Discussions about LiFePO4 cells and Battery University

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

  1. dan2l

    dan2l 2014 Prius v wagon

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    Hello everyone,
    In another thread, wb9k posted a link to.....

    I would like to have some discusions about LiFePO4 cells and about the information at Battery University. I think it would be best to do this in a thread on that subject, So I am starting this thread.

    Thanks,
    Dan
     
  2. dan2l

    dan2l 2014 Prius v wagon

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    One of the thinks that I would like to understand is what is happening when we top balance. I have heard and read many differing opinions. Some say that it is OK to run up to 3.8v or even 4.0v as long as you do not overheat the cell. Others say stay at 3.6 as the top charging limit.

    So my question is, What is happening inside the cell?

    The obvious is that if you stay at high voltages over time the cell heats and you get heat damage. The electrolite turns to a vapor and the cell swells and the spacers can melt.

    But what happens if you cut the current way back and keep it from heating while you give your top balancer the time it needs to drain the high cell?

    I have done this and it does seem to shorten the life of the cells. Is it Plating? Is it vapor bubbles?

    Thanks,
    Dan
     
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  3. wb9k

    wb9k 09 Gen II Prius w Hymotion Plug-In Batt

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    Hi Dan,

    Good questions, and it occurred to me a couple days ago that I should clarify what I've been saying here. 3.60V should be considered max for rested OCV. Charging Voltages up to 3.8 are generally considered permissible, but you don't want to stay there long enough that your OCV will relax to a point above 3.6, or you will suffer some level of Li plating. It may not be severe, but it is happening. If you are dealing with very high charge currents, pushing this envelope becomes more and more risky. Exceeding terminal Voltages of 3.8 is never advised, IMO, and our starter batteries are set up to never allow cell Voltages to exceed 3.8 under any circumstances. Li plating becomes more severe, and as you approach 4 Volts the electrolyte will begin to break down (again, the exact Voltage at which this begins and how severe it is depends largely on charge current). Above 4 Volts, electrolyte breakdown should be considered a sure thing. If charging current is high enough, gas pressure can build fairly quickly and cell rupture may occur. Add an ignition source at the time of rupture, and you could have some real trouble, but in most cases the event is pretty benign--often not even becoming apparent until many hours or even days later. The gas that builds up in the cells is mostly CO2, so it actually can have a fire retardant effect as it vents, but don't count on that. In some cases, the cell may remain intact and usable, though with noticeably decreased capacity--this is more common with cylindrical cells which typically take a little more internal pressure to crack open.

    Because I'm a hands-on guy who knows how things go in a lab, I always recommend float charging at 3.6 and just waiting for the cells to stabilize at that Voltage. You can save some time with a higher charging Voltage, but if you aren't baby-sitting your setup you risk damaging the cells while you're not looking--an expensive lesson you'd probably rather avoid. If you do experience a cell rupture, never touch the electrolyte with bare skin, allow it into your eyes, or breathe the vapors. It is a serious irritant, though it is not carcinogenic nor is it known to cause birth defects. If you must handle a vented cell, wear nitrile gloves. Do not let fumes build up in an enclosed space, use good ventilation of your workspace. And, as with any energy storage medium, have good fire suppression on hand. I like Halon or CO2, but any ABC extinguisher is OK. Do not use class D extinguishers for these cells.

    Hope that clarifies some things.
     
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  4. dan2l

    dan2l 2014 Prius v wagon

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    Hello wb9k,
    Thanks for the explaination,

    As for some of my details, I have two Enginer systems. One with the Pacific EV top balancing, On with the Enginer BMS.
    I have some Mottcells with internal resistors to drain at 3.6v and some Thundersky's and some Realforce cells.

    All of these have probably see some plating. I see that they "spike" more at ent of charge and end of discharge, but if I cut the currents way back then I can still get good cappacity out of them. I assume this is typical for cells that have plating. Is that true?

    Anyway, my next question is if there is any way to improve the performance of a cell that has had some Plating? Will the Plating desolve away over time? Would mildly exercizing the cell help it?

    Thanks,
    Dan
     
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  5. wb9k

    wb9k 09 Gen II Prius w Hymotion Plug-In Batt

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    Your observations are true, whether Li plating is present or not, and your strategy for dealing with it is spot on. Good work.

    Everyone has always told me Li plating is 100% permanent. My understanding is that there is no way to reverse the process. Please bear in mind, I have zero hands-on time with the other cell makes you mention here and cannot speak to what their precise behaviors will be.
     
