IF one is firing up a 120Vac generator, primarily to charge a set of batteries in dire need of recharging, then I recommend one gets a charging source which can nearly max out their generator.
A pair of healthy GC-2 6v golf cart batteries in series, depleted to 50%, can easily accept 80 amps if the charging source is seeking 14.4 to 14.8 volts.
When batteries are depleted, they have little resistance and can accept huge charging currents, efficiently. When they get to the 80% range they start resisting the current and efficiency goes down, and so does the charge rate.
While one can get to ~80% quickly with a large charging source, that last ~20% is going to take about 4 hours, No matter what.
Figure an hour from 80% to 90%, another hour to 94.5% and another hour to 98.5% and maybe another hour when holding 14.5+volts to reach 100% State of charge.
If the batteries have been cycled a lot without getting back up in the 95% range, preferably higher, regularly, then the time required to go from 80% to 100% increases, and can increase significantly, Double or more!. They might even require an hour or two after this absorption stage at 15.5 to 16 volts to be restored to their full capacity( equalization charge). Again the more abused the batteries are, the longer the equalization stage can take, upto 8 hours even, before specific gravity is maximized across all cells.
At the 80 to 100% state of Charge range, the voltage should be at 14.4v or higher at 77F when brought up to this voltage at a high charge rate, high being a minimum 20 amps per 100Ah of capacity. The higher the charge rate, the faster absorption voltage is reached, and is reached at a lower state of charge. Once absorption voltage is reached, the amps required to hold absorption voltage begin to taper. Once absorption voltage is reached the flooded batteries are either gassing or will be soon.
When low and slow solar brings the battery up to say perhaps 90% State of charge over a few hours, only then might the voltage climb/hit/reach 14.4v or higher and then it takes ~2.5 hours more to reach 100% .
When batteries are sulfated, it requires less amperage to get them upto Absorption voltage, compared to the same batteries when new and unsulfated. Again this lengthens recharging timespans.
Those watching a voltmeter closely might notice a very depleted battery, being fed with a high amp charging source, will quickly rise to absorption voltage, but soon the voltage lowers again. If one has an Ammeter one might have noticed 20 amps quickly brought the voltage to 14.5v, but then, as the battery wakes up, and resists the charging current less, the electrical pressure will fall as the amperage increases to 30 or more.
Running a generator to get to 100% is wasteful of gas as that last 20% can take four or more hours.
So generally a 50 to 80% strategy is adopted by those who charge via generator. Some might go another hour to try for 90%, But in this scenario, one should be using the highest amperage charging source that their generator can power, or that their batteries can accept, if they have a listed maximum charge rate( Inexpensive AGMS say no more than 30 amps initial charge current per 100AH of capacity).
Using a powerful charging source will shorten generator run times, and allow the batteries to reach a higher state of charge when it is shut down, and thus ultimately last longer.
If one also can plug their extension cord into the grid, but only for a few hours, one also then wants a powerful charger to quickly bring the batteries upto absorption voltage, and hold them there for as long as one can hold them there, for 4 hours. So if it only takes 45 minutes to get to absorption voltage, a whole heck of a lot more charging can get done in less time.
If one has Solar, and can get to 90% charged by midday using other higher amp charging sources, then the solar will likely have no issues holding absorption voltage, and hold it long enough to either max out the specific gravity on flooded batteries, or on AGM batteries, when amperage required to maintain absorption voltage, tapers to 0.5amps per 100AH of battery capacity.
Batteries remain happiest and live the longest when All available Charging sources are employed whenever possible, to keep the batteries at as high a state of charge as possible.
When I plug in, I can charge at ~65 amps with two charging sources in parallel. This is into a 90AH high $$ AGM battery which can easily accept this much current and actually prefers high charging currents.
When I drive, my alternator has produced as much as 110 amps into this depleted group27 Northstar AGM when it was new. Now it instantly reaches 14.9v absorption voltage( too high!!) at ~85 amps and tapers from there. Despite this being well over Northstar's recommended 14.46v@77f, it does not seem to have minded this voltage. I will be modifying me Van's voltage regulator soon so I can choose my Absorption voltage.
My Solar will contribute to charging amperage of either my alternator, or my Grid powered charger, as long as the battery has not yet reached the Solar's programmed absorption voltage. The Charging source whose voltage regulator is programmed the highest will keep going when the lesser voltage ones just stop outputting anything.
