And I can relate stories of short lived lightly depleted marine group 27 batteries that saw an insuffuicient time in absorption, sulfated early and died very prematurely. My first batteries after acquiring solar reverted to float as soon as 15 minutes after absorption was achieved. Depth of discharge was rarely more than 20%/80% state of charge for that frst 13 months. One battery shorted a cell in 13 months, the other made it to 23 months, the third, delegated to engine starting only at 23 months, lasted over 7 years total, and was still easily able to start my engine when I replaced it.
If I had used a hydrometer and extended time in absorption, I would be several hundred dollars richer from back in my early days in solar, when I foolishly believed float = full and never bothered with a hydrometer.
When I first acquired a hydrometer, on a 3 week old group31 battery whose morning voltage became extremely disappointing in those 3 weeks, it revealed that when the solar controller first reverted to float, the cells registered 1.210 to 1.215, well shy of the 1.285 maximum they were eventually able to achieve when I bumped up absorption voltage, and held it longer. That battery wound up getting over 500 deep cycles before I removed it from the van, and is still alive, but no longer being deeply cycled. Mex kept indicating I was frying it, that it was being fried by the voltages I was bringing it to daily and the durations I was holding it, with 16.2v equalization charges every 14 to 20 days, yet it still has not shorted a cell, and can still deliver some unknown portion of its original 130Ah rated capacity.
An accurate 20 hour capacity test is not easy to achieve, yet this is the only way to truly know how much capacity the battery has remaining. Everything else is a guess, educated or not.
Had I not verified with a hydrometer on that 3 week old group 31 marine battery, a quality 67LB USbattery, not a wally world/Interstate special, and bumped up absorption voltage and duration at the three week mark, it likely would not have achieved even 200 deep cycles before its capacity decline to the poitn od uselessness, and had i bumped up ABSvoltage and duration at cycle number one, perhaps it might have lasted 600 deep cycles before I removed it from daily deep cycle duty.
Recharging a daily deeply cycled battery is much different than one which gets a few/ several days off in between light cycles, and was not or rarely cycled deeply.
It is a whole different ball game bringing a deeply cycled battery back every day and expecting even half the lab rated cycles from it, and no way will a premature reversion to float help it in any way. A premature reversion to float is the number one battery sulfator/capacity killer/ life shortener of lead acid batteries. Premature float will likely kill a battery faster than holding absorption voltage an hour longer than required.
And for those needlessly worried about the absorption stage not requiring the full output of their solar array, well, premature reversion to float should worry them more, as less of that excess solar wattage is being utilized, AND their batteries will likely be sulfating quicker, losing capacity faster as that prematurely initiated float stage initiated by the charge controller tells them all is well. When it is not!
A hydrometer would prove it, if it was ever employed by a curious individual on a tight budget who wanted to extend their battery longevity.
Every battery will be a bit different in how many amps it can accept at a voltage at a certain state of charge and it will change as the battery ages.
I do notice sometimes, that my AGM battery only accepts 0.2 to 0.4 amps less at 13.6 compared to 14.7 at ~ 98% charged, yet other times the difference is more than an Amp. I can also know when to expect this and it is directly related to how many partial state of charge cycles have accumulated since the last true verified full charge, and the last time it saw a high amp recharge from its most depleted state.
Also 0.4 amps at 14.7v is 5.88 watts
0.4 amps at 13.6v is 5.44 watts
Not a huge difference, on paper, but for the battery struggling to reach that true 100%, it is, and every watt the battery accepts is one step closer to that battery orgasmic goal of 100% state of charge.
Many abused batteries will NEVER revert to maximum specific gravity, no matter how long float voltage is held.
Will some batteries in the 98%+ charged range accept nearly as much wattage at 13.6v as they will at 14.7? Perhaps some will, others will require only a fraction of the amps to be held at float vs absorption, and take exponentially longer to reach full charge, and the day very well might not be long enough before the next discharge cycle begins, equalling another partial state of charge cycle.
The person who believes their solar is barely adequate for their capacity and usage, would be a fool to allow premature reversion to float hoping to not have to water the batteries as often and that the batteries will get to 100% full in the same amount of time at this lesser voltage.
Use a temperature compensated hydrometer. Test.
Hope to see 1.275+ when the solar controller reverts to float. Hope that one can program their solar controller to hold absV longer, hope the day lasts longer, as few who actually dip a hydrometer when their charge controller reverts to float will actually see specific gravity back at its maximum. Those few who actually bother to test a floating battery late afternoon and extend the absorption duration will see much improvement over when the charge controller first reverted to float a few hours earlier, and this equals opportunity wasted, opportunity to begin the next discharge not from a partial state of charge, but from a full recharge.
