Best Charging Practices for FLA Batteries

MrM1 · August 2018

I am now a year into this Solar Beast I have created. Having it about where I want it and seeing it preform as expected, I am now settling into how I want to use the system long term, what my goals are and how long I want batteries to last long term (that is, how strong do I want them when I really need them). My primary goal is power during an outage and energy independence/self-sustaining if needed.

The MidNite Solar Classic CCs have a feature that allows the user to skip days between charge cycles called "Days Between Bulk". I could see where this might be useful in reducing the number of cycles on the battery if SOC permitted. Then I got to wondering, which is actually better for longer battery life?

Would it be better to skip a day if my daily discharge is 15% DOD and charge every other day? This would give an SOC of 70% between charges but the battery would sit uncharged for a longer period of time.
Or does it do more damage to allow the battery to sit for 2 days uncharged and thus better to charge daily from 85% SOC?

I could see where the first option would decrease the number of charges, but the second option would allow the batteries to be fully charged daily. The second option would also be shallower discharge/charge cycles, so it might be a wash between the two as it relates to battery life and longevity.

I am at this crossroad and interested in your thoughts.

MrM1 · August 2018

The batteries I have are the Trojan L16H-AC. Here is the Cycle Life chart for this battery (red line)

Image: https://us.v-cdn.net/6024911/uploads/editor/nv/g0a0jqmeyjmo.png

mcgivor · August 2018

There is a fine line between depth of discharge and cycle expectancy, some are under the false impression that if they cycle to a very shallow level, say 5% of total capacity, the life expectancy will be greatly enhanced, this actually not the case, shallow dischargeing has detrimental effects, which is why the percentage in the graph above starts at 20%. The truth is if you're not using them you're loosing them, remembering that all/most of the projections are based on ideal conditions, specifically temperature, elevated temperature, can reduce the cycle count significantly. The calculation would have to include many factors, making it quite complexed, but there might be some merit to slipping a day, since every discharge/recharge is considered a cycle, no matter how shallow. I'm not attempting to provide a definitive answer as there is much contraversy revolving around the subject, the attached pdf has some very useful information and we'll worth the read.

Estragon · August 2018

I have skip days set at 4 days, and may extend that to 7 or more for winter when the cabin is used less often. When occupied, DOD is typically similar to yours -~15%/day, and the classic will do a full absorb cycle. When unoccupied, skip days will only float, easily replacing self-discharge at float voltage.

IMHO, it's better to recharge from ~80-85% SOC than to deliberately discharge further. I think Mcgivor's correct that batteries will eventually die of old age anyway, so having a massively oversized bank relative to loads makes little economic sense, but I don't think shallow cycling per se shortens life. A car battery (admittedly different construction, but similar chemistry), for example, spends almost its entire existence shallow cycling.

BB. · August 2018

According to the chart above (and what I have seen for published for other batteries)... Cycling 20% every day vs cycling 40% every 2 days--The batteries will last exactly the same (3,000 20% cycles = 3,000 days; 1,500 40% cycles every 2 days = 3,000 days).

It appears to me that cycling the (flooded cell lead acid) batteries (actively charging or discharging) is better than letting the batteries set for 1 or more days at ~75% state of charge or less (i.e., active discharge for 2 days then charging is better than discharging 25% or more one day, then letting sit (no charge/discharge) for 2 days or more after 1 day of discharging). It is the batteries sitting (no charge/discharge) at less than ~75% state of charge when sulfate crystals start to form (hardening of sulfate into black crystals).

-Bill

MrM1 · August 2018

Thanks all. Good thoughts. My biggest issue with skipping days and the Classic is that when set at the recommended 27v float, if I discharge to 85% +/- SOC my loads through the day are so small that even in float I charge back up fully or close.
I have gone to setting float volts low at 26.3v so the batteries at the least doesn't go all the way back up.
But what are the downside sides of partial charging on skip days?

Estragon · August 2018

The conventional wisdom seems to be that you're in good shape if you can get into the 90's % every day or two, and to 100% weekly-ish. Cool/cold batteries can go longer than warm/hot.

My L16s float at 13v/52v.

BB. · August 2018

I think 100% is really close to equalization which is hard on batteries and should be done once a month or so.

Bill

Estragon · August 2018

I'm considering "full" as being when current at absorb voltage drops to ~2% of C. Not really 100% full, but it's what my various meters and devices call full.

BB. · August 2018

90%+ a couple times a week to once a week seems to be ok.

Equalizing (gassing) once a month seems to be recommended especially for tall cased batteries.

