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

vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
Recently in another thread Photowhit summed up:
Photowhit wrote: »
but most recommendations I've seen are to use the upper 20-30% of your batteries capacity.

I have noticed that most (all?) battery manufacturers that provide life cycle information define a cycle as reaching 100% SOC. The data always shows that shallow cycling will result in more cycles than deep cycling. Thus more cycles from 80% to 100% than from 70% to 100% SOC.

I have never seen data that shows how many cycles per lifetime can be obtained from shallow cycles that do not reach 100% SOC. In other words, will there be more cycles available from 60% to 80% than from 80% to 100%?

One other thing... although manufacturer's data shows more cycles per lifetime with shallow cycles, deeper cycles provide more kwh stored and released over the lifetime of the battery. The most cost effective system would use smaller (cheaper) batteries that are deeply cycled and replaced more often. Many off-gridders use larger batteries with shallower cycles because they occasionally need the greater peak current that the larger battery can provide.

But recently, in another thread, Chris Olson wrote:
ChrisOlson wrote: »
Lead-acid's don't really like to be cycled from 80-100% SOC all the time. They really prefer 50-80%.

What does this mean? Are you referring to more cycles per lifetime? Greater kwh stored per lifetime? Greater efficiency?

I certainly understand that batteries accept charge more efficiently at lower SOC, but my personal preference is to sacrifice efficiency for longevity... a few extra panels (one time expense) will make up for the inefficiency.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
«1345

Comments

  • PhotowhitPhotowhit Solar Expert Posts: 4,965 ✭✭✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?

    Just my opinion...

    I think batteries like to be used, and if shallow cycled and brought to full charge every day is now great for them. That said, normal use should also not include periods of more than a day or 2 when they don't reach absorption stage of charging (roughly 80-85% full).

    In the normal use of an off grid system, my batteries will find time of deeper discharge, I've been running an A/C over night and since I've been away most days and won't be around when the batteries reach float, I've run my water heater for an hour or so in the morning. I worry about the charging being fooled by the heavy load while charging the batteries. by loading while the batteries are still in bulk I prevent the load during timed cycles and likely drop the battery down to 60% DOD but also at a time when the batteries will be recharged soon so as not to leave them in a lower state of charge.

    In the winter and spring cloudy weather will bring the battery down to similar levels most weeks. I don't use a generator, so the need for the batteries to reach fully charged once or twice a week does require some thinking as to weather and what state of charge the batteries are in. I also don't normally use a battery monitor, but can kinda keep a running idea of the state of charge the batteries are in. I can get some confirmation of that from the system voltage with my known loads.

    I'm a firm believer in doing a maintenance equalizing cycle once a month with flooded batteries, now that I have a forklift battery, it is simply required.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?

    Considering that most Batteries will lose from between 8-10% per year in capacity just sitting unused on a float charge there is not reason not to use them and get as much as you can. Anyone ever try to hook up a 7 year old battery, pretty ugly. Of course a mission critical battery is considered depleted at 75-80 % of design capacity, solar folks take them to 30 % and swear they have good batteries, till they don't.
  • ChrisOlsonChrisOlson Banned Posts: 1,807 ✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?
    vtmaps wrote: »

    What does this mean? Are you referring to more cycles per lifetime? Greater kwh stored per lifetime? Greater efficiency?

    I learned from Surrette Tech Support that you can work a Series 4000 or 5000 battery for a month without fully charging it, and it won't hurt it and cause hard sulfation problems. I don't go that long - a week to 10 days usually between full charges. Cycling them below 85% SOC definitely is not a full cycle because it never causes any plate erosion (according to Surrette). And it's way more efficient - now that I have a ComBox I am able to see kWh into our bank and kWh out and compare to kWh to loads - and the efficiency drops dramatically (way less out than what you put in) when you absorb batteries every day. And I mean it's REALLY bad to the tune of 4-4.5 kWh/day lost in heat in the batteries on a bank our size.

    I don't make RE power to turn it into heat in the batteries every day. I'll sacrifice some of it once every week to 10 days or so to keep the batteries healthy. But absorbing batteries every day is about as efficient as starting up a 20 kW generator to make yourself two slices of toast and a cup of coffee for breakfast.
    --
    Chris
  • Lefty WrightLefty Wright Solar Expert Posts: 111 ✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?

