DuneElliot said:
And at what point is premature initiation for a float charge if you have a programmable charge controller? Are we talking percentages or levels etc? 80%, 90%, 98%?? I know this is system specific, but what is a good average?
This is difficult to quantify.
If the goal IS to end absorption prematurely, hoping the remaining daylight lasts long enough that float voltage can indeed fully charge the battery by the end of the day, Well this is a gamble.
With the inconsistency of solar, day to day, some days the gamble might pay off, and one can minimize water usage, on day X, but on day Y, fall seriously short of the goal of 100% before sundown and incur yet another partial state of charge cycle, which is not ideal, and could be far from ideal.
I Urge the use of a hydrometer. If one gets within 0.005 of maximum specific gravity at noon, and the day is sunny and forecast to remain so, and sundown is not until 8PM, then 13.2 to 13.6v float could very well achieve maximum specific gravity before the next discharge cycle begins, and reduce water usage somewhat.
If one is still 0.015 or more short of maximum SG at noon, and hopes that float voltage initiated then and there will allow the sun to finish the job, well that is much much less likely to occur. The only upside to a prematurely initiated float voltage, is using less water, the potential downside is chronic undercharging, sulfation and capacity loss and prematurely replaced batteries. Which is why I call Float a BS charging stage, at least on daily deeply cycled batteries with just enough solar, for a good sunny day 2 months each side of the summer solstice.
So while a premature reversion to float 'might' still allow a battery to achieve maximum specific gravity/100% full charge by sundown, Prematurely initiated float stage has Killed way more batteries than it has ever saved/ extended lifespans of.
The exceptions are those whose water level is not checked in time and the plates are exposed. The exposed portions of the plates rarely contribute anything ever again after exposure to air and seem to be a drag on the battery from that point.
Batteries whose plates have been exposed to air, then refilled, usually exhibit extremely poor performance afterwards. Much depends on how long the plates were exposed, and the health of the battery at the point they became exposed.
Poor performance meaning that they noticably drop voltage faster under the same discharging regimen, and also take longer to recharge( absorption takes much longer). They also cannot accept as many amps in Bulk, but raise to absorption voltage quicker. Once absorption voltage is reached, the amps required to maintain absorption voltage, start declining. Which of course slows charging even if one has a large amperage potential charger available.
I recommend only allowing float to occur once the battery has already achieved maximum SG/100% state of charge at absorption voltage. The extra water usage caused by holding the proper temperature compensated absorption voltage until 100% state of charge, is what it is, and with AGMs is not a factor.
Also 2% of capacity, meaning when a 100AH flooded battery can only accept 2 amps at 14.7v, is a ballpark amperage that one can use to know if the still healthy battery is near 100% state of charge.
One really needs to use the hydrometer to verify this, especially on an older battery. My experience with amperage based guessing on flooded batteries is often wrong, when I bothered to dip the hydrometer. I've found 2% at 14.7v on my previous flooded 31 yielded only a specific gravity of 1.235, when full on that battery is known 1.285.
This was later in the battery's useful life. A 16 volt equalization cycle was required to get specific gravity back upto 1.285, and it took hours for Sg to climb back upto the maximum, and only did so on 5 of the 6 cells. The 6th cell ( closest to ground post) has never gone above 1.265 again.
After the 16v EQ charge, 2% of capacity from that point was again much more accurate as to specific gravity being near maximum, but declines each accumulated cycle, until the next EQ charge. At least on that particular battery which was a 67LB group 31 Marine USbattery, not a 54LB group 29 wally world marine battery.
Over a period of months, dipping the hydrometer regularly, on that specific battery, I raised absorption voltage to 14.9v, and raised float voltage to 15.3v, as more of a finishing charge stage. This was the only way to get to near 1.285 on that particular battery in my usage, with my available solar. Still it needed a 16v EQ charge every 14 to 20 cycles. Kind of a PITA battery, but it did reach near 500 deep cycles, which is pretty darn good for a hard working 12v marine battery.
This battery is still not dead, lightly cycled, and will hold 12.79+ volts for days after a full charge. I bring this up as many people believe that a battery which can retain a high fully charged resting voltage is still somehow good. This battery retains full charge voltage well, but is very far from being healthy, and frankly I am surprised it has not yet shorted a cell. Any day now, i have muttered for the last year.
But voltage held under fairly loading is pretty bad when I do cycle it. It lives a pretty easy life now, and just powers LED lights and 12v computer fans in my dilapidated workshop, and is often held at 13.2 to 13.4v by my older schumacher 'smart' charger when doing so, and not discharged at all.
So, I recommend holding absorption voltage until 100% state of charge is reached, verified by a hydrometer on flooded batteries. Hoping to reduce water usage by allowing premature initiation of float could easily allow premature sulfation to set in, and this reduces battery capacity, causing deeper discharges in the same overnight discharge regimen, which just accelerates the capacity decline of the battery.
Treating a battery right, from day one/cycle one, is much better than taking a few months to figure it out, and what is 'right' when new, does not apply at cycle number 400. Absorption times usually need significant extension with age/accumulated cycles, even if recharged often to 100%. Without the extension the capacity decline accellerates.
Use a hydrometer, it is the battery polygraph. Dip it into some cells when the controller first reverts to float voltage. If less than 1.275*, then extend duration of absorption stage. One can figure out if absorption should be 3 hours, or 5 hours, or 1 hour. This number depends on so many factors that no universal average can be applied without knowing the exact variables.
*1.275 is not the maximum Specific gravity for each and every battery out there. Batteries for hot climates might only be 1.260, batteries for fridgid climates might be 1.310. Figuring out the maximum on specific battery X, is wise. Even trojan who says their SG at full charge should be 1.275 or higher, well this can vary, and will vary slightly depending on the electrolyte level in battery. A still healthy fully charged battery whose electrolyte level is just above the plates might read 1.290, but when refilled, and recharged fully for 3 cycles might only read 1.270. So electrolyte level is another factor to take into account when taking specific gravity readings.
When one variable changes drasticvally, so will the time required in absorption. If not taken into account, overcharging, or under charging is likely occurring.
A happy medium is usually good enough, and 'just fine', unless one is trying to get excellent battery longevity.
Just be suspicious of a solar controller initiating float voltage early in the day when overnight usage is heavy.
Also some solar controllers might see voltage from another charging source, like the alternator on a recently started vehicle, and drop out of bulk and absorption and go right to float.
My vehicle previous to feb '16, would seek 14.9v. When my solar was set for 14.7v. This would trigger the solar controller to only hold float voltage for the rest of the day, as some of my other settings were not programmed right. I knew it was not right, but had difficulty forcing controller back into bulk, and as my depletion levels were light, I did not think it much mattered. it did.
Now I have it set right, and can control the vehicle's voltage, and all charging systems work in conjunction to achieve 100% as often as possible. Now when my battery is full or nearly so around noon, My solar can partially, sometimes fully, power the engine's electrical demands taking ALL the load off the alternator.
Obviously the information on how to achieve 'Ideal battery longevity' can be overwhelming to a Newb who just wants electricity overnight and not wanting to kill their batteries prematurely.
Accepting 'less than Ideal' is a very good strategy too, but upto the battery owner/renter.
I just urge some basics, that the lead acid battery always wants to be 100% charged, and anything less than this ideal, is detremental, to some degree, to the battery.
When I was parked in a shady spot with electric for a few months, I turned on the converter for 6 hours every few days.