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  6. dan2l

    dan2l 2014 Prius v wagon

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    Hi w,
    These makes of cells are the various brands that Enginer has used over time. They are all LiFePO4 and I beleive they act symilarly to the A123 20ah prismatic pouch cells. Except that the supplier quality control may, or may not be at a symilar level.

    Anyway, I am also a "hands on guy" and a Mechanical Engineer. I have lots of data that I can share if we get to needing that.

    Let me discribe what the Enginer system does so that everyone can better understand why I think I have plating. The Basic Enginer system is 16 cells, 80ah each. It charges at 15a until some voltage. and then goes into constant voltage mode. Some systems at 55.8v and some as high as 58.3v. The discharge is at about 55a but as the pack voltage droops, the converter draws more current. Generally the converter turns off at about 46v and I have seen as high as 85a at that point. The Enginer BMS attemped to balance at the bottom but that did not work, so they turned balancing off. The BMS does turn off discharge if a cell hits 2.5v and turn off charging if a cell hits 3.8v.

    So it is typical for balance to drift and eventually the system becomes useless with one cell going high and another cell going low. Users generally manually balance or use a Mini BMS or a Pacific EV BMS.

    Let me discribe the Pacific EV BMS becuase I have hands on experiance using it. This system uses CellLogs to read cell voltages. Pacific EV settings are generally set at 2.7v and 4.0v. These give an alarm that is used to turn of both charge and discharge. Also there are draining resistors that turn on to drain any cell that goes above 3.6v. The level of draining is about 1/3a. This draining of a cell happens both when the charger is on and when it is off.

    So the sytem is charged and if the total voltage hits the charger top voltage, the charger turns off and it all stops. But if a cell is High, when that cell hits 3.6v the drain resistor turns on. Then we have 15a into most the cells and 14.67a going into the high cell, so it goes up to 4.0v and then the alarm turns off the charger. Then the low cells hold at their rest voltage and the cells that are still above 3.6v continue to drain the 1/3a. As the voltage goes to a "rest voltage" and the alarm goes off. After a little delay, the charger is turned on again and it cycles from then on. I think it is generally OK until the charger goes into constant voltage mode and the current drops. At that point the rest voltage gets closer and closer to the power on voltage because the curent levels are lower and lower. Eventually the rest voltages will be above 3.6v for some cells.

    I think that this type of a system is doomed to always have some point in the balancing process where we are at risk of plating. I can and have adjusted the alarm voltages but I fear that I can not acomplish risk free balancing without manual intervention.

    So what do you suggest for an automated balancing system so that we don't hive this plating process.

    Thanks,
    Dan
     
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  7. wb9k

    wb9k 09 Gen II Prius w Hymotion Plug-In Batt

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    You are learning that balancing is not always as simple as it looks. What will be the best scheme depends both on what cells you are using, and the nature of the application they are used in. The system you describe above is pretty typical of many balancing schemes I am familiar with. For A123 cells, I would change the 4.0V alarm level to 3.8 and probably leave it at that. The cells should get to 3.6 Volts for each cell group with few or--ideally--none of the balancers ever turning on. You are on the right path.
     
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  8. dan2l

    dan2l 2014 Prius v wagon

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    Hi W,
    I have done this. My experiance is that what you are suggesting is that it is fine for a while, But over time balance drifts. The Balancers do turn on. Then either we get to being at risk of plating or we have 15 of the cells getting into balance but one that just does not make it.

    I am now running at 3.85v but I am sure that I have some plating risk.

    Try as I might, I am seeing that cells only last about 3 years. I am guessing 600 cycles. Slowly over time I am not able to get the same capacity out of the pack. I now have 6kwrh of cells that droop to the point of being depleted after using about 2-3kwhr. This is all at below 1C.

    Thanks,
    Dan
     
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  9. wb9k

    wb9k 09 Gen II Prius w Hymotion Plug-In Batt

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    We should expect that the balancers will be required to do their job sooner or later, and that as time goes by they will do it more and more often. Li ion batteries--like all other types of batteries produced thus far--degrade over time, even when there is no Li plating going on inside. There are many possible causes for cell degradation, ranging from manufacturing defects to environmental abuse, to poor pack management. The only way to have a real chance at knowing what did a cell in is to tear the cell down post-mortem. In A123 cells, Li plating looks like a whitening of the black active material on the plates. Other makers, I wouldn't know. Li plating can also happen during charging at low temperatures, but it would have to be pretty darn cold to be a problem at 1C--at least with A123 cells.