    Here is what I've noticed about my batteries;

    I am in the middle of an electrical upgrade at my off grid site and my old batteries died. I made a temporary replacement with some rebuilt golf cart batteries.

    These batts didn't seem to have much capacity. Since they are temporary I decided to use all the power I wanted and run them down as low as they wanted to go and not worry about their longevity.

    At first they were going down to 12.4V (this is a 12V system) by bed time. But after "abusing" them for a month or so they seem to have much more capacity and now hold their charge quite well.

    Do lead acid batteries need a break in period?

    I do have good sun every day and the batteries are always up to 14V by 9AM. I haven't seen then drop below 12.6V in some time.
  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?

    They say it takes between 20-50 cycles ( define cycle ) depending on the batteries. Now that could have changed some as Manufacturers pre-form plates and different chemistry ( antinomy, calcium, selenium alloy ) would react differently. New batteries can be sulfated some and that depends on how they were commissioned. You also have the dip and recovery syndrome to consider, it depends on the use before the voltage reading was taken..
  • northernernortherner Solar Expert Posts: 492 ✭✭✭✭✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?
    ChrisOlson wrote: »
    I learned from Surrette Tech Support that you can work a Series 4000 or 5000 battery for a month without fully charging it, and it won't hurt it and cause hard sulfation problems. I don't go that long - a week to 10 days usually between full charges. Cycling them below 85% SOC definitely is not a full cycle because it never causes any plate erosion (according to Surrette). And it's way more efficient - now that I have a ComBox I am able to see kWh into our bank and kWh out and compare to kWh to loads - and the efficiency drops dramatically (way less out than what you put in) when you absorb batteries every day. And I mean it's REALLY bad to the tune of 4-4.5 kWh/day lost in heat in the batteries on a bank our size.

    I don't make RE power to turn it into heat in the batteries every day. I'll sacrifice some of it once every week to 10 days or so to keep the batteries healthy. But absorbing batteries every day is about as efficient as starting up a 20 kW generator to make yourself two slices of toast and a cup of coffee for breakfast.
    --
    Chris

    So, is that something you practice, preventing your system from going into absorb most days? If so, how do you accomplish this? There are 2 methods that come to mind. One is through system design, having slightly less solar (wind), and slightly more battery capacity. The other is to increase your loads, perhaps using waste not on those days.

    Going with the system design approach, would give one a slightly less healthy system, I thought. Is it not better to have more solar and slightly less batteries? This would be more likely to bring the batteries to float every day. The issue is that batteries are an expendable expense, and the more you have, the more you have to replace when there time is up. Of course, you do need enough battery capacity to handle your heaviest loads, and to get you through an entire day, at least.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?

    Optimizing battery cycles involves more than just the batteries.
    The deeper it is cycled the more power you need to recharge so you may have to invest in larger PV to bring it back up in a reasonable amount of time.
    Deeper cycling leaves less "reserve" room for when the sun doesn't shine, so again you have to add charging capacity and/or run the gen more.

    Deep cycle batteries benefit from being cycled rather than floated. Using only a small amount is a waste of money as the lifespan at 10% DOD is almost exactly the same as at 25%. Time itself will get batteries no matter what, so you may as well make use of them while you've got them.

    And yes there is an initial break-in period where the Amp hour capacity actually increases (up to rating) with cycling. After that it's all down hill. Like life.
  • ChrisOlsonChrisOlson Banned Posts: 1,807 ✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?
    northerner wrote: »
    Going with the system design approach, would give one a slightly less healthy system, I thought. Is it not better to have more solar and slightly less batteries?

    As 'coot mentioned in his post, you still have to have the generating capacity to meet your daily loads.

    I have designed our system to use "opportunity" loading on the inverter with electric water heating. I used to use Classic controller on the solar with Waste Not mode. I am now using XW-MPPT60-150's on solar with diversion mode, in cooperation with wind power on Classic 150's using Waste Not mode. As voltage approaches absorb the controllers bring AC SSR's online that turn on AC power from the inverter to the water heaters. This adds enough load to the system so it struggles to make absorb and sends a good chunk of energy to the water heaters every day.

