Discussions about LiFePO4 cells and Battery University

Discussion in 'Prius PHEV Plug-In Modifications' started by dan2l, Dec 16, 2012.

  1. wb9k

    wb9k 09 Gen II Prius w Hymotion Plug-In Batt

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    I don't believe active balancing removes the usefulness of CV at top of charge. It just makes charging and balancing more efficient--and not by as much as you might think. 50% or more of charge is lost as it gets shuttled through the capacitors that carry it from cell to cell. The CV portion of a charge cycle addresses differences in cell impedance as much as capacity. This becomes more and more important as a pack ages. An HVC of 3.5 sacrifices the nth degree of capacity, but the longevity gains are surely well worth it, and good balancing should be possible anywhere above 3.45 or so. I would prefer this methodology to bottom balancing, which usually will sacrifice even more capacity and requires much more sophisticated software capable of assessing cell SOH (State of Health) dynamically over the life of the cell.
     
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  2. NortTexSalv04Prius

    NortTexSalv04Prius Active Member

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    wb9k
    In regards to A123 and you I will defer ......for the most obvious reason/logic being that you have direct working knowledge. In large part I will agree with all of your statements. However, where does this leave the owners(myself)
    of RealForceEnergy packs aka RFE 24vdc 80ah Lifepo4. I am going to follow the OEM specs for charge/discharge of that HV lifepo4 pack. That being of a top balance method process. The stipulations I have to the OEM methods is some owners have likely overcharged their packs .Root causes being a poorly design/built BMS and unattended charging of packs. Additionally the OEM specs are weak at best...........
     
  3. wb9k

    wb9k 09 Gen II Prius w Hymotion Plug-In Batt

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    I agree the specs are not generally well written; they tend to be much too vague. There can be many reasons for this, but most common seems to be a lack of knowledge on the part of the manufacturers themselves, or a lack of a sense of responsibility. A123 is unique in that we try to deliver whole systems to customers rather than just supply cells and leaving it to the customer to figure out how best to use them. I would imagine cell makers that don't do this are both less able and less interested in providing details on how to best manage a pack. That's not to say this is OK, or that A123 has done a great job in this area either....the resulting confusion over what is OK to do to a cell or not is one of the main reasons I post here. It's bad for EVERYBODY for this kind of mysterious atmosphere to reign. The industry needs to do a better job of getting this info out there and of conveying the subtle complexities of electrochemistry that come into play when designing a control system.

    That said, I see nothing in the RFE spec that says it's OK to charge above 3.65 under any circumstances--so why are guys doing it? The specs also show standard discharge at 1C down to 2.0 Volts, which seems to directly contradict the practice of many here who are afraid to ever dip below 2.5 Volts, even under load. Maybe other docs out there conflict with those provided here by lopez, but it seems to me if users had just been a little more conservative in their approaches and obeyed the same limits as RFE does in their own spec, the apparent Li plating issues folks have had could have been avoided. This isn't damage from top balancing, per se. It's damage from top balancing done improperly.
     
  4. pjc

    pjc Member

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    Great thread. I recently had a cell in my RFE start to show signs of plating. I've only had the system for ~1 year. I rarely discharge the pack completely, so it seems pretty clear to me that the problem is overcharging, especially since the balance function doesn't work. Manually balancing is for the birds and very difficult to keep it truly balanced (as opposed to a rough approximation of balanced). But it sounds to me as if the overcharging problem could be mitigated simply by lowering the HVC level of the BMS, albeit with some loss in capacity (a bad cell causes a much larger loss in capacity, though!)

    Does anyone know if you can adjust the HVC voltage in the Enginer BMS16D (v4)?
     
  5. dan2l

    dan2l 2014 Prius v wagon

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    Hi wb9k,
    Thanks for this coment and a NUGET.

    You state capacitors. In acutallity all the top balance BMS systems I have seen are not capacitors but rather resistors.

    Clearly in my mind, capacitors shuttleing power to the lower cells would make the top balancing work way better. What happens with systems like the Mini BMS and the Pacific EV BMS is that the high cells are slowly drained but nothing is added to the low cells. Then you eventually get to 15 cells at 3.6v and one cell at what ever is left at the charger CV voltage. So....

    15 x 3.6v=54v. So a CV level of 55v or 56v will never top up the lowest cell. 57.6v would be required to get that last cell balanced at the top. In acuallity at 57.6v you will eventually settle out with a distribution of cells between 3.5v and 3.7v However. The charger is now trickling charge into the pack with the lowest cell still comming up and the highest cells in danger of plating. The real problem is how to get the last cell up without the highest cell getting some plating.

