What is the optimal SOC range for cycling lead acid batteries?

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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?

    Ben, if you are using the FM80 the Absorb time will not be 4 hours every day; that is a maximum time limit. It counts up during Bulk and Absorb will last only as long as the Bulk stage, up to the maximum 4 hours. Likewise you can set Absorb to end once the current goes down using End Amps - this is a good idea but takes some experimenting to find the right setting for your system. Normally it is 1 to 2 percent of the battery capacity, but it also has to include the draw of concurrent loads.

    You most definitely should have a remote (battery) temperature sensor on the system.

    Get a hydrometer with built-in temp compensation. It's basically a thermometer which measures the temp of the electrolyte while you're measuring the SG, and then you adjust accordingly. A higher temp does indeed mean the SG is actually higher than indicated (reduce temp to 'normal' for the chart and the fluid density increases, thus a higher real SG).

    The problem most people have with Surrette batteries is using too low a Voltage for Absorb. Whereas most batteries are fine with 14.8 maximum, Brand S tends to demand >15. Some inverters will have trouble with that, and scream "HIGH VOLTAGE!" just before they shut down. The RTS is going to adjust the 'read' Voltage against the parameters to compensate for high/low temperatures.
  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    Because there are a few items that get added each day maybe 14 would be correct. So your suggestion is to keep the 14.7 absorb, add an end amps time of 14 amps. Let it run for a few weeks and check SG's see how things are looking. If things look good then bump up the voltage to 15? Float voltage would be set to 13.1 and that would be correct?

    It all depends on the SG and what the load is for that end amp setting. The only way to get a true end amp reading is with systems like our XW where this is all measured at several different places so it knows what real amps are to or from the battery, or with a shunt that feeds information back to the controller. Otherwise it's a guess because with widely varying loads during absorb it can change a lot. If your loads are fairly stable every day, then end amps will work without a measurement system to precisely end the absorb stage.

    The hydrometer is what you have to use to determine if it works. 1.265 for Surrettes is basically fully charged for 4000-series. They will go higher if you over-charge. And if you have sulfated plates, in my experience with 4000-series the SG will be below 1.250. I've never seen ours ever get to 1.280.

    The "test" is if you get normal amp-hours on discharge. And you should cycle those batteries down to at least 50% SOC or below every once in awhile to keep them healthy. They won't last if you only go to 80% DoD all the time. Surrettes in the same class as Trojans have thicker plates, so they're more like a traction battery. They don't like to be pampered and maintenance charged all the time if you want to get maximum kWh stored and delivered over their life. People that have them and don't work the snot out of them end up with batteries that go "flat" after awhile, and that refuse to get the SG anymore without extremely high voltage and long corrective EQ's. After I learned how to run 'em, and with a lot input from Surrette on "quit pamperin' em" ours have been fine for the last two years.
    --
    Chris
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    ChrisOlson wrote: »
    1.265 for Surrettes is basically fully charged for 4000-series. They will go higher if you over-charge.

    You've mentioned this in a couple of other threads also. I don't understand how that can be. I thought there is a certain amount of sulfur in a battery, either in the form of sulfate (on the plates) or sulfuric acid (in solution). I also thought that when all the sulfur is in solution (as sulfuric acid) the battery is fully charged. If "overcharging" can raise the SG above "fully charged SG", where is the extra sulfur coming from?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    vtmaps wrote: »
    You've mentioned this in a couple of other threads also. I don't understand how that can be. I thought there is a certain amount of sulfur in a battery, either in the form of sulfate (on the plates) or sulfuric acid (in solution). I also thought that when all the sulfur is in solution (as sulfuric acid) the battery is fully charged. If "overcharging" can raise the SG above "fully charged SG", where is the extra sulfur coming from?

    --vtMaps

    There is also water. Boil enough of it off and the SG goes up. If there is sufficient fluid capacity in the battery cell you can have elevated SG (without exposed plates) due to decreasing the amount of water the sulphur is mixed with.
  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    There is also water. Boil enough of it off and the SG goes up

    Yep. Our batteries hold about 1.5 gallons of electrolyte over the plates at normal fill level. If I boil off 4 quarts of water the SG goes up to 1.270-1.275 between services, and is a good indication that they've been severely over-charged. At normal water consumption of 1.5 - 2.0 quarts per battery between 3 month service intervals, the SG never changes much.
    --
    Chris
  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    Ben, if you are using the FM80 the Absorb time will not be 4 hours every day; that is a maximum time limit. It counts up during Bulk and Absorb will last only as long as the Bulk stage, up to the maximum 4 hours.

