Switching from Wet to Lithium?

rpvietzke
rpvietzke Registered Users Posts: 48 ✭✭
Hi,

I have a 12xT105RE 24 battery bank that is 7-1/2 years old. It was probably undercharged and undercycled the first 3 years and I believe is showing its age now both in using a lot more water and in relatively shorter discharge cycles. I gave it an EQ charge off the generator a month or two ago and it didn’t seem to make a difference.  For background, the first few years had 6x250W of charge through a MPPT60. I now have 3 banks of 6x250W panels and three MPPT charge controllers in the winter, so should have plenty of charge capability (off grid in VT).

Unless someone has other tricks, I’m looking at a new battery bank and am curious how people think about sizing. Currently 224 Amps per string times 3 = 672 Ah of T105RE’s.

I’ve been told that I can get away with 2 x 3.8Kw lithium due to the deeper DoD, but it seems a little light to me, both on capacity and perhaps on max draw at any given moment. Are there any lessons learned or tips that I should be thinking about? (Rest of system is a Schneider CSW4024, AGS, battmon, ComBox, 3xmppt60-150)

thanks

Rob

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    edited February 2022 #2
    Converting units:
    • 672 AH * 24 volts = 16,128 WH = 16.1 kWH of Lead Acid storage capacity
    • 2 * 3.8 kWH = 7.6 kWH of Li Ion storage
    It sounds like you have received a "reasonable" life from your Lead Acid Trojan bank... Overall, well treated.

    Using our "rules of thumbs"--We can make some estimates of energy usage:
    • 672 AH * 24 volts * 0.85 AC inverter usage * 1/2 days of storage * 0.50 max planned discharge = 3,427 Watt*Hours of average daily loading (2 days of usage to 50% discharge rule of thumb)
    For Lithium Ion, let's start with with some assumptions.
    • Capacity range (best life): 90% State of charge to 20% SoC = 70% of battery capacity
    • 2 days of "no sun"
    • 70% capacity available * 1/2 days = 35% usage over two days of "no sun"
    • 7,600 kWH capacity * 0.35 daily usage = 2,660 Watt*Hours per day
    That is a bit less than your "rule of thumb" usage estimated from your present Lead Acid bank...

    Next, for the lead acid bank, we suggest 50% minimum planned discharge--But you can go down to 20% SoC on occasion--With the suggestion that the "next day" you have good recharging (sunny weather, genset running, etc.)... Does that "extra 30% capacity" help you for those random times during bad weather that you use that "cushion" of battery capacity?

    With the above Li Ion bank suggestion, at 2 days--You are out of battery power and will need sun and/or genset power for the "third day".

    Solar charging wise... 6 * 250 Watt panels for a 1,500 Watt array. For a lead acid bank, suggest 5%/10%/13%+ rate of charge with 10% minimum for full time off grid usage (technically 10% is recommended by most FLA battery mfg... Li Ion do not have a "minimum rate of charge" specification):
    • 672 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 2,531 Watt array "nominal"
    Array size, you are at quite a bit less than 10% suggested rate of charge:
    • 1,500 Watt array / 2,531 Watt 10% charge = 0.59 of 10%
    • 59% * 0.10 rate of charge = 5.9% rate of charge
    You are a bit under paneled--But with your battery bank life, you have been keeping it relatively happy (state of charge wise)... So you "average daily load" is probably less than 3,427 WH per day (at least during winter).

    The "break even" hours of sun per day:
    • 3,427 WH per day loading * 1/0.52 off grid FLA AC system eff * 1/1,500 Watt array = 4.4 hours of sun "break even"
    For somewhere in Vermont, hours of sun per day:
    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Burlington
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 46° angle from vertical:
    (For best year-round performance)

    JanFebMarAprMayJun
    3.08
     
    4.00
     
    4.49
     
    4.51
     
    4.50
     
    4.72
     
    JulAugSepOctNovDec
    4.89
     
    4.77
     
    4.33
     
    3.41
     
    2.46
     
    2.40
     

    Your break even power usage is only barely exceeded in summer months... Unless you are using a fair amount of generator run time, 3,427 WH per day is probably too high of estimate of your energy usage.