    If we had cappaictors in the BMS we would top balance much better.

    So my question is if there is any BMS on the market that has cappacitors and transfers power to the low cells as you have indicated. Maybe this is the answer to making our systems more reliable over time.

    Thanks,
    Dan
     
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  6. wb9k

    wb9k 09 Gen II Prius w Hymotion Plug-In Batt

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    Hi Dan, (What's a NUGET?)
    Resistors are used in systems with "passive balancing". In "active balancing", excess charge from high cells is shuttled through capacitors to adjacent cells. This is a separate idea from top or bottom balancing. Note that while this is more efficient and can achieve a bit of range improvement while driving in some cases, it's not always as neat and clean as we would like. Maybe the energy transfer you want to make is not from one cell to the one right next to it. This means the charge has to be shuttled across some distance in the pack, one cell at a time. The losses can add up quickly to the point where the extra expense hardly seems worth it. I've devised a scheme that I think is better in at least some applications, but I digress...

    The simple answer is to make your CV current no greater than that which can be drawn off by the balancer load, and to limit all cells to a max of 3.6 during the CV portion of charging . This way nobody can be overcharged to damage, but the downside is that the straggler takes forever to catch up. That's why we want to balance as often as possible--so things can never get that far out of whack to begin with. Passive balancing can work very well--we make a hybrid bus pack with 192 series elements in it. Even after considerable aging, the passive balancers and BMS in that pack usually give you a pack with <5mV of delta from max cell to min cell. 2mV spread is common in that system. This is a 650V, 18 Ah pack with balancers that can draw only about 300 mA--and it's not a plug-in either. These systems CAN work remarkably well.

    I've seen the concept on paper, but don't know of any systems that actually use it. I've heard that we are developing systems here with active balancing, but I don't expect to see one in production any time too soon. Maybe some others know of an available system with active balancing.
     
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  7. dan2l

    dan2l 2014 Prius v wagon

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    Hi wb9k,
    Thanks for the added comments. I am not knowledgable on the active system hardware so my thinking on that was only theoretical.

    So the typical system I have seen has 250-400ma drain resistors, but the chargers do CV ramping down from 15a to about 2a and then turn themselves off. During that ramp down, an out of balance pack will always push the highest cells too high. Now, the Pacific EV BMS could be set to 3.65v and then be safe if the Zieners were adjusted to turn on the drain resistors at a lower voltage. But it would then be cycleing the 120v ac on and off for hours (or even days). I expect that sooner or later that power cycling would break some thing.

    Thanks again,
    Dan
     
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  8. lopezjm2001

    lopezjm2001 Senior Member

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    Even if you managed to set the BMS16D HVC to a lower value of 3.6 volts you still have a problem in that the BMS16D draws different current values from each cell causing imbalance. Last I heard Enginer service was still advising their users to manually balance their RFE packs (before they shut up shop).

    I expect that the HVC value would be set in a firmware download. Only Enginer can do this or maybe MD-Tech.
     
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  9. wb9k

    wb9k 09 Gen II Prius w Hymotion Plug-In Batt

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    Ok, I gotta know....why is this?
     
  10. lopezjm2001

    lopezjm2001 Senior Member

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    Each cell voltage is measured using a voltage divider and a ADC. Each voltage divider draws a different value of current. The BMS16D was originally designed to do permanent active balancing so the imbalance was always being corrected by the permanent active balancing.

    Over time there were issues with the BMS16D and newer improved versions. Then the active balancing was made switchable via a jumper setting on the PCB and was delivered with a jumper set to disable balancing.
     
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  11. lopezjm2001

    lopezjm2001 Senior Member

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    Hi wb9k,

    I have a question about the A123 20AH pouch cells. I have come across two different C ratings for the same A123 20AH pouch cell and I would like to know which one if any are correct. I know that you are bound by a A123 NDA but maybe you could make a comment.

