Announcement

Collapse
No announcement yet.

Winston absorb

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Winston absorb

    Hello everybody,( sorry for my English I am Italian) Just want to share my past few month of understanding LYP winston cells. I have being playing with them since 6 years or so in a complete hybrid solar/grid system based in Rome Italy. Like u guys I have been a pioneer in this game and I learned as I went. I tried so so hard to retrieve all the info about this cells operating voltage like first charge from factory, absorb voltage,floating and so on. Made many mistakes in the run but at the end I have learned a lot. I feel very confident to say after the past few years winston as exceeded my expectations and I hope it will do the same in the future. I have played very safe in regard of voltage and 0,5 C was my max in both discharge/charge rate. Many and many times even when I understood all the info gathered from the net I had to go back and and read again and again just to contemplate and think about it. I have posted already one of my past bank achievements . If I remember correctly I have performed over 16000 micro cicles and over 500 cycles down to 80% in a period over 5 years. Recharge was performed with 3,45 volts per cell With end current down to 0,25 C at that stage there was no floating, so just interrupting current from panels to mppt and then re feeding current when the SOC was down to 95%. Either wrong or right it worked form me and when I upgraded my bank I performed a full discharge cycles from full down to 0% SOC which gave me the tag capacity. For those who r wondering how I measure the the capacity I use a shunt counting Ah with a precision of 0.01 Ah. Temperature of cells was maximum 33 Celsius in the summer. Now I am here to ask u your opinion about recharging voltage. I use now 3,4 voltage but I have noticed that around 70 % SOC the mppt is cutting lots of power to keep that voltage constant. I thought to increase the absorb voltage to 3,45 just between 70% and 95% and then reduce to 3,40 and let it absorb till current drops down to C/20 and than float at 3,4. My understanding so far is that voltage has a far meaning when within 20/95 SOC. what we should be away from is overcharging or over discharging not over voltage or low voltage when within 20/95 SOC. It's like when discharging at 0,5C and we are near 20% SOC and the voltage drops below 2,9. This is absolutely normal and no harm is given to the battery in terms of cycles because the battery is not being over discharged. What is ur opinion regarding this matter? Thanx so much in advance Simon

  • #2
    Simon if you are only going to 3.45 volts, you are only charging them to roughly 80% SOC. At that voltage you can float them. FWIW you never want to fully charge a LFP battery to 100%. If you do , you cut cycle life in half
    MSEE, PE

    Comment


    • #3
      Originally posted by Barba View Post
      Hello everybody,( sorry for my English I am Italian) Just want to share my past few month of understanding LYP winston cells. I have being playing with them since 6 years or so in a complete hybrid solar/grid system based in Rome Italy. Like u guys I have been a pioneer in this game and I learned as I went.
      Your English is very good, much better than my Italian. You are one of the real pioneers having used LYP batteries for more than six years, over twice as long as I have been using them.

      Now I am here to ask u your opinion about recharging voltage. I use now 3,4 voltage but I have noticed that around 70 % SOC the mppt is cutting lots of power to keep that voltage constant. I thought to increase the absorb voltage to 3,45 just between 70% and 95% and then reduce to 3,40 and let it absorb till current drops down to C/20 and than float at 3,4.
      My maximum charge current is only C/10 and cutoff current is C/50 so I find that the 'absorb' time is not very long, with my setup I don't need a larger charge current and have enough hours of sunshine throughout the year . If you need to get more power into your battery in a shorter time then yes increasing the charge current to 3.45 volts/cell will help and I don't think will decrease the battery life substantially.

      There are quite a few people in Australia who do a bulk charge and absorb at 3.45-3.50 volts/cell, and then float at 3.34 volts/cell which keeps their battery at close to 100% full for the rest of the day. I think this is a good strategy.