Bill

MrM1 · August 2018

Wow you have told me some stuff I have never heard. I have been pushing to a true SG 1.277 100% almost daily. But if I set any of my chargers to 2‰ of C my SG is only about 1.260 at the end of absorb.

Estragon · August 2018

With new batteries, it may be closer to 1%, but the general idea is the same. I EQ about every 6-8 weeks in summer. With the end amps I have set, there's some mild gassing in absorb.

mcgivor · August 2018

Hoppecke states to immediately fully recharge after discharge to prevent sulfation, then again the recommend a DOD of 50% cyclical for thier solar range of batteries, this however leaves little in the way of reserve should there be a couple of days of incliment weather. Reduced SG means the sulphur is not in solution, the only place it could be is on the plates, I doubt there is any single setting for all situations, each needs to be evaluated, the leading cause of failure is undercharging so my personal choice is to avoid the most common problem.

Vic · August 2018

The Classic-charged battery banks here are usually set to Skip one or more days, depending upon the expected DOD per day.

There is significant recharge of these batteries on Skip Days, but very little heating of the batteries, as Vflt is well Gassing voltage. Have also tried setting Vflt up to 2 volts below the recommended Vflt to try to minimize the amount or recharge on Skip Days, but considered this to be not such a good idea.

We are presently using an early Surrette-recommended Vflt of 52.4 V. This is a voltage that will keep a fully-charged battery, fully-charged ... barely.

Some days, the SOC is a bit above 90% at the beginning of the day. This, plus the reduced battery heating by skipping a day or so, seems (to me) to be easier on the batteries.

IMO, Vic

MrM1 · August 2018

Thanks again for such timely responses. I have a lot of things to consider now. I think I may set the CC to do 2 skip days and set the Vflt to just above Vic's 52.4 (which would be 26.2v on my system and is .1v lower than what i have now). Then I will set the absorb time to 9 hours, BUT set the end amp to 1% of C and set Absorb volts to 31v (with temp comp it usually charges at about 30.6v and we have been so cloudy in the afternoon lately that it hardly every sustains those hi Vabs for very long). I did Vabs at 31v today and made it to 100% (no idea about the SG) in a heavily clouded and afternoon rain day and I was down at 75% SOC this morning after skipping yesterday. So I think I am on the right track. The batteries are really way cooler on the skip days, never getting much above 27*C

Adding the PSX-240 Autotransformer really has changed the way I am able to operate the system. It has given me a lot more flexibility, even in how I now charge / discharge.

Estragon · August 2018

> @mcgivor said:
> Hoppecke states to immediately fully recharge after discharge to prevent sulfation, then again the recommend a DOD of 50% cyclical for thier solar range of batteries, this however leaves little in the way of reserve should there be a couple of days of incliment weather. Reduced SG means the sulphur is not in solution, the only place it could be is on the plates, I doubt there is any single setting for all situations, each needs to be evaluated, the leading cause of failure is undercharging so my personal choice is to avoid the most common problem.

Interestingly, the paper you linked to above reports grid corrosion as responsible for a majority (86%) of FLA failures. That was, however (I think), in a UPS application. My guess is undercharging is probably a bigger problem in deep cycle applications like ours.

jonr · August 2018

From what I've found, 100% charge (defined as completing a IUI cycle) every day should be the goal (for solar+flooded lead-acid). The only exception is very light discharges (like a UPS). Then some skipping is beneficial. Don't know what the best definition for very light is - 5%? But possibly Vic's < 10%.

Taslim · April 2019

I have 600 watt solar panels with 12 v 150ah battery. (One 50 ah and other 100ah lead acid floaded batteries connected in parallel). How maximum voltage should i give to charge the battery for safe use and safety of batteries. I have connected 30 amp PWM controller with panel. I have conneced 12v tripping relay to charge bank at high current(directly panels to battery) along with controller. This relay is connected with voltage adjustable setting circuit. I have set its setting in that way that it give power from minimum 13.2v and disconnect power on 14.4v.Is it right to do this for batteries????

Estragon · April 2019

What is the purpose of the relay?

Normally a pwm controller would have the ability to limit voltage, and to manage the transition from absorb to float voltage (generally based on time and/or pwm duty cycle).

Using a voltage controlled relay for this means the transition to float happens as soon as absorb voltage (14.4v) is hit. This would normally happen when the bank is ~80-85% full, and at float voltage (13.2) there likely won't be enough time available with solar to complete the charge.

Best Charging Practices for FLA Batteries

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