    A seller on Ebay quotes : "Discharge power: 3C Continuous, 10C Peak (10 sec)"
    16 A123 20ah Nanophosphate® AMP20M1HD-A | eBay

    A seller on Alibaba quotes: " Continuous discharge C-rate: 5c, Peak discharge C-rate: 20c" Lifepo4 A123 20ah Prismatic Cell - Buy A123 20ah Cell,A123 20ah Prismatic Cell,20ah Prismatic Cell Product on Alibaba.com Read more: Discussions about LiFePO4 cells and Battery University | Page 4 | PriusChat

    Thanks,

    John
     
  12. NortTexSalv04Prius

    NortTexSalv04Prius Active Member

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    This old discovery youtube video explains a mfg process which looks domestic USA mfg. for cells possible A123

    Wb9k if you could or possible comment on this thanks also just curious what is mindset on Headway round blue lifepo4 15ah cells and Calb prismatic lifepo4 100ah
     
  13. wb9k

    wb9k 09 Gen II Prius w Hymotion Plug-In Batt

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    This video is definitely not from A123--there is no Li metal in our cells at all. All our Li is suspended in the electrolyte in an ionic state, which I believe is the case for all LFP chemistries. Interesting stuff though.

    I have no familiarity at all with the other cells you are asking about, but given a choice, I would tend to shy away from very large capacity cells unless they come with a really good warranty. Cell anomalies and defects can have a larger potential impact on the performance of your pack than would usually be the case if you used several smaller cells in parallel. Cell impedance will generally be higher as well, but this may be OK depending on your exact application.
     
  14. wb9k

    wb9k 09 Gen II Prius w Hymotion Plug-In Batt

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    Hi John,

    Hard numbers are difficult to produce for a naked cell that can be packaged any number of ways, but if you go to

    Prismatic Cell | Pouch Cell Battery 20Ah | A123 Prismatic Cells

    there is a link for getting the data sheet for the 20Ah pouch cell. Similar sheets for other cells are also available here. You have to submit some info to have these emailed to you. The sheet does not give a continuous discharge rating, but does show (indirectly) a 10 sec pulse capability of 20C. A continuous discharge capability of 5C seems reasonable to me, though operating like that all the time will cost you calendar life. The spec sheet shows capacity derating over some 3200 cycles at +1C/-2C. Hope that helps.
     
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  15. NortTexSalv04Prius

    NortTexSalv04Prius Active Member

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    Okay however please explain these issues if possible. A123 has overseas mfg(aka china) now own by wangzang . The A123 pouch lifepo4 20ah is the product I have in my PHEV enginer kit or maybe a china (knockoff).. ...with a 24vdc 80ah Lifepo4 unbalance pack what technical suggestion do have to rebalance. My thinking is one of several . ONE teardown pack and balance each cell(which will be a pain in the bootie) TWO attempt to balance /charge the whole pack...?Since this is a 8S jst connector to the BMS and the with 2-7s connector okay but black negative 8S darken and cellog could not make contact can I just replace the jst connector...................?
     
  16. wb9k

    wb9k 09 Gen II Prius w Hymotion Plug-In Batt

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    1) It's spelled Wanxiang, pronounced approximately "wahn-SHUNG", and the entity that own A123 LLC is more specifically known as Wanxiang Clean Energy USA.

    2) Ownership of A123 has nothing whatsoever to do with any technical question anyone may have, so why do you bring it up?

    3) To my knowledge, A123 has only manufactured prismatic cells in two locations--at Enerland's now shuttered facility in Korea and in Livonia, MI. Only cyclindrical cells have ever been manufactured in China by A123, along with powder for coating electrodes. No need to fear that your A123 prismatic cells were made by the dastardly Chinese.

    4) The technical portion of your comments is indecipherable to me. To me, a "24VDC" A123 module is 7s, not 8s. Let's say your pack is 8s....how is it constructed? Can't you get to the busbars that tie the cells together? What are you using for a BMS? How far out of balance are you? What does "black negative 8S darken" mean?
     
  17. pjc

    pjc Member

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    Nortex, are you sure your cells are A123? If you got them from Enginer (Jack Chen), to my knowledge he never sold A123 cells. Maybe you have the RFE packs (pouches in the stainless steel boxes)? I think they are different manufacturers...
     
  18. NortTexSalv04Prius

    NortTexSalv04Prius Active Member

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  19. NortTexSalv04Prius

    NortTexSalv04Prius Active Member

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    Yes these are RFE 24vdc 80ah that has an issues. The molex(bms connector ) has shorted on one the pins.??
     
  20. NortTexSalv04Prius

    NortTexSalv04Prius Active Member

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    Sorry bout the grammar and mispoken statements. Sad thing is the A123 tag /identity has been so mispoken (by others) one pouch lifepo4 20ah cell from other mfg. If anything A123 trademark/patents, copywrights have been walk all over. My cells are Powin or some1 else that idk. I still have the issue of two battery packs Lifepo4 24vdc 80ah that have been overcharge and a teardown will be required. Any instrutment suggestions are welcome.......................
     
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