    You definitely are having lifespan issues if you lose 30 to 50% of your capacity in 600 cycles. 10 to 15% wouldn't be shocking under certain operating conditions, but at 1C or less, even that seems severe. Are the cell capacities fading equally across the packs, or is it just a bad cell group here and there limiting the rest of the pack? Any cell teardowns? How far out of balance have these packs been allowed to get, and has this led to greater chance of cells being exposed to overcharging or overdischarging? I'd need quite a bit more detail before I could make anything like a confident statement about what may or may not be your problem.
     
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  10. dan2l

    dan2l 2014 Prius v wagon

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    Hi W,
    They all degrade symilarly, but of cource the worst one shows up as realy bad. If it is replaced then the pack goes a little longer and the next wost shows up looking really bad.

    2Z16.jpg
    This is today's data showing #11(red arrows) going bad after 40-50ah on 120ah cells. It may be hard to see, but the full file with data in about 12meg in size. I can Email all the data to you if you get me your Email adress.

    I have never opened a cell. I did not want to get into the mess.

    Thanks,
    Dan
     
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  11. lopezjm2001

    lopezjm2001 Senior Member

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    I really appreciate your experienced input to this forum. Could you please provide a photo of what Li plating looks like compared to one cell which does not have Li plating.

    Do you have access to a full specifications data sheet for the A123 20ah pouch cell, does one actually exist. The one provided by the A123 website is very defficient.

    Yeah, I am probably asking for too much.

    Thanks
     
  12. wb9k

    wb9k 09 Gen II Prius w Hymotion Plug-In Batt

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    Can't blame you for not wanting to cut cells open. It's not something that should be undertaken casually.

    On to the data....

    It's not clear to me whether or not all this data comes from the same charge/discharge cycle or not, but if the "end of charge" and "end of discharge" windows are typical (same cell hits top and bottom of charge first all the time), then you have indeed identified the weakest cell in the pack. Did you manually balance in the end-of-charge window where cell 11 is held flat around 3.495 Volts? You can do that, but it's easy to overdo it and overcompensate, and this can make your results harder to interpret properly. However, looking closely at the charts suggests to me you did OK here and you are interpreting properly and cell 11 is the weakest link. Do you know the capacity of the strongest and weakest cells in the pack?


    A few comments--the mismatch here doesn't look that severe to me, but maybe a straight 1 or 2C steady discharge would look worse. I wouldn't call this a case of one radical outlier--many cells are showing degradation in varying degrees. The change in the shape of the curve for cell 11 is a strong indicator that there have been changes in that cell's characteristics--can you measure individual cell group impedance? Do the same cell positions seem to fade fastest, or is capacity loss completely random wrt pack position? Not sure what the minimum allowable Voltage for your cells is--I would have no qualms about taking A123 cells as low as 1.5 Volts under load. Your upper balance points look good, I see no reason to think Li plating from overcharging is much of a problem for you, if at all. Are the cells really 120Ah each (or are there, say, 4 30Ah cells in a parallel group)? Several smaller cells in parallel is preferable to one big cell, IMO, because any variation in cell quality is going to be exaggerated with one big cell. One bad cell will cause the pack to decline much more quickly if that bad cell makes up a whole series element all by itself.

    How confident are you in the absolute accuracy of the Voltage measurements you're getting here? Is this the exact same data the BMS is using to make its decisions for balancing? Precision is very important with this, and erroneous readings can cause the balancers to mismanage cells.
     
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  13. wb9k

    wb9k 09 Gen II Prius w Hymotion Plug-In Batt

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    I'm glad you realize the sensitive nature of your requests. :) I am not able to post any A123 documents here, and I am equally reluctant to post photos from lab reports. I tried to find a photo on the internet, but all I found were photos taken at very high magnification. There really isn't too much to describe here. A healthy electrode will have an even black sheen to it when first exposed to the air. The electrolyte evaporates quickly, right before your eyes, and the black then has a slightly lighter "dry" look as opposed to the fully black "wet" look it has when soaked with electrolyte. If Li plating is present, the black color will be uneven, with a whiteness that ranges from semi-transparent to fully opaque over the black surface of the electrode. The plating is never evenly distributed, IME. In more severe cases, the black active material coating may be flaking and peeling off the electrode as well.

    This causes not only loss of capacity, but also impedance rise in the cell because now the Li that can still be cycled through the cell has to take a more and more tortuous path through the cell to do it's work. The plated areas are effectively dead, and the Li plated there is no longer available for cycling. That said, correlating impedance rise with capacity loss is, as far as I've been able to tell, an exercise in futility. The relationships between standard parameters as a cell degrades are wildly unpredictable. This can probably all be unraveled, but I believe doing so would be a major undertaking requiring teardown and detailed analysis of hundreds of cells that have been exposed to carefully controlled and documented abuse conditions.
     