    Lets look at your 2 day available power from Li Ion bank of 2,660 WH per day estimate... And instead of the 52% FLA AC off grid system efficiency, FLA which average at 80% efficiency vs Li Ion of 98% efficiency or overall system eff of ~0.64 overall system efficiency:
    • 2,660 WH per day loading * 1/0.52 off grid FLA AC system eff * 1/1,500 Watt array = 3.4 hours of sun "break even" FLA bank
    • 2,660 WH per day loading * 1/0.64 off grid FLA AC system eff * 1/1,500 Watt array = 2.8 hours of sun "break even" Li Ion
    Again, guessing at your daily loads as being 2,660 WH per day--A lithium bank will be more efficient at charging/discharging, so it will make your solar energy harvest "larger" (makes array 1.25x "larger" due to less battery bank losses). And gets you pretty deep into winter before needing genset usage.

    Also, the charging ability of Li Ion is generally "better" than FLA (higher rates of charge, no "wasted fuel" running for 2-6 hours to "absorb" charge the battery bank... Li Ion typically take "full charge current" until they are at 90%+ SoC, then turn off--Saving you fuel and wear on genset.

    The only functional "down side" at this point--You have a "hard" battery cutoff of 2 days where the FLA had a "soft cutoff" of 2 days and a hard cutoff of 3 days of storage (at ~3,427 WH per day energy usage).

    Other than higher battery costs and a BMS (battery management system requirement/complexity) for Li Ion, the other tripping point for Li Ion is that they need to be kept >~40F bank temperature for cycling... Below that temperature, Li Ion batteries should not be cycled. You will have to keep the banks "warm" during winter for daily usage... FLA batteries, as long as you keep them >50% SoC, they will function well at below 0F very nicely. And cycling them will tend to keep them warm (Li Ion batteries will need heaters/warm room... Not just an insulated battery box).

    In a cold location like Vermont--Winter battery temperature could be a major issue for you.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rpvietzke
    rpvietzke Registered Users Posts: 48 ✭✭
    Thanks Bill,  very complete and helpful.

    Reading your note, I realized I probably had stats on this and went back to look at the combox over the last 3 years. It looks more like this: 25+ days a month are something like .75 to 1.25 kw. These are weekdays and weekends we don’t get there. I have been lazy the last few years and leave a lot of stuff on. Web cameras, remote WiFi switches, etc. When we are there, it looks like the average day is more like 9-12 kw of load. That is probably 6-8 days a month,

    One other note: after a couple of years of being under-paneled, I added two more mppt 60-159’s and two more 6x 265w arrays. We are over paneled in the summer now, and I have one set of panels set up with a knife switch to power a dc mini-split for the times we are there and need a little cooling, haven’t had any concerns about not enough panels in a few years, except, of course, when it is cloud for a few days straight,

    i am actually surprised the loads when we are there are as much as they are, but probably helps sort out why the batteries don’t feel like they hold a charge as much too. As I think about it, three of us bring laptops/phones/etc with us during the pandemic and we are sometimes online for remote work, We’ve grown our loads a lot!

    Rob

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Rob,

    OK, I missed on the size of your array... Should be 3 arrays * 6 * 250 Watt panels per controller: = 4,500 Watt Array

    Yea--That is a lot of panels for your battery bank (672 AH @ 24 volts)... Do you have a remote battery temperature sensor setup on one of the batteries? You want to make sure that you do not cook them with that high of charging current (batteries get hot, their charging voltage drops, charge controller thinks batteries need more charging current, batteries get hotter... potentially overheated bank or worse).

    I like to "do the math" to make sure that all is makes sense:
    • 4,500 Watt array * 0.52 off grid FLA AC system eff * 4.33 hours of sun per summer day (September) = 10,132 WH per average summer day FLA
    • 4,500 Watt array * 0.64 off grid Li Ion AC system eff * 4.33 hours of sun per summer day (September) = 12,470 WH per average summer day Li Ion
    9-12 kWH per day--You are pushing it--But it is possible during sunny summer weather.

    Now you have to look at the battery bank "useful" capacity of 3,427 WH (FLA) to 2,660 WH (Li Ion) per day... If most of your loads are during daytime (water pumping, cooking, laptop usage, even some Mini-Split time)--And you reserve your lesser loads to evening/sun down (lights, entertainment, cell charging, overnight fridge, etc.)--Then you would be OK... And your present FLA bank 7.5 year life seems to indicate that you are not badly treating your present bank...

    You are not going to see a huge improvement in harvest with Li Ion over FLA with a "small battery bank"--Most of your loads appear to be "day time" which has zero battery cycling (i.e., 100% efficient) and only a (relatively small amount) of night time/bad weather usage.