    There's a fundamental flaw in that algorithm. Say you get limited power in the morning, like from overcast conditions, where the battery takes a long time in bulk stage and struggles to get to absorb, maybe just a volt short and it hangs there for several hours. Then the sun comes out in the afternoon and suddenly you have all sort of charging power. It's going to absorb a battery for four hours, that probably only needs an hour in absorb because of the extended time being charged at only 1 volt short?

    I've seen absorb with our XW system last anywhere from 5 minutes to 3.5 hours, depending on how the battery was charged, and what the loads were, during bulk stage. If the battery is slow charged during bulk at like half of normal charge rate, the absorb will be very short. If it was fast charged during bulk at C/10, the absorb will be quite long. I've seen situations where our system is running in Float and a heavy load comes on that causes voltage "sag" to the re-bulk level. The XW system will rebulk and absorb is done within 5 minutes and it returns to Float.

    Timer based chargers are less than ideal. And that's why the MidNite guys are working furiously on a shunt system so the Classics can accurately do it based on amp-hour capacity and ending amps instead of using timers.
    --
    Chris
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?

    That is why there is also the End Amps function.

    What many people fail to grasp is that it is all part of the same thing, not an either/or choice. This includes getting the Absorb Voltage right; if that is increased then the Bulk stage takes a little longer and so it stays in Absorb a little longer - and at a higher Voltage so there is more charging effect.

    As soon as MidNite gets the shunt interface done we will finally have available a SOC control for the recharging which will negate the crazy juggling we have to do now. :D
  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    As soon as MidNite gets the shunt interface done we will finally have available a SOC control for the recharging which will negate the crazy juggling we have to do now. :D

    The full XW system has always had that. But it only works with the inverter and MPPT's together. Without the inverter, and the MPPT60's standalone, it has no way to know what the net current is to the battery. The MPPT's report their output to the XW, the XW knows what the DC load is, and it reports net battery current back to the MPPT's so they exit at the right amps. All the components of the system coordinate charge stages over Xanbus so the MPPT's don't do something different than the inverter if the inverter is also being used to charge battery.

    Previous, I had problems because the Classics would never agree with the inverter or the MPPT60. Using the complete XW system on the solar and letting the solar charge batteries and carry loads, while the wind on Classics does the water heating, works MUCH better and got rid of all my problems with getting components of the system to work together. All I can use wind power for right now is to assist with bulk stage, then switch it to water heating when absorb voltage is reached, and let the XW System handle the battery charging. When MidNite gets a Modbus interface done so the Classic will talk Xanbus, then I can put the wind back on battery charging full time.
    --
    Chris
  • inthejungle
    inthejungle Solar Expert Posts: 91 ✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?

    Well it seems that there is a lot of different thoughts, I want to clearly understand everything so that I can have a long service life on the batteries.


    At the end of the day today, 4 hours of absorption time, my SG's were at 1.270 across all cells. I would think that is pretty good.

    Now I hear several things:

    1- Rolls batteries are tough and like to be work
    2- There is a problem with charging and DC input and output


    I have a victron monitor and so I have a pretty good idea of what is going in and what is going out. My loads are fairly stable during the day
    our sundanzer freezer is really the only heavy load, maybe 5a

    So what would be everyone's recommended care and maintenance of this system?

    Based on what everyone is saying it seems to me that I really only have two options

    1: If I am using 20% of the system each day, my thoughts would be that I should let the batteries carry the load for a day and a half, then turn on the charger and let them come up to charge. This would bring the SOC to 60-70% before recharging them. But in this case what to do about the absorb time, absorb voltage and such?

    2- I can keep the system the way it is and adjust the absorb end amps allowing the heavy wear on the battery.


    What is everyone's thoughts?
    In Niger, trying to keep a LG FMA 102NAMA fridge(This has the inverter compressor) backed up with solar using a Victron Multi-Plus Inverter/Charger Compact 12v 1600w with a 70a charger built in.I want to back it up for 4-8 hours. I am also running a few O2 cool fans and a few Thin Lite LED's of my batteries for when the grid is down so my kids can sleep.
  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    1: If I am using 20% of the system each day, my thoughts would be that I should let the batteries carry the load for a day and a half, then turn on the charger and let them come up to charge. This would bring the SOC to 60-70% before recharging them. But in this case what to do about the absorb time, absorb voltage and such?