    Typically, the voltage will hang just below absorb for hours so the batteries get sort of "float charged". This does not raise their temperature at all, or even stress them. But it pumps amp-hours into them. Over time, it seems we get the "exceptional" day about three times a month or so. On that day (usually caused by excessive wind power) there is so much excess power that we can run the water heaters plus absorb the batteries. Those are the days when we can generate 40-50 kWh in one day.

    It's been working fine here for about two years. Although the water heating system has undergone constant "tuning" to match the load to the available system input power. I have 5 kW of aux load available with the new setup with MPPT60's and Classics driving SSR's. I used to have only 2 kW with daisy chained water heater elements and thermostatic relays.

    Electric water heaters work just as good (and are a little more efficient) as batteries for storing energy. Our water heaters can store up to 100,000 BTU, or 29.5 kWh of energy - at way less cost per kWh of storage than batteries.
    --
    Chris
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?
    ChrisOlson wrote: »
    Cycling them below 85% SOC definitely is not a full cycle because it never causes any plate erosion (according to Surrette).

    This (if true) is the key to the puzzle. I've never seen that in any Surrette literature. It makes sense, though.

    It is at odds with the advice over at gbbattery.com:
    Each time you charge a battery, regardless of how long, it constitutes one cycle.

    In designing my system I followed most of the rules-of-thumb that are endorsed by our forum moderators, and that has resulted in a system that has me floating by noon whenever I have consecutive sunny days.

    How would you recommend that I improve my charging? I can limit my controllers maximum charging current so that I don't reach absorb every day, but that will limit my ability to run opportunity loads. I can reduce my absorb time to near zero... but this will still get my voltage up to absorb every day, although not for long. I can reduce my absorb voltage... is this the best strategy to avoid plate corrosion?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • ChrisOlsonChrisOlson Banned Posts: 1,807 ✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?
    vtmaps wrote: »
    This (if true) is the key to the puzzle. I've never seen that in any Surrette literature. It makes sense, though.

    It is at odds with the advice over at gbbattery.com:

    It is not in Surrette's literature. You have to talk to somebody at Surrette Tech Support that knows batteries. Steve Higgins is one of those people.

    And I don't necessarily agree with GB Industrial on their cycle theory, and neither does most of the rest of the battery world.
    How would you recommend that I improve my charging?

    If you want to try 50-85% SOC daily cycling, the best way I have found to achieve it is to put load on the inverter. I have tried it with DC loads in the past and that is like beating you head against the wall. There's nothing really useful that you can use DC power for, except to feed it to an inverter. And the aux load typically makes your inverter more efficient too - if you look at the efficiency curves of most inverters, they don't reach peak efficiency until they're operating at minimum 25-30% load. Most off-grid people I know have their inverters "idling" all the time at 200-300 watt load where the inverter is maybe only 80% efficient (it's zero percent efficient at zero load). Put some load on the thing so it can do what it was designed to do.

    You'd be surprised how Large you can live off-grid when you get the efficiency of your batteries and inverter up to their peak and quit turning your daily energy production into heat and losses in equipment. You can use that power off-grid to run stuff to make your life better.
    --
    Chris
  • NorthGuyNorthGuy Solar Expert Posts: 1,925 ✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?
    northerner wrote: »
    So, is that something you practice, preventing your system from going into absorb most days? If so, how do you accomplish this? There are 2 methods that come to mind. One is through system design, having slightly less solar (wind), and slightly more battery capacity. The other is to increase your loads, perhaps using waste not on those days.

    I'm also going to skip absorptions most of the days. I believe bubbling and heating is harmful to batteries and doing this only when absolutely necessary is beneficial. I don't have big loads as Chris does, so I simply going to force them into float if they're not discharged enough to justify an absorption. They're still going to get some charge during float, so you may end up your day with 90-95% charged batteries. We'll see. It was a thread here (can't find it) where Sunny Boys where used on an island following the same strategy.

    It is a certain risk in this, of course. You don't fully charge today, and you're going to hit with real bad weather tomorrow. You wish you would've charged yesterday, but it's too late. However, I believe the impact of this is minimal.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?
    ChrisOlson wrote: »
    You'd be surprised how Large you can live off-grid when you get the efficiency of your batteries and inverter up to their peak and quit turning your daily energy production into heat

    As I mentioned earlier, efficiency is not a concern. I want battery longevity. I can make up for inefficiency by adding a couple more panels.

