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  • #16
    I use a shunt to count Ah.
    Where do you read this info of not charging fast below 3,0 volts?
    Thx

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    • #17
      3.0v when you look at most discharge graphs, is at about 80% DOD where the discharge knee starts to drop off.

      At the risk of name dropping, I can't remember if it was Profs Jay Whitacre, Jeffrey Dahn or some other reference advising caution in regards to coming out of the discharge knee slowly.

      Safety concerns about not hitting up your batteries when they are nearly dead abound - cell degradation from sitting around at very low voltages where dendrites and such tend to grow are pretty much everywhere - even in the non-lifepo4 crowd who try to safely revive laptop pulls and other junk cells back to life without setting their bench on fire.

      Aside from the extremes of cell degradation due to neglect, if you catch them in time you still want to bring them back slowly if under 80% DOD. Why? The efficiency of intercalation is not good at low voltages. Basically what happens is that if you hit them with a normal charge current, a massive release of ions tries to intercalate, but the sei layer and whatnot just can't handle the overload. Now the ions have nowhere to go but fight amongst themselves in the electrolyte.

      It would be like trying to go to the world cup championships with only two or three ticket-gates operational.

      The situation is particularly of interest to those of us with "large prismatics", which only have about a 3C burst capability. Part of this is due to the limitations of the materials ability to intercalate. Cells such as A123 are designed to intercalate faster (ie, the nano-phosphate - more nooks and crannies in your english muffin allow for butter to be absorbed more readily!). Our large prismatics while being improved upon, aren't the world's fastest when it comes to intercalation. And at low voltages that becomes a problem.

      This is something I have always wondered if any EV'ers who use larg-prismatics ever took into account when they seemed overjoyed to discover they can take their cells down to 95% DOD, drive up to the garage and HAMMER them at very low soc's! While they may not have been at immediate damage levels, the amount of cell degradation / balance due to poor intercalation during this phase may not be something they thought about. Perhaps it took many cycles off a pack, I don't know - but then again, this isn't EV land.

      I saw that coming out slowly is logical, since it is the inverse of how we get into it in the first place. That is, once you go beyond the event-horizon of 80% DOD, voltage starts to drop pretty quickly. Accordingly, coming out of it is the inverse by applying low current until we reach the flatter part of the curve.

      How much of an improvement taking it easy coming out hasn't really been measured to my knowledge. As 80% DOD users, we'll never know.

      I put this whole subject in the back of my mind until I saw a commercial Lifepo4 charger for the lifepo4 powersports crowd come out from Optimate. These guys aren't stupid. Many of these powersport batteries are abused well beyond 80% (if they don't have an internal lvd), and the Optimate charger detects when a battery is below about 80% DOD, and if so, the charger *gently* brings the cells back to about 3.2v before applying a normal charge rate.

      Once I saw this in action from a respectable manufacturer, I felt it was a very desirable thing to do. First off, don't go beyond 80% DOD. But if you do, come out of the knee slowly. They do, and some of those powersport batteries contain A123, which are very robust in the first place.

      Basically, in our kindler-gentler "Sub-C" application, being nice when merely going beyond 80% for some reason fits in logically, especially for cells that aren't designed for super efficient intercalation at low voltages.

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      • #18
        Originally posted by PNjunction View Post
        Safety concerns about not hitting up your batteries when they are nearly dead abound - cell degradation from sitting around at very low voltages where dendrites and such tend to grow are pretty much everywhere - even in the non-lifepo4 crowd who try to safely revive laptop pulls and other junk cells back to life without setting their bench on fire.
        My understand of this is if you do leave the cells at low voltages for a period of time that the copper foil acting as a current collector and connecting the carbon anode to the battery terminal starts ionising and the copper becomes copper ions. The copper ions can plate out and become copper crystals (dendrites) when the battery is recharged again. The problem is that these dendrites can grow in areas they are not supposed to and cause short circuits or damage to the structure of the cell, also as some of the copper current collector has dissolved its resistance will be higher and some sections of the carbon anode might become disconnected from the battery terminal. This damage can turn the cell into a resistor which will heat up if voltage is applied to it. If the current going into the cell is large enough it will heat up enough to destroy the cell and maybe even cause a fire. This is why you should limit the current going into a cell that has been left at a low voltage for a period of time.

        Aside from the extremes of cell degradation due to neglect, if you catch them in time you still want to bring them back slowly if under 80% DOD. Why? The efficiency of intercalation is not good at low voltages. Basically what happens is that if you hit them with a normal charge current, a massive release of ions tries to intercalate, but the sei layer and whatnot just can't handle the overload. Now the ions have nowhere to go but fight amongst themselves in the electrolyte.
        I can't see how low SOC would effect the rate that the lithium ions could be intercalated (inserted) back into the carbon anode. I would be interested in information that describes the mechanisms you are talking about.

        I put this whole subject in the back of my mind until I saw a commercial Lifepo4 charger for the lifepo4 powersports crowd come out from Optimate. These guys aren't stupid. Many of these powersport batteries are abused well beyond 80% (if they don't have an internal lvd), and the Optimate charger detects when a battery is below about 80% DOD, and if so, the charger *gently* brings the cells back to about 3.2v before applying a normal charge rate.
        I would think that Optimate are being careful here. They have no way of knowing if the battery they are charging has been left for a period of time at a low voltage or not, so assume it has been and start charging it up slowly. If the battery is damaged and turns into a resistor, by limiting the current they are limiting how hot and therefore how dangerous it will get.

        Are you sure they are detecting 80%DOD? I would have thought they would be looking at a voltage rather than %DOD.

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

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        • #19
          Karrak - you said it best!

          The key issue is one of time. You can take cells down to near nothing (provided they don't go into reversal), and recover gently, provided you get to it in time, like asap, and recover gently before the parasitic reactions have started to really do damage. Even if you do catch it asap, it is wise to recover gently - again being conservative in nature.

          Even at very low currents, highly discharged cells tend to recover quickly enough back to 3.2v to be practical even at C/50, so there really is no need to hammer them when they are this low. The process is amazingly fast, so no need to give them the bum's rush.

          You are correct about the Optimate. As a 2-terminal charger for a 4S battery, they are going by voltage as an approximation to determine 80% DOD. RESTED voltage that is. Upon recharge they watch more than just voltage, (IR, charge slopes etc) and if the battery falls out of the ideal charge slope for lifepo4 as programmed in firmware, it knows that something is up and will stop.

          They are being conservative, but wise about it considering that it is only a 2-terminal charger. AND, being especially conservative when these powersports cells can contain up to 16 individual cylindrical cells - which they are honest about saying that when going over 4 cells, the effectiveness of the monitoring can be made more difficult by a rogue cell being hidden by so many other good ones. That honesty was refreshing, and stating that up front to the consumer tended to make me take their charge procedure a bit more seriously.

          So this was an indication of how being conservative was a wise thing to do regardless of the cell construction. It is an indication that being conservative with both DOD and being conservative while in *either end* of the charge discharge knees should lead to a longer life.

          I'll try to pull up that reference with more digging if I can find it.

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          • #20
            Without the ability to edit, did I seem to repeat, repeat, repeat myself there a little bit?

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