    It shouldn't change anything except cut the number of cycles the batteries are going thru now in half. It will take more amp-hours to charge them, and bulk stage will last longer. But the batteries charge one heck of a lot more efficient from 60-80% than they do from 80-100%.

    You got that Victron monitor - try it. How many amps-hours do you put in right now for every one you get out? Then cycle them down to 60% and try it. If the battery efficiency proves better by cycling to 60%, guess which cycling regimen will get you the most useable capacity over the life the battery? Your off-grid cost for batteries is in dollars/kWh over their lifetime, not in how many years they last.

    One you get a handle on battery efficiency, then try cycling from 60-80% for a week and only take them to 100% once a week. Then re-figure your battery efficiency - amp-hours in vs amp-hours out. Then if you still have no problems getting them to 1.255-1.265 SG after that week of 60-80% cycling, re-figure your cost/kWh again, and how many cycles you're putting on your batteries over a year's time.

    Edit:
    Had to go retrieve this from Rolls Battery Engineering's tech pages. This is the cycles v DoD for the 4000-series:

    Attachment not found.

    Consider a 400ah 6V battery. Cycle it to 50% 1,280 times. You'll get 1.2 kW per cycle, or 1,536 kW over the battery's life.

    Cycle the same battery to 20% 2,000 times. You'll get 480 watts per cycle, or 960 kW over the battery's life - only ~63% of what you get out of your money spent on it if you actually put it to work instead of pampering it.

    Now, reduce the number of cycles this battery has to go thru by charging it less often, and discharging it deeper. The light bulb should come on over your head as to how much your off-grid energy storage capacity is going to cost you if you pamper the battery vs using it.
    --
    Chris
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    ChrisOlson wrote: »
    Consider a 400ah 6V battery. Cycle it to 50% 1,280 times. You'll get 1.2 kW per cycle, or 1,536 kW over the battery's life.

    Cycle the same battery to 20% 2,000 times. You'll get 480 watts per cycle, or 960 kW over the battery's life - only ~63% of what you get out of your money spent on it if you actually put it to work instead of pampering it.

    Now, reduce the number of cycles this battery has to go thru by charging it less often, and discharging it deeper. The light bulb should come on over your head as to how much your off-grid energy storage capacity is going to cost you if you pamper the battery vs using it.

    All of these published cycle-life graphs are based on a cycle that discharges to a particular SOC and then charges to 100% SOC. That's not what happens in a RE application where there are partial-state-of-charge cycles that do not reach 100% SOC.

    I'm not saying that your approach isn't more cost effective or won't result in longer battery life, I'm saying that I haven't seen published studies that show the lifespan of batteries under conditions of partial-state-of-charge cycles that do not reach 100% SOC.

    One thing seems clear... partial-state-of-charge cycles increase stratification in flooded batteries. Stratification is harmful to batteries. I am waiting to be convinced that the benefit of partial-state-of-charge cycles outweighs the harm of stratification.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    vtmaps wrote: »
    All of these published cycle-life graphs are based on a cycle that discharges to a particular SOC and then charges to 100% SOC. That's not what happens in a RE application where there are partial-state-of-charge cycles that do not reach 100% SOC.

    Correct. And that's what this thread is about. Almost everybody that lives off-grid partially charges their batteries, whether by design or because mother nature don't cooperate. Few have done it by design.

    As far as studies, such a study could take potentially 10 years, and would have to be duplicated several times to be published as a real "study". I think the writing is on the wall - nobody reads it. A battery rated for 1280 cycles should only last 3.5 years if it's discharged to 50% and recharged every day like most people preach. However, they usually last 5-7 years being discharged to 50%. Why is that? I put and two and two together, added 12 and carried 5 and came up with the fact that it's because the battery don't get fully charged every day so it doesn't go thru 365 complete cycles in a year. There's your "study".
    One thing seems clear... partial-state-of-charge cycles increase stratification in flooded batteries. Stratification is harmful to batteries. I am waiting to be convinced that the benefit of partial-state-of-charge cycles outweighs the harm of stratification.