    You use a lot of generator power and inefficiency would cost you dearly. I expect that over time I will spend more on batteries than generator fuel.
    ChrisOlson wrote:
    Typically, the voltage will hang just below absorb for hours so the batteries get sort of "float charged". This does not raise their temperature at all, or even stress them. But it pumps amp-hours into them.

    So then the best way for me to emulate your charging profile will be to lower my absorb voltage a bit, and have a longer absorb time?

    By the way, I think you may be onto something with the way you charge your batteries, and I thank you for documenting it on this forum. Hopefully Surrette will also document it at some point.

    The down side of your charging regimen is that lower voltage does nothing to stir the electrolyte and break up stratification. I wonder if taller batteries need higher voltages more often than shorter batteries...... Battery advice is probably not one-size-fits-all.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?
    vtmaps wrote: »

    It is at odds with the advice over at gbbattery.com:



    --vtMaps
    Thats because they are protecting their 1500 cycle warranty. A much larger Battery Co than GB , Enersys Industrial Battery says something a little different.

    9. FAST CHARGING AND
    OPPORTUNITY CHARGING
    If a single battery is being used in a lift truck for multiple shifts or is
    partially recharged during breaks, lunches and other idle periods, it
    may be in a fast charge or opportunity charge mode of operation.
    Opportunity charging can be used to keep the battery’s state of
    charge above 30% depth of discharge during the daily discharge
    cycle thereby reducing or even eliminating the need to change out
    spent batteries in a heavy single shift or multi shift operation. The
    total accumulated discharged ampere-hours should not exceed
    120% of the batteries designed capacity rating per day. Discharge of
    more than 120% of the batteries designed capacity rating in a
    24-hour period will shorten battery life. If engaging in opportunity
    charging, the battery must be returned to nameplate specific gravity
    at least once per week. However, more frequent recharges to
    nameplate specific gravity are desirable. Charge rates during this, of course, refers to the actual temperature of the cell and not the ambient temperature. Thus a battery may be operated
    in quite low ambient temperatures for short periods without the actual battery temperature falling to a point where the capacity is seriously curtailed. For example, batteries used in cold storage
    plants or similar locations will deliver close to normal capacity if they
    are moved into warmer areas for charging and whenever not in
    actual use.

    http://www.enersysmp.com/documents/28.00-InstructionsforFloodedBatteries_000.pdf
  • NorthGuyNorthGuy Solar Expert Posts: 1,925 ✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?
    The
    total accumulated discharged ampere-hours should not exceed
    120% of the batteries designed capacity rating per day. Discharge of
    more than 120% of the batteries designed capacity rating in a
    24-hour period will shorten battery life. If engaging in opportunity
    charging, the battery must be returned to nameplate specific gravity
    at least once per week. However, more frequent recharges to
    nameplate specific gravity are desirable.

    Interestingly enough, Trojan recommends very similar thing for my batteries, except they allow 300%. That is from the manual:
    Trojan recommends any automated system should be set to equalize Industrial batteries at least
    every 30 nominal charge throughputs. A full charge is recommended at least every three nominal
    charge throughputs. A charge throughput is when a battery is taken from the end of charge, is
    discharged, and then taken to the end of charge again. A nominal charge throughput is reached
    when the sum of the discharge currents corresponds to the nominal capacity of the battery
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?

    A couple of things to think about:

    Sulphation becomes a real problem at <70% SOC for extended periods. Hmm.

    Much of a complete charging cycle occurs at a slow rate above 80% SOC. Hmm.

    Float Voltage is above full charge resting Voltage. Hmm.

    (Okay, so it's a ménage à trois of things to think about.)
  • BB.BB. Super Moderators, Administrators Posts: 29,368 admin
    Re: What is the optimal SOC range for cycling lead acid batteries?

    If I understand the arguments (no chemistry professor here)--Letting Lead Acid batteries set below ~75% SOC for days/week/months without cycling => Sulfation.

