A real world example of a maintenance top charge. Just yesterday, after leaving the travel trailer in storage from a trip to Yellowstone in mid June, I decided to drive out to the storage yard to run the A/C unit. Always a good idea to use and run things every now and then on your rig.
Had a couple of Eu2000i Honda generators paired and warmed up, turned on the A/C in 97F temps and set the thermostat to 78 and on "low" fan, probably 10-11 amps being drawn, both on "Eco" after the intial surge to start up the compressor in normal mode.
Took a Voltage reading on the T-1275 150 ah capacity, a 2 year old used model that spent it's life in a golf cart. 12.62 volts after being top off charged and in storage for 7 weeks in summer heat. This is a particular battery that has never quite held 12.73 volts... 12.68-12.69 seems to be it's sweet spot after a top charge and 24 hours to lose the over voltage afte a charge and stablize.
Like Mena, I too, on Mex Wanderers recommendation, due to me being a dry camper and off the pedastal, bought a Mega Watt /Mean Well switch power supply rated at 30 amps and 15.5V with an adjustable pot. I tweaked my adjustment down to 15.05V, at the pot.
Hooked it up to the battery, plugged the Mega Watt in, and instant 30 amps drawn, with a bit of grunt on both generator motors. In under 10 minutes, amperage had dropped off to 8 amps, and I was showing around 15.0V at the battery, a .05V drop from the power supply. Yes, 9 minutes, and a drop to 8 amps, and I am certain, with that kind of voltage and amps, that T -1275 was topped off. Battery was off gassing slightly the whole time, electrolyte was stirred up. I am certain, based on past experiences, that with a complete disconnect and 6 or 7 weeks storage, the job, via a honda generator, even a Eu1000I would work, of top off charge was complete.
I can thank Mexicowanderer for teaching me how not to monkey around and do it right, a top off charge, without wasting time, a generator, fuel for it, or battery life, or anything else important.
Dry camping has different charge controller needs from those at an RV park with a pedestal and 120V access 24/7.
For me, for the shape and construction of my Trojan T-1275, and ratio of battery acid to lead plates, and thickness, it seems to take the force of around 15.0V to get things truly top charged. YMMV, GC-2 batteries seem to be more forgiving, due to construction and shape and volume of battery acid. 14.4V often seems to get the job done on them. Trojan does recommend 14.8V, BTW.
Another individual, who has tinkered with his group 31 battey at various top off and float voltages, has found out that, indeed, 15.3 V is what it takes to do a top off charge, and that 16.0V every 2 weeks is what it takes to prevent his SG from slipping downwards. In his case 15.0V was close, but not quite enough.
Every battery is different. There is no substitute for time in the field, taking notes, and seeing what really is going on, with a couple of basic tools. Get the device for reading your SG if you have flooded lead acid batteries. Use it. Get a digital multi meter. Also get a DC amp meter. These three instruments will tell you all you need to know, 100%, what is going on with your batteries. Use them, don't guess. Your batteries health depends on it. Especially if you are off the grid or you full time it.
Once you know what to look for, the pattern of your batteries, with a specific charger the charging procedure, time alloted etc, amps it takes, gets predictable.
For me, that Power Supply unit is the Cat's Meow, for my application, with the slight need to watch the time it's turned on. Generally speaking, if I see the amps down to about 8 amps charge rate, with my battery, she's darn close to, or fully topped off, with the V set at 15.05 and the terminals on the battery showing 14.95 to 15.0. Seems to be consistent, in summer temps, where I camp. All bets are off, come cooler weather and fall, winter or early spring temperatures. It will take more voltage to git 'er done then.
In my case 8 amps on a 150 ah battery is right around 5%. Voltage being 15.0 at the battery is telling me that since the voltage it's taking at the terminals is up, and pretty close, within .05V of what the power supply unit is putting out, and the amps dropping, that there's not much left in terms of sulfur on the plates that can be put back into battery acid solution into H2 SO4. That's what requires the force in volts, the lower the amount of acid on the plates, the more battery resistance, the less capacity the battery acid has to take on the sulfur. Electricity is used to overcome the natural chemical reaction between lead plates and sulfuric acid. There is a loss, that needs to be overcome. Voltage, at 15V, is what overcomes that. Once you get it to 100%, or close, 13.4V for maintenance, is fine. I never see that maintenance charge. My system is stored fully charged, and fully disconnected. Bulk charge is what works for me on the generator. Solar panel is for when camping. I still set up for 15.0V on the solar panel. The closer I can get to 100% charge, daily, the better. Not 90%, not 80%... I wish to get as close as possible to 100%.
A commercial grade battery, like a Trojan, is a different beast. It's a work horse. Ridden hard is fine, but don't put it away wet and dead... Top it off with a full charge. Absolutely the best thing you can do for longevity of the battery's life. It can, and it will take a hard charge, without protesting or claiming abuse. It is truly built into the design considerations. Group 24, 27, 29 and 31 deep cycle, car shaped batteries need not apply for the job.