YOu might notice the repeated use of the term 'Absorption Voltage'
This is the highest voltage generally allowed and the voltage at which batteries can be fully charged, in the least amount of time. Battery manufacturers list their recommended absorption voltage at 77f. Higher temps require lower Absorption voltage, colder temperatures require higher voltages.
Attaining the proper absorption voltage quickly, when one has all the time in the world to recharge, is not necessary.
When one has limited generator run times, or times to plug in, then reaching proper absorption voltage quickly AND then holding/maintaining absorption voltage will be the most effective at not only returning the required energy into the batteries, but keeping them as healthy as possible for as long as possible.
Most Automatic/ Smart charging sources DO NOT HOLD absorption voltage for long enough, or they will drop to a lower voltage, like 13.8v a second after 14.8 is reached on the charging source output. When the charging cables from charging source are long and thin the charging source might think it sees 14.4v, but the battery might only be 14v due to voltage drop on the inadequate wiring.
When the voltage drops, so does the electrical pressure and the charging amps can slow by 1/3 or more! And you might still be powering the generator putting 8 amps into a battery which could accept 30 if it was getting 14.8v at the battery terminals.
So, again, in the interest of minimizing generator run times, one should have a powerful charging source, which can seek and Hold 14.8v, to 14.4v AT THE BATTERY terminals, and hold this absorption voltage the entire time the generator is running. Almost no automatic charging source can do this.
In steps the adjustable voltage Converter/power supply.
Powermax makes Converters with an adjustable voltage which go upto 15.5v( good for EQ charging flooded batteries, and conditioning lifeline AGM!)
These can be had in 45 amps flavors, upto 100 Amps. At 60 amps they are power factor corrected( more efficient) use less AC electricity to make more available DC charging output.
http://www.bestconverter.com/Boondo...ower-ConverterCharger_p_587.html#.VpHVoLYrLUI
I would Contact Powermaxconverters.com and order directly from them over the phone and stress you want the adjustable voltage version! I had the above proprietor try and blow some smoke up my keester regarding a '100 amp' model maxing out at 74.82 amps.
These are NOT Automatic chargers. There are no charging stages, it just goes full output until absorption voltage is reached, and then it holds it until either it is unplugged, or the voltage is manually lowered
My Friend now has a 100 amp version of the powermax and i have measured ~96 amps from it, perhaps a little low due to an older distant 15 amps household circuit and low input AC voltage.
My personal plug in charger is a Meanwell RSP-500-15 adjustable voltage power supply @ ~130$ . Rated for 500 watts it will output 600. It is capable of 40 amps output and has a voltage range of 13.12 to 19.23v. But I modified mine with a better voltage adjusting potentiometer and increased ventilation and heatsinking. I also have a Wattmeter on its output that counts displays, volts, amps watts, watt hours and amp hours Amp peak, watt peak, and minimum voltage.
https://vanlivingforum.com/Thread-My-newest-electrical-toy
Side by side with that 350$ Xantrax, it will outperform it. The Xantrax would however not be able to overcharge the battery. My Meanwell would fully charge it faster to 100%, but then it would begin to overcharge, unless I either turn it off, or lower the voltage to an acceptable 'float' voltage .
In terms of quickly recharging batteries, one can also have an appropriate sized engine spinning a powerful alternator. If this alternators voltage regulator is adjustable and set to 14.4 to 15 volts, then one can quickly get a battery back upto 85% charged.
Many one wire alternatores have an internal regulator which only allows 14.0 or perhaps less. 14.4 will cause more amps to flow, and 14.8 might allow 35% more amps to flow than 14volts at 75% charged
But that last ~20 % of charging is Always going to take hours, and achieving that last 20% of charge to as near a true 100% as possible, is Key to achieving acceptable battery longevity.
Although 'Acceptable' is subjective.
Some might remember Grandpa's advice about a slow trickle charge always being best. While this is the least abusive to a discharged battery when it is left on for long enough(days), it is more important to get to as high a state of charge as possible, before the next discharge cycle begins.
So Do not fear the high amp charge rate when that next discharge cycle begins just before sundown. The depleted battery, when tricked charged , will never have enough time to even approach 80%, and then the next discharge cycle likely drops the state of charge even lower, and once a battery is hovering in the 50% charged range, it is doomed, drowning and crying for life sustaining charging amperage, all it can get until bursting full.
The clock is always ticking.