  14. dan2l

    dan2l 2014 Prius v wagon

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    Hello W,
    Yes, all the data is from the same 15 hour cycle shown in the "All Data" chart. The End of charge and End of discharge charts just allow changes in scale so that I can better focus on the detail.

    I did not do any manual balancing. In fact there is no balancing at all in this 15 hours. Days prior to the data, the Pack had been top balanced. The 5 charge pulses at the begining were because the converter had been turned on for a short time every 20 minutes to top up the Prius HV battery. When this short energy transfer is done, then the charger comes on to top up the Enginer pack again. This is done with out any top balancing resistors. Then you see the drive with the discharge in 3 intervals. The "end of charge" is the third interval. I drove a total of 27.5 miles geting 93mpg on the Prius display.

    After the 3 drive discharges, there is a full charge for about 5 hours. You see that while the charger was putting out 15a, the voltage on cell 11 rises. When the Charger goes into constant voltage mode, the current slowly ramps down and the voltage of Cell 11 goes flat at about 3.495v. In the end 14 of the cells spike up in voltage together and turn the charger off. Note that during this time cell 11 and cell 16 do not spike up in voltage.

    Yes, I am confident in the voltage readings. It is from 2 CellLogs. I have about 6 CellLogs and have changed them and check calibration on ocation and they have always been reliable.

    Thanks,
    Dan
     
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  15. wb9k

    wb9k 09 Gen II Prius w Hymotion Plug-In Batt

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    OK, I kind of figured that, but wanted to be sure the data was of a single cycle (not just from a single day). I'm much more used to looking at data from HEV's that are not top-balanced....getting zeroed in on your data, I can see we have a top-balanced PHEV pack with SOH ranging from near 100% to ~50% (OK, I take one comment back here--that's a moderately severe spread, but am I correct in seeing that the worst cells are 9, 10, 11, and 12? If so, that could be significant--thermal issues come to mind as a possibility right away. Knowing the larger history of the pack would be helpful [what cells have been replaced and when, etc.], and while I'm asking for more info, I'll just note that I asked several questions in my last post that you've left unanswered. If I'm off course with my line of questioning I need to know so I can actually try to be helpful. Please respond one way or another to any questions I may ask.

    Reading on into the next paragraph, things get more interesting:



    OK, a picture is crystallizing here. The data is making more sense to me now...Are you sure there was no balancing at all? Several cells exceed 3.6V multiple times, albeit briefly. Just asking...the devil's always in the details with this stuff, hence all the questions. The phrase, "This is done with out any top balancing resistors," is a big red flag for me. Are you saying that this pack runs a cycled float charge (for lack of a better term) during which balance resistors never come on no matter what? Can the pack top Voltage float around? How much? I'm wondering what the data looked like a couple days ago prior to balancing.



    Your description clarifies a lot. Is that cell 16 or cell 12? Either way, this behavior the two cells are exhibiting usually indicates a high self-discharge condition in the cell. It may be only intermittently "leaky" based on SOC or temperature, but it's just as likely constant and only plainly visible "at the tails" where Voltage changes fast enough for us to notice in a pack that gets used every day. Again, understanding of long term behavioral cycles (if you manually balance from time to time, there must be cycles) through your data is important.



    It's great that you are taking all this data. Do you log every day in the car like this? What else are you collecting besides Voltage? I'm not familiar with "CellLogs", must be some kind of tool for taking pack data?
     
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  16. lopezjm2001

    lopezjm2001 Senior Member

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    Thanks. I guess it was worth asking. :) I expect that maybe you have signed a NDA. However A123 has become bankrupt and since the cells have ended up on the hobby market and the general public I think that A123 are obliged to publish a full data sheet for these cells so people like myself have a better idea of what we are dealing with in terms of designing and building a large A123 battery pack. At least a MSDS.
     
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  17. wb9k

    wb9k 09 Gen II Prius w Hymotion Plug-In Batt

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    If A123 was actually dissolved it would be one thing, but it's not--A123 has simply reorganized and is continuing on under new ownership. Yes, I've signed an NDA as has everyone who has ever come by anything from A123 through legitimate channels. Grey market cells are not the doing nor the responsibility of A123. There isn't much of value in the MSDS that I haven't already shared here. One tip I'll share is that if you get electrolyte on you, wipe off all you can with a rag BEFORE washing it off. The electrolyte can form hydroflouric acid (very nasty, very corrosive) when combined with water. That said, this stuff isn't nearly as scary as it sounds. The volume of electrolyte in the cells is very small--far below what I would consider "dangerous" from a chemical exposure standpoint unless someone was a real idiot about it.
     