    However, if you are actively cycling the battery every day--You can easily operate the battery at 50-80% State of Charge range as the active cycling is doing the Sulfate<=>Lead cycle... And going 5-10 days before recharging >90% SOC is just fine for battery cycle and overall life (or Chris' 30 days per Surrette). And certainly more efficient as you avoid the 80-90-100% charge range where efficiency falls as you start generating hydrogen/oxygen gasses (and create more heat in the batteries).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Eric LEric L Solar Expert Posts: 262 ✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?
    Typically, the voltage will hang just below absorb for hours so the batteries get sort of "float charged".

    That's interesting Chris. I started doing something similar with the Waste Not function and hot water heating this Spring. This was after concluding like others here that I didn't need/want a near-daily full absorption cycle.

    As long as the typical daily loads are fairly constant (and large enough), simply changing the voltage offset on the Waste Not Hi function does a pretty good job of letting me determine how often the batteries are going to complete the absorb cycle per week for a given season of year (it's still subject to weather variations within that season, of course).

    It's a nice added "perk" of the already great Waste Not Hi + SSR system, and further increases capacity utilization of the solar panels.
  • ChrisOlsonChrisOlson Banned Posts: 1,807 ✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?
    BB. wrote: »
    However, if you are actively cycling the battery every day--You can easily operate the battery at 50-80% State of Charge range as the active cycling is doing the Sulfate<=>Lead cycle... And going 5-10 days before recharging >90% SOC is just fine for battery cycle and overall life (or Chris' 30 days per Surrette).

    Bill, that's a pretty good summary. Leaving batteries set around at low SOC is a lot different than working them at lower SOC levels. The sulfate on the plates does not harden into crystal formations for a very long time when the battery is chemically active and constantly changing state. The easy way to tell how much sulfation you got is with the good ol' hydrometer. If you can run 'em 10 days without a full absorb, then absorb with normal voltages and time and get SG to spec, you are not sulfating your plates. That sulfate has to go back into solution in the electrolyte to make it more dense and raise the SG. If it's stuck on the plates, the SG is going to be low.

    Lead acid batteries are not mysterious things. They have been around for over 100 years with little changes in how they work. You can forget all the mumbo-jumbo market speak about hydro-turbo-super-adSorb separators or plates, or whatever. There ain't nobody that's come up with any real changes in how the dang things work. Different manufacturers have used various plate chemistry - but they are still all basically lead antimony construction. More calcium is added to severe-duty batteries because it makes the plates tougher than whale snot so they stand up vibration and excessive heat, as in traction, marine and automotive applications. Stationary batteries usually have less calcium so the plates are more porous and the battery has higher capacity for the grid size, but overheating this type causes plates to warp and then the battery is junk.

    But still, nobody has changed how they work.

    Any manufacturer out there can build batteries that will last 50 years. The thing is, how much do you wanna pay for 'em? So pay attention to the two things that kill lead-acid batteries in the long term - sulfation and excessive heating. If you can reduce the daily heating of the grids by reducing the number of absorb cycles, and still not hard sulfate the battery and get normal SG on a periodic full charge, guess what you get? Batteries do not live their lives on a 24 hour cycle like humans do, based on the rising and setting of the sun. So don't make 'em try to fit your daily cycle.
    --
    Chris
  • nsaspooknsaspook Solar Expert Posts: 396 ✭✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?
    ChrisOlson wrote: »
    As 'coot mentioned in his post, you still have to have the generating capacity to meet your daily loads.

    I have designed our system to use "opportunity" loading on the inverter with electric water heating. I used to use Classic controller on the solar with Waste Not mode. I am now using XW-MPPT60-150's on solar with diversion mode, in cooperation with wind power on Classic 150's using Waste Not mode. As voltage approaches absorb the controllers bring AC SSR's online that turn on AC power from the inverter to the water heaters. This adds enough load to the system so it struggles to make absorb and sends a good chunk of energy to the water heaters every day.

    Chris

    I have basically the same "Waste Not" design in my system.

    I use several control factors to optimize the Solar power output in a mainly automatic manner. What you optimize for can be power efficiency to battery life by varying several control points that I adjusted to power for testing.

