due to heaving cable as opposed to heaving the anchor when it leaves the bottom).

(3000/12 = 250 250/(3/60) = 12.5)

a 12.5Ahr drain on a battery bank of (presumably) 120Ahr (possibly 240Ahr) is only 1/10th or less of its total capacity.

Most leisure boat batteries will charge from either an onboard charging system (mains powered) or direct from the engine. If from the engine then we're talking potentially shoving 60A into them(!) which will take around 20 minutes to recharge the batteries. Hardly an inconvenience but, initially, not that great a demand on the batteries as you would imagine.

What usually happens though is that boat owners neglect the batteries and after a few years find them discharging more than they feel happy about and look to solutions to either recharge them more often or reduce the load on them - rather than addressing the problem directly by fitting new batteries!

Not saying that this is the OP's issue but just indicating the facts as I've found them on the many boats I've attended with 'battery problems'.