    The smaller Li Ion battery bank would function for your needs as they are (i.e., can supply the surge current much better than even your larger FLA battery bank)--But look at what happens when battery bank hits bottom (~20% SoC) when clouds roll in--Is that going to be an issue for you (2,660 Watts--Then hit BMS shutdown)--Or when you get the warning, either cut power usage to near zero (lights, cell phone charging) and/or crank up the genset to carry you through to the next sunny day (and topoff the battery bank).

    That is something only you can decide... Watching your energy usage and placing loads in "base loads" (those that must run 24x7 (fridge/lights) and those that can be put off to next sunny day or use genset (well pump, A/C, cooking, washing machine, etc.)....

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Horsefly
    Horsefly Registered Users Posts: 470 ✭✭✭✭
    BB. said:
    .....
    For Lithium Ion, let's start with with some assumptions.
    • Capacity range (best life): 90% State of charge to 20% SoC = 70% of battery capacity
    .....

    -Bill
    Not to nitpick you @BB. but I think you are being over conservative on the usable capacity for LiFePO4. 

    I did several discharge tests on some 230Ah LiFePO4 cells, discharging at a constant 25A until the voltage dropped to 2.5V. The graphic of one of these tests is attached below. The LFP discharge curve has an very steep voltage drop from 3.65V (what is considered "full") and 3.3V. Then it is very flat until it gets to somewhere around 3.15V where it starts another steep voltage drop down to the 2.5V "empty" point.  The best way to make sure no single cell in a battery goes too high or too low is try try and stay "between the knees". That is, Between about 3.3V and 3.15V. 

    The cell in the graph actually delivered 238.6Ah in this test. Of that total, there was 2.8Ah from 3.65V down to 3.3V (or 1.2%) and 17.4Ah from 3.15V down to 2.5V (7.3%).

    So the real usable range while still staying "between the knees" in the discharge curve is from almost 99% down to around 7.5%. However, I think it is wise to be a little more conservative to make sure no single cell gets above the 3.65V limit or below the 2.5V limit.  So I think a very conservative range of available energy from an LFP cell is from 95% to 10% SoC.

    Steve

    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    edited October 2021 #6
    I am certainly no Lithium Ion expert...

    I was "derating" for better battery life, not pure capacity. As I understand, charging to 100% shortens Li Ion battery life (as does storing >90% SoC)... There is also the issue of not exceeding maximum cell voltage (as you said--Lead Acid batteries are much more "forgiving" with cell/battery over voltage/over charge conditions--But they have their limits too).

    And the 20% SoC at the bottom was to prevent any one cell from going to 0% or negative SoC which destroys most rechargeable chemistries.

    There are a wide variety of Lithium rechargeable batteries with lots of variation in chemical and physical make-up. The above was just a back of the envelope starting calculation. And will change based on actual batteries chosen. And what type/brand/model of BMS is used.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Horsefly
    Horsefly Registered Users Posts: 470 ✭✭✭✭
    I understand your thinking, and respect your approach. Heck, I think I learned more from you and a couple of unnamed folks here than anyone else when I was looking to build my system in late 2016.

    There is a fair amount of debate about what harms LFP life, but the scientific material seems to say that charging repeatedly to 3.65V does NOT shorten the life, nor does discharging to 2.5V. Fact is, those voltages seem to have been arrived at based on the fact that the do NOT shorten the life of LFP cells. There is a fair amount of literature that says even charging to 4.0V per cell doesn't really hurt LFP. Discharging below 2.5V is more problematic, but staying at 2.5V or above is 100% safe. So repeated charging to 3.65V and repeated discharging to 2.5V is not the problem. (Note that it does cause a problem for cells to be stored long-term at the bottom or the top.)

    Having one cell that hits the lower knee earlier than the others is certainly a problem. Most commodity cells that people can buy from sellers in China are not matched, and are likely grey-market lower grade cells. Some are even used cells. You can't know much about those, except that they will not be matched in capacity or internal resistance. That would argue for a higher cut-off voltage on the lower end.

    The problem is, if you pick 20% SoC as a cutoff, how do you do that? The difference between 25% SoC and 20% SoC in the above graph is 0.001V. The fact that I said 3.15V isn't just too eek out more Ah from the battery, but so that the voltage of the battery has enough slope that it can be used as a reasonable way to choose a cutoff.

    It is really handy that 90% of the usable capacity of LFP (and most other Li-based chemistries) is in the flat part of the discharge curve. Stay in there, and all is well. 
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    No problem... I also try to give a bit of cushion too... As cells wear, their capacity drops, colder temperatures, etc... Don't want to see a "normal daily load" "hit the limits" when new or a year or two old/cold weather/etc...

