HEV battery management system

Discussion in 'The Projects Forum' started by Flip101, Jan 4, 2012.

  1. Flip101

    Thread Starter New Member

    Jan 4, 2012
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    Hello,

    I am an electrical engineering student and my colleagues and I are building a HEV for a degree project. We recently received GBS-LFMP40Ah Lithium Ion battery cells.

    My question is can anybody provide me with any schematics or circuit diagrams that provides proper connection between the generator to batteries, batteries to electric motor?

    At the very least could somebody please provide me with any helpful links that will allow for a proper charge circuit design. I have went through many sources where block diagrams are provided but I am more interested in actual connection diagrams and components connected to and from the Lithium Ion batteries

    Any feedback is greatly appreciated
    Thank you
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    This is really complicated stuff. I just worked with somebody doing development on EV technology. The lithium battery packs have to have individual monitoring for EACH CELL so that they can be balanced and monitored. He had developed some higher level software and control architecture to monitor the individual cells and do various things.... that never worked right BTW. The Li batteries also have to be charged in a VERY precise way to keep from damaging them.

    Be very careful.
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    IMHO, Li-Ion cells are not well suited for use in electric vehicles. The new development on EV that I have seen are using Li-Fe type batteries (Lithium Iron) not Li-Ion.
     
  4. Flip101

    Thread Starter New Member

    Jan 4, 2012
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    Hey, I am in charge of building a battery bank and charging system for a hybrid vehicle for school. We went with lithium polymer batteries and I am unfamiliar with how they operate and what some of the basics of lithium polymer batteries. I was going to start small scale which is why i'm posing my question here and figured the small scale application theory could be applied to a larger scaled application in the end. But I'm going to ask about the big picture here and see what you think.

    Here are my initial questions and was hoping someone could help if they have any experience in this field ---

    I am required to have 32 battery cells in series to get a high output voltage, but need to design a charging system for it. Is it as simple as just applying constant current to all these batteries while monitoring voltage and current to make sure they do not break any limits? I explain my question in more detail below ---->

    I need to know if charging lipo batteries in series requires you to consider the current limit of one battery or all of them? Example: If you have 32x 40Ah batteries in series will you have to charge at only 1C of one battery so 40A? Is that the limit? Another question about the charge curve --> Do you HAVE to have constant current going through the series batteries until the batteries charge to their maximum voltage? So would you have to alter voltage to the batteries in the above example to keep the current at exactly 40A in order to charge the 32 batteries in series? Like do the lipo batteries in series keep relatively equal resistance in order to maintain constant current themselves while charging to their max voltage? After they reach their max voltage, from my understanding, you then have to keep the voltage constant and keep charging until the current through the batteries drops to approximately 10% of the charging current (40A) so 4A?

    Would it be a good idea or recommended to charge so many batteries in series like that?
     
  5. evilclem

    Member

    Dec 20, 2011
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    I would highly recommend charging the cells individually as bountyhunter mentioned previously.


    There is plenty of literature on the internet regarding the charge method for lithium-ion cells however the datasheet from the actual battery manufacturer should be your first and last point of call (sometimes I will avoid a battery if the manufacturer cannot recommend optimum charge voltages/currents for their batteries). Start with http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries.


    If you must charge the batteries in series then I would highly recommend temperature and voltage monitoring of every cell. If any single cell hits the maximum temperature or maximum charge voltage then the entire charge must be stopped, which potentially leaves several cells undercharged. (Edit: As mentioned by strantor, the weak cell discharging first then becomes a problem, as such you need LVD monitoring on all cells, which in a vehicle system then requires a backup battery for emergency braking/lighting when this happens etc.).

    As for the charge current, 36 x 40 Ah batteries in series increases the voltage, not your Ah rating as they will all have the same current drain (as they are in series).

    I am not entirely sure how to apply a charge to each cell (or battery) individually but someone here will know.
     