    Stratification doesn't happen overnight, or even in a month as long as the cell is active. Not that I've ever seen yet, and our batteries are about as tall case as they come. Cells that aren't worked, undergoing shallow discharges and charge cycles, and sitting around not being active, can have a serious stratification issue. And stratification is another thing that's not even mysterious or hard to detect - the SG will be low. You get water at the top of the cell and more dense electrolyte at the bottom. If that happens, bump the voltage up. Higher charging voltages promote more vigorous mixing of the electrolyte during charging.
    --
    Chris
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    ChrisOlson wrote: »
    Almost everybody that lives off-grid partially charges their batteries, whether by design or because mother nature don't cooperate. Few have done it by design.

    But if one has adequate solar resources to reach 100% SOC every day, should one intentionally (by design) NOT reach 100%? --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?

    vtMaps might mean sulphation, not stratification. The latter rarely happens in any standard size battery with a reasonable amount of 'activity'. Sulphation is a natural process that happens to all types of FLA's and is accelerated by the amount of time spent at lower SOC. Below 70% (some say 75%) SOC it occurs much faster (nothing about batteries is linear). Of course I am referring to the 'hard' type which is not reversed by charging and causes permanent loss of capacity.

    This is why it is most economical to use 25% of capacity on average. Overly-large battery banks tend to be a false economy and may even mask deficit charging. Planning on 100% charge from solar on every sunny day should also work out best (not too much invested in solar). The occasional day of not reaching 100% will not have any seriously detrimental affect on battery life.

    Much of the economics of partial recharging is related to using generators when solar is not sufficient: running a gen to supply the low amounts of power needed to do that last 10-20 percent of charging is a waste of fuel and hard on generators.
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    vtMaps might mean sulphation, not stratification.

    No, I meant stratification. Stratification is bad, and occurs in just a few partial cycles. Part of what makes stratification bad is that it does cause sulfation on the lower parts of the plates.

    reference: http://www.battcon.com/PapersFinal2013/11-Larry%20Meisner%20-%20The%20Effects%20of%20Mechanical%20Electrolyte%20Mixing%20VLA%20Cells%20in%20Renewable%20Energy.pdf

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    vtmaps wrote: »
    No, I meant stratification. Stratification is bad, and occurs in just a few partial cycles. Part of what makes stratification bad is that it does cause sulfation on the lower parts of the plates.

    I have yet to ever see that in an active battery. Like I said, it's very easy to detect because the top of the cell where you pull your sample from will be watered out and the SG will show low. When that happens, you figure the charging isn't done yet, so you set the voltage up, or absorb it longer - whatever it takes and the SG comes up. People that use a hydrometer as part of their battery maintenance program are going to naturally takes steps to prevent any sort of stratification issues because they become worried about low SG.

    From a "study" standpoint, I've been at this off-grid game for going on 13 years. The first set of batteries we ever bought was 8 off-the-shelf, cheap $72 each, Group 29 marine deep cycles from Farm & Fleet. Six of those original batteries failed starting at 4 years, one lasted 7, and two or the originals still tested at about 50% of their original capacity in March 2010 when the whole works got replaced with our new bank.

    Back in those days I didn't know ANYTHING. I didn't even own a charge controller. The first solar panels we bought were so incredibly expensive that we couldn't afford them. So I started building wind turbines. Those first wind turbines didn't work very good either. Those batteries were really, really lucky if they got charged once a month - and usually with our gas engine driven car alternator/standby generator that we had back in those days. How could cheap marine deep cycles last up to 9 years with that sort of treatment? They never got charged, that's how. They basically got maintained at about 12.5 volts, and if we got 'em up to 13.5 it was a glorious day, and if they made 14 we figured they were fully charged.

    Granted, only 25% of those original 8 made it that far. And 25% of them barely made it four years. But still, it was the most mis-treated and cheapest set of batteries, for what we got out of them, that we have ever bought.
    --
    Chris
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?

    I've seen a lot of batteries over a lot of years. Stratification is a non-starter of a problem.

    Tall case batteries that aren't used much (discharged and charged - it takes both) will show signs of it. I would not agree that it is likely to occur in, say, a set of GC2's that don't reach 100% daily and/or are only discharged 10%.
  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?

    Who's farther ahead ?? If you live on the power from your batteries. Guy A & B have a identical set of batteries Guy A uses a DOD of 50% and gets to 100% when he can, Guy B uses 25% and recharges to 100 % everyday. Guy A gets 3 years out of a set, Guy B get's 6 years if he's lucky. Guy B spends the last year of his 6 years nursing a set of low capacity batteries. As batteries age ( 7-10% a year ), use them or lose them. In the end who comes out ahead ?? If your a person with a back up system and make a set of batteries show voltage and don't care then 10-25 % DOD probably makes sense. You'd be surprised how many people just disconnect their system when they find out they have to buy a new set of batteries.
  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    Guy A uses a DOD of 50% and gets to 100% when he can, Guy B uses 25% and recharges to 100 % everyday. Guy A gets 3 years out of a set, Guy B get's 6 years if he's lucky.