    E = efficiency (0-100%)
    PVE = current solar array power vs max power from the solar array
    CCE = current load (battery+inverter) power on the output of the charge controller vs the input of the charge controller
    USE = current load (battery+inverter) power USAGE vs max power from the solar array (can be greater that 100%)
    A battery health (weight factor, the bigger the better) factor that combines several measurements or conditions into a single number that gives the health of a battery like Ah capacity, charge cycles, depth of discharge, internal resistance, etc ...

    From the PVE I also generate a PQ (for power quality) two day running tally of daily energy collection so the software can decide using all these factors if TODAY is a good day to try diverting power to loads or to waste energy fully charging batteries.
    If the software decides it's a good day it chooses between one or more of two methods of power diversion.

    1: A PWM channel that sends the PV DC output before the charge controller to a DC (water) heater load controlled by making the CCE factor as close to 100% as possible during absorption or float.
    2: A AUX power relay that switches AC power from the grid to inverter power (via a 120VAC isolation transformer) on several circuits with non-sensitive loads (mainly lights) or are connected to a 'online' UPS that only keeps it's internal battery charged from the incoming AC power.
  • ChrisOlsonChrisOlson Banned Posts: 1,807 ✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?
    Eric L wrote: »
    It's a nice added "perk" of the already great Waste Not Hi + SSR system, and further increases capacity utilization of the solar panels.

    I'm using my MPPT60's to do the same thing with voltage-based trigger firing up the SSR and operating a 1,000 watt element. I use the Waste Not on the turbines and their Classics, each driving their own SSR and 2,000 watt water heater element. This works really good - in 8 hours I can dump 8 kWh into the water heater, which keeps the solar busy all day, along with other normal house loads. If the turbines try to raise the charge stage voltage the Waste Not kicks in and fires up the element, adding up to additional 4 kW load to use up the turbine power and still not raise the charge stage voltage.

    When the water heaters get up to temp, then there's no place for the power to go, so the system goes into absorb.

    On 110 gallons @ 165°F, and one of those mixing valves on the primary heater that only lets water come out at 125°F, we can easily go a week without any power input to the water heaters. If we get 3-4 nice days in a row where the system produces more hot water than we use, the next day it will probably get caught up and the battery gets an absorb cycle. Or sometimes (usually more in the winter) we get a "Noreaster" off Lake Superior and then the turbines flat out stomp the solar right into the snow bank - there's days we could get 80-100 kWh from those turbines if there was someplace for it to go.
    --
    Chris
  • VicVic Solar Expert Posts: 2,970 ✭✭✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?

    Over the past few years have gone through periods where purposely avoided daily full recharge, primarily to force discharges to a lower SOC, and to reduce the heat dumped into the batteries by the Absorption stage. Initially would do this by just switching the PV input breaker to OFF. This worked fine, although on unattended systems variable loads like A/C added uncertainty to just how many days of no charging would equate to a certain, approximate SOC.

    More recently, beginning about three months ago, have changed to avoiding daily full recharges, by setting the CC's Absorb voltage to Vfloat +0.1 V, and then manually resetting the Absorb V to the correct value for the individual bank every 5-7 days. This was primarily done to reduce battery heating in HOT weather. It works quite well for this , of course, on the full recharge days there is still battery heating, but the average battery temp is quite a bit lower than would result from daily full recharges.

    Regarding the minimum DOD, usually, for FLA batteries the number of discharges vs DOD start at 90% SOC. And it has been specifically stated by a number of battery manufacturers that a "Cycle" should be to a point of 90% SOC or lower. The personal strategy here is to frequently discharge banks to about 75-80% SOC, and less frequently discharge to about 55% SOC, or so.

    Believe that Chris might have been making an indirect reference ("plate erosion") to the SUrrette Bulletin #614, where it states that a battery can be discharged to 50% SOC, and on occasion considerably low (20% SOC), and this is GOOD for the battery, as it forces electrolyte deeper into the pores of the plates, which maintains Capacity, and improves Charge Acceptance, and so on:

    http://www.altestore.com/mmsolar/others/Charging_and_Discharging_BU-RS-614.pdf

    There has been about three versions of that Bulletin over a period of a year or two.
    While this Bulletin was on the Surrette site, some of their Tech folks, would mention that this bulletin was "controversial" within Surrette (Engineering).