      I think that the main factors that degrade the battery when charging is heat produced by the charging process and higher voltages. Both these factors accelerate the unwanted side reactions that break down the electrolyte. When we charge the battery the amount of energy that is stored as chemical energy and available later is around 3.25*(charge current) If we are charging at a voltage of 3.45 volts/cell the amount of energy wasted in the charging process and converted to heat is (3.45-3.25)*(charge current). The amount of heat generated in the battery if we charge at 3.65 volts at C/2 rather than 3.45 volts at C/10 is ten times as much for the higher charge rate and voltage. If some research that I read is correct there is also the fact that the charging occurs unevenly in the cell at higher currents which results in localised hot spots within the battery.

      My understanding so far is that voltage has a far meaning when within 20/95 SOC. what we should be away from is overcharging or over discharging not over voltage or low voltage when within 20/95 SOC. It's like when discharging at 0,5C and we are near 20% SOC and the voltage drops below 2,9. This is absolutely normal and no harm is given to the battery in terms of cycles because the battery is not being over discharged.
      I think it is voltage and charging current that are the important factors at high SOC rather that SOC itself. At low SOC I am not so sure. I think that taking a cell to a low SOC say < 10% for a small period of time will not do any harm, leaving a cell at less that 20% for extended periods should be avoided. I would avoid taking the cell voltage below 2.8 volts.

      Simon
      Off-Grid LFP(LiFePO4) system since April 2013

      Comment


      • #4
        I basically agree with Karrak, but have a tip about the low voltage point.

        IF you discharge below 3.0v, then do just the opposite of the steep discharge "knee" curve to get back to say 3.1v or higher under charge. In other words, reduce current until you reach 3.1v or so while charging, and THEN resume with full power from the panel array. Say start with .01C until 3.1v is reached.

        At very low voltages, intercalation is too rapid with full current applied, basically a championship soccer crowd of ions is trying to go through the turnstiles and get jammed up, heating the electrolyte. Too much pressure at this point is inefficient and harmful in the long run.

        So I take a tip from the discharge knee itself. Under discharge, the battery cannot chemically sustain the voltage when it nears the end. Conversely when charging from a deep discharge, limit the current until your voltage is close to the flat part of the knee, about 3.1v.

        Reducing current automatically when charging cells that have been discharged below about 80% into the steeper part of the knee might be easier to accomplish by just not exceeding that level of discharge in the first place.

        Although intended for EV use, our GBS, Winstons, and low-end versions of the CALBS, share basically the same characteristics of small LFP solar garden lights. They are really "energy cells", and not "power cells", like A123, Headways, and some others, so I try to treat them gently at the bottom, or the top - and not exceeding about .5C like you are wisely doing.

        You are doing great so far. But instead of a possible failure at the 8 year point, I'd want you to go to 16 years by not doing full current charging when below about 80% DOD. Another reason to just not go beyond!
        Last edited by PNjunction; 01-29-2016, 04:25 AM.

        Comment


        • #5
          Yes of course I will not go down 80% from my relative 100%.
          At the beginning I always charged to 3,45 volts and from there count 80% down which result in a static voltage of 3,2 volts with no load.
          Now I am playing with 19 1000Ah winston cells to power my house and garden.
          At the moment I have 3 inverters giving me 24kw power .
          Towards the end of my 20% SOC I see cells dropping down 3,11 at full load( 390/400 amps) but when 20% soc is reached and the inverters stop drawing amps from the bank the rest voltage is around 3,2.
          I think this values are pretty good for me.
          What do u think?

          Comment


          • #6
            Originally posted by Barba View Post
            Towards the end of my 20% SOC I see cells dropping down 3,11 at full load( 390/400 amps) but when 20% soc is reached and the inverters stop drawing amps from the bank the rest voltage is around 3,2.
            I think this values are pretty good for me.
            What do u think?
            I think the minimum voltage of 3.11 volts/cell at full load is fine. The minimum voltage I have taken my battery to in the last few years is 3.04 volts/cell. I wouldn't do this on a daily basis but every now and again is OK.

            Simon
            Off-Grid LFP(LiFePO4) system since April 2013

            Comment

            Working...
            X