    Actually, Surrette's guidelines indicate there is not the double the lifespan in years - only about 1.5x in years if you cycle to 20% vs 50%. So if Guy A is only going to get 3 years, Guy B will get 4.5 years before the batteries fall to 50% of their original capacity (Surrette's end-of-life). Theoretically Guy B could get another year, but any battery that I've had that only had 50% of its capacity left didn't last another year. When they get that bad they get so hot when you try to charge them it basically wrecks what's left.
    --
    Chris
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    ChrisOlson wrote: »
    Actually, Surrette's guidelines indicate there is not the double the lifespan in years - only about 1.5x in years if you cycle to 20% vs 50%. So if Guy A is only going to get 3 years, Guy B will get 4.5 years before the batteries fall to 50% of their original capacity (Surrette's end-of-life).

    If you look at cycle life versus cycle depth charts for Surrette 5000 or Trojan RE, they suggest that to get more kWh stored through the battery life, you need to cycle as deep as possible.

    Similar chart for GC batteries (sorry cannot find it) suggests the opposite - you need to cycle as shallow as possible.
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?

    I have begun to force full recharges of the banks here ONLY every 5-7-ish days, at least for this Summer. Can almost always get full recharge every day during the nine months of the non-Winter period. Am hoping to be a bit easier on the batteries, and part of this intended easiness is from the REDUCTION OF HEAT PUMPED INTO THE BANKS from the Absorption stage.

    In hot weather, this reduces the average battery temperature by more than five degrees F, and additional cooling is available for the battery bank and the electronics in the power room because these short charges are only to Vfloat, which makes much more power available to this power room A/C unit due to the charge "cycle" being reduced by about 90 - 120 minutes on those days.

    To me this is an additional win. Will keep an eye on the SGs and see if these 24-inch tall Surrettes are happy with this approach.

    This Thread brings up good issues to discuss. Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    Vic wrote: »
    I have begun to force full recharges of the banks here ONLY every 5-7-ish days, at least for this Summer.

    This is from GB Industrial on the efficiency of a 1200ah 36V forklift battery, showing efficiency vs DoD:

    Attachment not found.

    GB Industrial does not recommend charging a battery at all unless it is discharged to 80% DoD. They claim that "opportunity charging" does more harm than good because the heat generated during an opportunity charge is more detrimental than discharging the battery to 80% DoD. However, while this may apply to industrial constant-current chargers that are designed to recharge a 1,200ah battery in 8 hours for the next shift, and will bulk the battery at C/5, it is different for RE systems on a "bad" power day.

    The RE system might add enough energy to the battery to trickle charge it and carry loads on the "bad" day, and charge at max C/10 on the "good" day while carrying normal inverter loads. So the "opportunity" charge from the RE system on the "bad" day is basically only going to delay what would otherwise be a more rapid discharge. And this does not create any heat in the battery like an industrial constant-current charger does on an "opportunity charge".
    --
    Chris
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    Vic wrote: »
    This Thread brings up good issues to discuss. Vic

    This discussion got me thinking about the 'old' concept of undercharging brought on by insufficient PV vs the Ah needs of the battery.

    "Use it on the weekend and then let the PV 'catch up' over the time you are not there."

    The only difference I can ferret out is that it would be the length of time, maybe > 4 days, until the battery sees enough Ahr's to 'fill' it to a useable level, >80%, so deficit charging results over a month or 2 in summer and the battery would start to build hard sulphation. The start of a slow irreversible death.

    Is this it?
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    ChrisOlson wrote: »
    The RE system might add enough energy to the battery to trickle charge it and carry loads on the "bad" day, and charge at max C/10 on the "good" day while carrying normal inverter loads. So the "opportunity" charge from the RE system on the "bad" day is basically only going to delay what would otherwise be a more rapid discharge. And this does not create any heat in the battery like an industrial constant-current charger does on an "opportunity charge".
    --
    Chris

    Am not exactly certain what you are saying.