    Have gotten the feeling from speaking with Surrette Tech, that every cycle (at least for series 5000 batts) really should be to 50% SOC ... the old thing of "Our batteries are designed to be worked".

    However, deep discharges usually require large RE charge resources, or the willingness to burn fuel for recharges. Everything is a balance.

    Am with vtmaps, generally, efficiency, to me, is not a huge thing, although heating the battery is not a good thing, and should be avoided to the extent possible.

    My opinions, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 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.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?

    The odds of discharging a battery exactly the same amount every day for seven or eight years are pretty slim. Some days it will be less and some days it will be more. So in reality when you're talking about determining the right DOD to use you are talking about an average, not an absolute.
  • ChrisOlsonChrisOlson Banned Posts: 1,807 ✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?
    The odds of discharging a battery exactly the same amount every day for seven or eight years are pretty slim. Some days it will be less and some days it will be more. So in reality when you're talking about determining the right DOD to use you are talking about an average, not an absolute.

    That's exactly right, 'coot. Ours will vary a LOT from day to day - sometimes getting to 85-90% SOC, then the next day only 70% by the end of the day. On the days when we only get 70% SOC the red lights might be flashing the next morning due to the XW complaining about low system voltage. But with the system under load, that's normal. If you take the load off and let the batteries set at-rest for a couple hours they'd pop right back up to nominal voltage, so they're still not under 50% SOC.

    The dang XW, unlike the old SW Plus, complains to high heaven about the voltage dropping below 47.5V, even when it's under load. There's sometimes that we cycle our batteries down to 35-40% - still above the LBCO, but with the Gen 2 hour low voltage auto-start trigger counting down.

    So, basically, there is no way known to man, unless you want to babysit your system 24/7, to cycle batteries to a specific SOC every day. They either got enough energy stored in 'em for the next day, or they don't. If they don't it causes generators to start (and I HATE using gensets to charge batteries). Every system and everybody's loads are different. So it's a deal where one does not fit all. So that's a warning for anybody who tries to copy what I have done - use my setup as a guide only. Don't try to copy it verbatim because it probably won't work. We have in excess of 15 kW of RE generating capacity on the "perfect" RE day. Few off-grid people got that kind of raw power on tap. But that's what it takes in the winter time to bring the system back from the doldrums without running a genset many days.
    --
    Chris
  • VicVic Solar Expert Posts: 2,970 ✭✭✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?
    The odds of discharging a battery exactly the same amount every day for seven or eight years are pretty slim. Some days it will be less and some days it will be more. So in reality when you're talking about determining the right DOD to use you are talking about an average, not an absolute.

    Marc,

    This quote may have been aimed in my direction.

    If it was, when speaking about the target SOC numbers for the banks here, it often will take several days to one week to accomplish, in typical conditions. Often the target SOC is actually measured, or estimated from experience, so in these cases the SOC is fairly well known.

    To reach 50% nominal SOC, this is usually forced by "Waste Lots" (electric heaters, usually heating the outdoors).

    There are heavy loads on occasion, however.

    This may not be responsive to your comment, though. Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 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.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?

    Vic;

    My comment was more generalized, aimed at anyone who might be "reading along" and trying to understand the whole rather than the parts.

    Sometimes we forget we have a "silent audience" that can easily get lost when the technical stuff starts being bandied about. :D
  • PhotowhitPhotowhit Solar Expert Posts: 4,965 ✭✭✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?

    I think the information at GB and Enersys Industrial Battery, has more to do with using large traction batteries in Forklifts, charged at high rates from grid tied chargers.

    Chris has the right idea in minimizing the heating of the battery. I cycle my batteries pretty shallowly, but they also don't heat up too much in reaching fully charged on most days. they do get cycled down to 60%SOC a few times a month. I'll stick with my system charging to full when it can, and not worrying about it. I hope to tell you it's worked well in 13-18yrs.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?

    I find this thread interesting, albeit paradigm altering. We spend top dollar on a charge controller that is supposed to know how to look after our batterys, weve programmed the exact right ending amps to get that perfect charge. And now thats all kind of tossed out ;)

    Here our system is designed to operate without a generator, hence theres tons of spare capacity generally. This winter there was only a dozen days when it didnt float. An average day sees the bank in float by 9am. Cycle depth i would say averages 5%. So these batterys spend almost all of their time at or near full charge.