    But I always try to include my "deratings" numbers and reasoning, so that the orginal poster can adjust them for their user profile.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rpvietzke
    rpvietzke Registered Users Posts: 48 ✭✭
    Update: I made the switch to 3 x 3.8kw lithium @ 24v. I couldn't be happier so far. I had to make sure every schneider component had the latest greatest firmware. (some settings wouldn't take in the MPPT or Inverter charger/battery settings), but other than that the installation was a breeze.  There is something truly amazing about seeing these batteries sit at a constant voltage, under load, in the overnight and cloudy morning for what seems like eternity compared to the way the T105's behaved during a typical day. That will take some getting used to.

    After having disconnected the 3 strings (4x6v) from each other and metering out the T105's, it was interesting to note that two of the batteries clearly have bad cells. The "good" batteries were reading 6.7v, which isn't terrible for the state of charge it was at when I disconnected them, but two of the 12 batteries were down at 4.7v. Ouch.

    Rob
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Rob,

    I think everyone would be very happy to see the details of your new system (and anything you have learned along the way)... What batteries/BMS/what your plans are for winter/cold weather operation with Li Ion/etc... Some photos and such too.

    -Bill "always learning" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rpvietzke
    rpvietzke Registered Users Posts: 48 ✭✭
    I need a day of a cleanup before sharing photos, but will put something together in the coming weeks. Thanks for the suggestion, Bill.
    One of the things I wish I had saved were some of the battery cycle profiles from the T105's -vs- the new simpliphi 3.8M's. They are remarkably different!

    Rob
  • rpvietzke
    rpvietzke Registered Users Posts: 48 ✭✭
    Here are some pictures of the install. The battery box to the left is where the T105’s lived for the last 8 years. Will need to figure out what to do with that once i haul them out of there.


  • nkin
    nkin Registered Users Posts: 26 ✭✭
    edited March 2022 #13
    Greetings Rob, Bill and Horsefly,

    i find this thread very interesting.  Thank you for the public discussion, photos.  Looks good.

    I’m not far away - up in the hills of central VT - thinking about upgrading from aging XW4024, 20 Trojan SPRE 06 255 and only six 170w panels - to maybe XW6848Pro and perhaps 16-22 panels of 375-400w.  I like most everything I’ve read about lithium.

    Past experience with dismal (vanishing) Schneider XW customer support, cursory review of current Schneider XWPro online guidance and warnings heard about the difficulties of lithium BMS software make me wonder if I should try to keep things as simple as possible.  That might mean Rolls AGM batteries —- or pay extra for professional installation (and hoped for good follow through service)?  Especially at age 73, I don’t want to plow into a minefield of BMS maintenance issues.  [Our system has been manageable so far - and first set T105RE lasted ten years.  I’d like to simplify maintenance for when my son takes over.]

    I’d be very interested in thoughts on the practicality of BMS these days, especially for lithium with XW6848Pro.  Has it evolved enough for someone who is not skilled with handling such sophisticated control software?

    Thanks,  Norm
  • Horsefly
    Horsefly Registered Users Posts: 470 ✭✭✭✭
    @nkin - Sounds like you would be a good candidate for one of the pre-packaged 24V LiFePO4 batteries (if you keep the current XW4024) or pre-packaged 48V batteries (if you upgrade to the 48V Schneider).  There is a BMS internal to the pre-packaged batteries, but you don't need to know anything about it. The key will be to change the charge settings of your solar as well as the charge-settings and low-voltage cutoff of the inverter/charger.

    I personally wouldn't pay the premium for the few batteries that operate with Schneider "closed-loop".  There are not that many things that the extra communications between the battery and the inverter/charger provide.  As you pointed out, Schneider builds quality stuff, but they are not very consumer friendly.  Adding another interface just introduces another potential lack of customer support. 
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
  • nkin
    nkin Registered Users Posts: 26 ✭✭
    Horsefly,

    Thank you.  I didn’t realize there are different modes of operation.  Will look into it.

    Norm
  • Wheelman55
    Wheelman55 Registered Users Posts: 233 ✭✭✭
    nkin. I am by no means a pro. I did have Dave Angellini’s help. Dave is invaluable. 

    We used Schneider gear and Discover AES 48 volt batteries with BMS. The Discovery AES battery install is literally plug and play using the Schneider gear. 

    All you do is connect the 4/0 battery cables to XW, plug in the XANBUS network cables from battery to battery to XW, plug in the COM cables from battery to battery and turn on the power. The XW system and AES BMS figure it out from there. 