    Last edited: Jan 4, 2012
  6. strantor

    AAC Fanatic!

    Oct 3, 2010
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    google "lipo fire". LiPo batteries are the most volatile batteries I know of. charge them incorrectly, they catch fire. over-discharge them, they catch fire. look at them with the "stink eye", they catch fire. They are the best batteries out there though, in terms of just about every performance aspect. You just really need to be careful, educate yourself, and know wht you are doing before you do anything, especially with those big cells.
    no
    all of them (each individual battery)
    forget charging them in series, that's how you catch them on fire. Each battery needs to be charged individually. There are systems out there called battery management systems (BMS, one such example) which control the charging, and monitor the discharging as you drive. You will also need a special lipo charger that will balance the cells, which needs to be done periodically as maintenance. If you really want to do it all yourself, you will need to make one (BMS, and possibly balancing charger). It's no small task. the reason why, is that if you charge them all in series, they will not all charge at the same rate. one will get charged first, and as the other ones are coming up in voltage, the first will catch fire. Also, while you're driving your HEV around, one cell will be the first to go flat, and when it does, all the current from the other batteries will still be flowing through it, and it will catch fire. when one catches fire, they all catch fire, and it is violent self oxidizing chemical fire that cannot be extinguished, it must burn out on its own. DO NOT CHARGE INSIDE. Common advice is to charge outdoors, out from underneath any overhangs, inside a ceramic flower pot, with a steel plat on top. This is the reason why mainstream EVs are not using LiPo; they opt for the bulkier and far safer LiFePo4 batteries. The crowd who is really using the LiPo a lot are hobby airplane crowd and ebike crowd. check out www.endless-sphere.com/forums. They play around with LiPo all day long and give you specific advice for the situation you are in, that I cannot.
     
  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    You charge them in series but each individual cell has monitor lines coming out to the controller that can activate a shunt to bypass the cell when it is full and other cells in the string need more charging.

    Like I said: it's complicated.
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Don't forget to bring the marshmallows.......

    Seriously, stick with Li-Fe batteries.
     
  9. strantor

    AAC Fanatic!

    Oct 3, 2010
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    too late for that; sounds like he's already got them on hand. So, here's to the trial by fire;)
    It'l be like drinking from a fire hose ;)
    ah gosh I'm too funny
     
  10. Flip101

    Thread Starter New Member

    Jan 4, 2012
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    I would like to start off by saying thank you to everybody who has responded to the posts thus far as it has been extremely beneficial for my research. My comprehension continues to grow with the input and references that are provided.

    I was wondering if there are any pre-existing designs of a battery management system that could be implemented with the GBS-LFMP40AH Lipo battery cells?
     
  11. strantor

    AAC Fanatic!

    Oct 3, 2010
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    the one I linked to is specifically designed for that battery
     
  12. Flip101

    Thread Starter New Member

    Jan 4, 2012
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    Yes I understand the link that you provided me with is specifically designed for that battery but I am looking to build my own BMS without purchase of that one in particular.
     
  13. strantor

    AAC Fanatic!

    Oct 3, 2010
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    The best designs are likely going to be proprietary; you could always revers engineer them. There are home brew designs out there if you google; I found some by googling "BMS circuit". Again, I would direct you to www.endless-sphere.com/forums, as the members of that forum are all about LiPo and BMS. Several members of that forum have designed their own BMS's and the info is there if you search the site, along with the advise of the creators themselves. I must warn you though, they are not a noob friendly as here. If you don't familiarize yourself with the designs already available on the site and pose your question the right way, you will either get flamed or recieve no help. When I say "pose your question the right way" - I mean, do some research and make it obvious that you have done said research in your post. Do not go there and ask your question in a way that it seems you want them to design your BMS for you; ask specific questions, and be humble. some of those guys design BMS and motor controllers for a living; make sure you know who you are talking to (by reading up on their previous posts) before you accept or discard the info they give you.
     
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