    The battery heating that I was referring to was from the completion of an Absorption stage to 1% of C, which is what these banks experience on a FULL RECHARGE. The full recharge is where the heating that I was referring to comes from. Very little heat results from Bulking only to about Vfloat on the short cycle days (especially when using PV power only)

    So, every day that sees a short charge cycle is a day that sees lower battery temperature to the tune of about eight degrees F or more, when increased cooling from more A/C being available is counted.

    On the full recharge day, the battery temp does rise a bit more than would be the case of a daily full charge, as the delayed recharge occurs from a lower DOD, so more Abs time is required, thus more battery heating. But the average battery temp is lower, a good thing, and so on.

    FWIW, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    westbranch wrote: »
    The only difference I can ferret out is that it would be the length of time, maybe > 4 days, until the battery sees enough Ahr's to 'fill' it to a useable level.

    It also a slow rate of charge, which makes it difficult to elevate voltage to bubbling levels and get a good absorption.
  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?

    "...As soon as MidNite gets the shunt interface done we will finally have available a SOC control for the recharging which will negate the crazy juggling we have to do now. "

    boB posted over in the MidNite forum, that they're real close, this week. I hope by Christmas...

    While it is hard to determine SOC while batteries are charging, voltage tends to run up very quickly when you reach the max bulk voltage, and it can be assumed that you have reach close/around 80% charged. This is significant since it would represent the upper end of the scale if one desired to keep the batteries between 50(or 60%) and 80%. I suspect if someone wanted to keep it in this range they might check out the computer geeks section of the MidNite Classic forum.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • northerner
    northerner Solar Expert Posts: 492 ✭✭✭✭✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?

    My understanding is that heat is one of the biggest killers of all lead acid batteries. If your batteries are kept in a hot environment and/or they are charged or discharged often at high rates, they will degrade faster due to the effects of temperature, than if they are kept in a cool place and charged and discharged at moderate rates. If you have a small battery bank relative to your system size, it will be more susceptible to heavy usage, and higher operating temperatures. Of course if you keep your batteries too cold, what you gain in cycle life, you will lose in storage capacity. I believe 50 deg F (~10C) is a good compromise.

    I keep my batteries in a relatively cool place, and am about to expand the storage capacity, which will also lessen the load on the battery bank.

    The fellow that handled the sale of my system is off grid and has been for years. He uses forklift batteries and only cycles them on average down to 80% DOD. He says he can get 20 years out of the batteries that way, and they need very little maintenance!

    I really believe that the days of the traditional lead acid battery are numbered. Better alternatives will be here soon, although they have served us relatively well up to now. The same thing is happening with the incandescent light bulb and the CRT!
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?

    I detect stratification doubters amongst the contributors to this thread. Here are some excerpts from the paper I referenced back in post #76:

    From: http://www.battcon.com/PapersFinal2013/11-Larry%20Meisner%20-%20The%20Effects%20of%20Mechanical%20Electrolyte%20Mixing%20VLA%20Cells%20in%20Renewable%20Energy.pdf
    The main properties of a battery used in a renewable application, is its ability to cycle in difficult operating conditions. This means time limited cycling at shallow depths of discharge as well as operation in a partial state of charge condition.
    <snip>
    After only 5 cycles a large concentration gradient has developed in the electrolyte (0.16 kg/l) and the capacity has dropped by 12%. This subsequently has an influence on the electrical performance of the cell and causes an uneven utilization of the active mass.
    <snip>
    The key effects of electrolyte stratification are clear:
    1. Corrosion at the top lead and upper parts of the cell as a result of low specific gravity
    2. Undercharge of the lower part of the plate due to higher specific gravity resulting in sulfation and loss of capacity.
    <snip>
    Flooded lead acid batteries in applications with daily discharge and recharge cycles generally require a higher charging factor of around 120% to help compensate for the development of electrolyte stratification.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?
    vtmaps wrote: »
    I detect stratification doubters amongst the contributors to this thread. Here are some excerpts from the paper I referenced back in post #76:

    From: http://www.battcon.com/PapersFinal2013/11-Larry%20Meisner%20-%20The%20Effects%20of%20Mechanical%20Electrolyte%20Mixing%20VLA%20Cells%20in%20Renewable%20Energy.pdf



    --vtMaps
    Did you buy their Electrolyte mixer yet ??
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
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    Re: What is the optimal SOC range for cycling lead acid batteries?

    I have a question to the people who say they never seen the stratification. What would you need to see to say: "look, this is a stratification"?