  18. dan2l

    dan2l 2014 Prius v wagon

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    Hi W,
    You sound just like me. I have had to repeat questions for people that I was trying to help. I am sorry for skipping some things. The system is very complicated and not standard in some ways. I guess I am working too hard to try and keep the conversations simple. Also there are so many questions in each post that sometimes I forget some of them.

    Anyway, maybe it is best to first give a simple answer and then go back to the beginning and go through the detail in an order that may make more sense. The detail will most likely take 10 to 20 posts in order to get the detail at the level that you seem to be asking.

    The simple answer is that this data is from a 2kwhr "helper" Pack that is in parallel with the main 4kwhr pack that is has a BMS controling charge and discharge. I know that this is dangerous, However I made sure that the helper pack would always be less extreem in cell voltages that the main pack, and I do monitor it every day. I know this will bother most people and so let leave this alone for now and go through the bigger story.

    I started with a stock 4kwhr Enginer system in early 2010. It had MottCells, 80ah X 16cells. Mottcells are LiFePO4 cylindrical cells. 3ea 13ah cylindars in parallel for 39ah in each plastic box, but in China they stretch 39ah to 40ah. Then they Buddy cell'd these to get 80ah. I measured about 35 plastic MottCell boxes and got from 36 to 40.5ah when they were new. The celles in this system had a drain resistor and electronics in each plastic box that are supossed to turn on at 3.6v. They said it would drain 2a, but I am sure it is more like 1/2a. Those resistors do a successfull job of top balancing over time.
    BMS16CWeek3.jpg


    You can see this data from when the system was first top balanced.

    I took repeated top off charging to keep this system balanced. The system was discharged at 45-60a and charged at 15a for a year or so and then I could see it degrading in capacity.

    More Later,

    Thanks,
    Dan
     
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  19. dan2l

    dan2l 2014 Prius v wagon

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    I promised More...

    I tent to keep track of system cappacity using "electrical miles" That is, I take the MPG on a trip and compare that to the standard average MPG that I would have gotten. With some calculations, I can estimate that todays 28 mile trip had used about the amout of gas that would normally get me 16 miles without the PHEV system. So that means that I got about 12 electrial miles from the Enginer pack today.

    The best I did on the origional 4kwhr Enginer system was about 18 Emiles in the summer of 2010. By the summer of 2011 that was down to 14 miles or so. I collected this data every day. During this time, I had made a Y harness for the sensor wires in the main pack with one connection to the Enginer BMS and about 3 feet of 22awg wire giong up to CellLogs so that I could put them where I could easily access them.
    Alarms.jpg
    When it got to the point that all my trips were running out of juice, it was clear that I needed to do something. About that same time the 5k converter replaced the 3k converter and that meant that the discharge was at 55 - 85a. That made the cappacity problems worse. There was not physical room in the system for more capacity.

    What I did was to build a realforce 40ah X 16cell pack to put in parallel. Realforce are also LiFePO4 cells. 20ah in pouches symilar to the a123 pouches. These are put into a sheet metal box. Stacks of 16 pouches slip fit into the box. If the pouches expand then the box keeps them under pressure. The box has 3awg cables for the power and 22awg wires for sensing the cell voltages.

    I set up a second Y in the sensor wires so that I could have both packs feeding information to the CellLogs at the same time. This gave bleed wires that would slowly let current flow between packs for each cell so that the top balance on the MottCells would also keep the Realforce pack in balance. It was noted that the voltage readings on the CellLogs varied a little when curent flowed in those wires, but the CellLog voltages were not used for controling charging or discharging. The voltages read on the Enginer BMS was very close to the Mottcell voltage and not effected much by the Realforce cell voltages. The Enginer BMS controled both the charge and discharge.

    This all worked very well for about a year and a half. It was noted that the Realfoce cells not only added the Emiles as expected from the added 2kwhrs but the lowering of the C level helped the MottCells to give Emiles symilar to what they had when they were new. I was now getting as moch as 28 Emiles from a charge.

    More Later....

    Thanks,
    Dan
     
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  20. wb9k

    wb9k 09 Gen II Prius w Hymotion Plug-In Batt

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    delete....doubled post below.
     
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