    I do have a water heater ready, but dont have the inverter capacity, nor dc elements yet. Its on a list. But what about this for an idea. They are agms, so i dont eq. I could set absorb=float, and set eq=absorb, at say 7 day intervals. Would that work?

    On a semi-related matter, the classic says that its charge voltage setting defaults err on the low side, (better to under then overcharge). Turns out midnite was wrong. We have 2v agms, and the classic defaults for agm are 28.8V/ 27.4V. My initial look at the battery specs said the classics voltages would be about right. However after recently talking to the battery manufacturer again they recomended using voltages quite a bit lower 27.9/26.9. Thats a whole volt different. Since lowering them there is now no (feint) squealing during absorb, and no drop in rest voltage.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?
    zoneblue wrote: »
    An average day sees the bank in float by 9am. Cycle depth i would say averages 5%.
    That's not good. From the wind-sun battery FAQ:
    a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. This happens because at very shallow cycles, the Lead Dioxide tends to build up in clumps on the the positive plates rather in an even film.
    Over at batteryfaq.org they say much the same, except they recommend cycles be at least 10%.

    I'm not sure how applicable this advice is to AGM batteries... my understanding (which is limited) is that they tolerate floating better than flooded cells.
    zoneblue wrote: »
    I find this thread interesting, albeit paradigm altering.
    Me too. My batteries are almost 4 years old and are cycled about 20% per day, and for most of the year go through a daily full absorb at 29 volts. My SG is good and my water usage is reasonable, but I am starting to notice more voltage sag with even moderate loads. I take this to mean that I am developing increased internal resistance because of positive plate corrosion.

    Stephendv has written about how the sunny island will not do a daily absorb unless the batteries are discharged far enough. I don't think I can easily mimic that with my controller, but I am hopeful that lower absorb voltage with longer absorb time will have much the same beneficial effect.
    zoneblue wrote: »
    On a semi-related matter, the classic says that its charge voltage setting defaults err on the low side, (better to under then overcharge).

    Over at the Midnite forum I recall boB writing that if one must err, it is better to slightly overcharge than undercharge. I think I agree with that.
    EDIT-- here's the link: http://midniteforum.com/index.php?topic=628.msg4130#msg4130

    I am still trying to figure out this whole battery thing... I am thankful that I have flooded cells and an hydrometer. If I was learning on AGMs I would be a nervous wreck.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • northernernortherner Solar Expert Posts: 492 ✭✭✭✭✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?
    vtmaps wrote: »
    Stephendv has written about how the sunny island will not do a daily absorb unless the batteries are discharged far enough. I don't think I can easily mimic that with my controller, but I am hopeful that lower absorb voltage with longer absorb time will have much the same beneficial effect.
    --vtMaps

    Another useful feature for controllers would be predicting the weather. If that was possible, they could alter their charging and usage patterns based on the weather predicted for the next day or so. For example, if the controller knows that the next day is going to be sunny, it could make fuller use of the battery capacity the day before, perhaps with opportunity or waste not modes. It could also signal to hold back on generator charging if the battery state was getting close to 50 % or so. And if the next day is predicted to have heavy cloud, the controller could charge more aggressively the day before, and hold off on using the opportunity modes.

    Also, would it be possible to monitor the SG of a battery cell, and feed that info to the controller? Ideally, it would be good to know the SG at various levels within the cell, one would think, in case the electrolyte has not recently been mixed. Perhaps then the controller could learn and figure out what the battery needs?
  • NorthGuyNorthGuy Solar Expert Posts: 1,925 ✭✭
    Re: What is the optimal SOC range for cycling lead acid batteries?
    zoneblue wrote: »
    I do have a water heater ready, but dont have the inverter capacity, nor dc elements yet. Its on a list. But what about this for an idea. They are agms, so i dont eq. I could set absorb=float, and set eq=absorb, at say 7 day intervals. Would that work?

    This might be different for AGMs. Flooded batteries need absorptions to mix the electrolyte and avoid stratification. This clearly doesn't apply to AGMs because it's nothing to mix. I would guess AGMs would generally benefiit from lower absorption voltages.
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