"lab cycle" numbers posted by battery manufacturers, like 1200 cycles to 50%, assume that each and every discharge cycle begins from a 100% state of charge, not 95%, not 98%, not 99.98%, but 100%.
How to ensure 100%? Ensure absorption voltage is held each day as long as required for specific gravity to return to maxiumum density with flooded batteries, or with AGM, when amps required to hold the proper absorption voltage taper to 0.5% of capacity or less. Determining full on an AGM cannot be done at float voltage, one needs to bring it back up to absorption voltage. if amps taper back under 0.5% of the 20 hour rated capacity, then it is full and float voltage can be re initiated
Without actual testing with the ammeter or hydrometer, one is blind and running on faith, and more likely than not, will be getting many less total cycles from their battery, and might find themselves in a tight spot financially, or not be able to find an adequately priced quality replacement battery in location X, when battery replacement simply cannot be put off any longer.
And holding batteries at too high a float voltage when fully charged does more than simply use more replaceable water, there is positive plate shedding occurring during any overcharge. the degree to which positive plates shed active material depends on battery quality, and the degree of overcharge, but how many systems actually are indeed bringing the battery to 100% before the next discharge cycle begins?
How many people actually test this? My guess is very very few.
How many people just assume and make claims with nothing to back them up, and ring the 'just fine' bell, as failure ( possible very premature) has yet to occur? A much much greater percentage.
Premature reversion to float ensures the total number of cycles is nowhere near as good as it otherwise could be if absorption voltage was held as long, and no longer, than required.
Only those who only lightly deplete their batteries, or have a big solar wattage to battery capacity ratio will ever achieve 100% regularly where float stage is a requirement for not overcharging, causing excessive water usage and positive plate shedding.
Also drying out of an AGM battery by it venting from a solar overcharge is not instant death of the battery. The main worry with AGM is when bulk charging at high amperage with an already hot battery, not holding absorption longer than required. If this was the case my AGM would be recycled long ago as I have inadvertently overnighted at 14.7v more often that I care to admit.
Instead it is over 500 deep cycles and the only indications it was not as healthy as it was when new, are slightly slower engine cranking, and the absorption stage taking much longer before amps taper to 0.5% of capacity. In terms of voltage held for Ah removed under X amount of load, well this battery is holding voltage very similar to that as cycle number one, but at 500+ deep cycles, many of them down as low as 20%, this battery's capacity has to have degraded, and without performing an accurate 20 hour capacity test, I can only guess as to the remaining capacity.
And overcharging is extremely rare when one's solar is barely adequate for their capacity/level of depletion. In such situations, such batteries would be much better off being held at absorption voltage all afternoon, unless a hydrometer dictates reversion to maximum specific gravity.
Some/most/many? solar controllers will not allow one to program duration of absorption or the amperage at which float voltage is then triggered. the easiest way around this, is bumping the float voltage upto absorption voltage whenever their batteries have little chance of achieving true 100% before the next discharge cycle begins.
The OP is worried that her electrical demands will not be met by solar alone, and that lead acid batteries are a bad choice for her because they might not ever get the true 100% recharge by sunlight alone. This might be the case, or might not as the system is still theoretical at this point, but if the solar wattage is lacking in its ability to get to and hold the proper absorption voltage, as long as required, premature reversion to float will only help destroy the batteries that much faster.
The 100% recharge applied as often as possible, is as best as one can do, other than putting a fully charged battery in a fridge held at the exact proper float voltage for that specific battery, at that temperature, and never discharging it at all. One could expect 30+ years of life from such a battery. But it will have served no purpose either.
The 100% recharge by solar, on a daily deeply cycled battery, is not going to be helped by a premature reversion to float voltage. The only people it helps are the executives at battery manufacturers. They only care to have the battery last the warranty period, and not one day longer. Premature reversion to float voltage is their best friend.
Test. Get a good hydrometer, use it regularly, and adjust settings for longer or shorter absorption stages based on what actual testing reveals, in that specific usage pattern with that specific battery, at that stage in its lifespan. If the usage pattern changes, so must the absorption duration, if one wants to achieve good very good or excellent battery longevity.
And people need not seek ultimate battery longevity, but they should know what it would take to achieve it if they chose to go this far.
Maximum lead acid battery longevity is accomplished by the true 100% recharge, As Soon as possible, as often as possible, and a prematurely initiated float stage is the enemy of this 'possible' goal, and especially so on daily deeply cycled batteries on a solar only recharge regimen with limited available solar.
I will never attempt solar, or a house bank, I'm done