    We’ve got the older XW+6848 NA, with Schneider MPPT 80-600. Works great. 
    Off-Grid in Terlingua, TX
    5,000 watt array - 14 CS 370 watt modules. HZLA horizontal tracker. Schneider: XW6048NA+, Mini PDP, MPPT 80-600, SCP. 390ah LiFeP04 battery bank - 3 Discover AES 42-48-6650 48 volt 130ah LiFePO4 batteries
  • nkin
    nkin Registered Users Posts: 26 ✭✭
    Wheelman,  Good to learn.  Thanks.  Nkin
  • rpvietzke
    rpvietzke Registered Users Posts: 48 ✭✭
    edited March 2022 #18
    I am a month or so in and absolutely happy with these simplify batteries. They do not have a closed-loop BMS and I went through many of the same thoughts you are going through. I run a schneider CSW4024 and thought about whether this would be the moment to go to 48V with either the CSW4048 or the XW+. In the end, the benefits of "closed loop" and all the hassle of changing things out just wasn't worth it. Ultimately, my CSW4024 is meeting our needs and has been rock solid. 48V would be "nice" for wire size and upgradability, but the reality is what we have works, so it just didn't make sense.  

    Left over from my wet-cell installation, I do have a Schneider battery monitor, but with the Lithium discharge curve and built-in BMS you probably don't need it. One other neat thing I found was this: https://www.ebay.com/itm/151823544093. It allows me to do a low-voltage cutoff for my direct DC loads at the higher voltages that the lithiums want. I have it set at 25.2V for cut-out on the direct DC loads for my wifi router, fiber modem and a couple of other things. The inverter is set for a 25.3 cut out and such. This is meant to protect the lithiums from a full discharge.

    The one thing that could have made a difference on the 48 vdc upgrade if I did this over or was starting from scratch was the charge controllers... It would have been nice to take more advantage of the charge controllers, which I could have done if I was at 48V. Those things are not cheap and it would have been nice to spend the dollars on more panels to collect the sun than the controllers to split up the charging in to little 24V*60A chunks. If you really are doing a major panel expansion, then going to 48V might make sense. (And I am a fan of over-panelling for the December/January time period in VT. In the summer time I take one of the 6x260W array sections and redirect it to a DC-powered mini-split for air-conditioning. In the winter when there is less sun and no need for cooling, I send those panels to a charge controller. This is what the A/B switch over the inverter.)

    I had AGM's on my very first 12v system, then these T105RE's on the CSW4024 and now the Simplify 3.8KW-24V batteries with the built in BMS. I've had only a couple of "heavy load" days so far, but have been immensely impressed with how simple and low maintenance these new batteries are. One simple example is after lugging 6 Gallons a month of distilled water to the basement, I now wonder what I'll do with the 2 gallons that are left over!


    The lithium discharge rates and graphs take a little getting used to. I'm used to seeing wild voltage fluctuations throughout the day and these things won't give me any satisfaction. steady, steady, steady...


    Rob 
  • JRHill
    JRHill Registered Users Posts: 278 ✭✭✭
    rpvietzke said:
    I am a month or so in and absolutely happy with these simplify batteries. 

    Three of my 48v PHIs will be two years old in November. I so liked them that I added a 4th two months after the original three. The total is more battery than I need but the reasoning was that if one of the original three failed and I had to deal with warranty stuff I really didn't want to get by with just two PHIs. But with four of them, they have a really easy life. The heaviest load they see is the start-up load from the 8" bench grinder in the shop.

    I charge them to 55.4v. Absorb is set for .3h and this is usually enough time for the Victron's SOC to reset to 100%. One of these days I'll drop them back another volt. Yeah, its not a true 100% SOC but its close enough. I rarely see the SOC drop below 70% so I should be good for PHIs warranty of 10,000 cycles. Its a knock-on-wood thought but its my hope that I'll never see another battery bank replacement in my lifetime.

    I still use lead cells for the water well. It is a good reminder of the old days when I have to water them up and check SGs. When even one of the eight give up the ghost I'll disable the solar for the well and move those panels to the house.
    Off Grid. Two systems: 1) 2925w panels, OB VFXR3648, FM80, FNDC, Victron BMV-712, Mate3s, 240 xformer, four SimpliPHI 3.8; 2) 780w, Morningstar 30a, Grundfos switch, controller and AC/DC pump, 8 T105. Honda EU7000is w/AGS. Champion 3100. HF 4550, Miller Bobcat.