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Thread: Failure modes of Li-po cells in FSAE context

  1. #1

    Failure modes of Li-po cells in FSAE context

    Hello All,

    Greetings from IIT Bombay Racing Team.

    I am Mukul Jangid, Chief Engineer of our team. Our team builds electric cars for competing at Formula Student UK. We are from Mumbai, India's Indian Institute of Technology, Bombay (IIT Bombay).

    Context:
    We use LiPo cells from E Propulsions system based in Hong Kong. Our battery pack is of 96s1p configuration and cells are of 22 000 mAh. Our powertrain is from Compact Dynamics with nominal working voltage of ~350 V and peak power output of 80 kW. Our cells are connected to each other through busbars and mechanical clamping using screws and nuts.
    One major issue in our last cars has been the unreliability of our BMS. We make our own BMS system using LT6804 IC from Linear Technology. Because of unreliable BMS, many a time the cells get into unfavorable situations which render them useless for rest of the season. This leads to a lot of cells being wasted, an unreliable battery pack and a very small testing time. This year we
    want to make sure that these things are not repeated and hence we need to be sure of all the failure modes so that we can take an informed decision regarding the number of cells that we purchase as the purchase cycle is very long and ideally we would like to buy all the cells at once apart from the fact that the cells form a big slice of our budget as well.

    So we need to be sure that once we use the new set of cells, they will not be abused. For this, we referred to a lot of online sources, clubbing those details with our own experiences we have listed down the following points.
    • Overvoltage and Undervoltage protection through BMS (3.3 V to 4.1 V is regulated in our team)
    • Operating in safe temperature limits regulated through temperature monitoring (50 deg C according to cell datasheet)
    • Keeping charging and discharge current within safe limits as specified by manufacturer
    • Storage strategy: should not be left charged while unattempted (should be down to ~3.6 V if unused, should not be left fully discharged as well). Also, not storing them at very high or very low temperatures.

    Since our cars have not seen a lot of testing there could be several other practical issues that other teams might have observed with experience. If you guys could share some of this valuable information as to what other practical issues are we missing in the above list. This would really help us in designing a solution for such issues.

    This information would be useful to other teams starting with EVs as well, so we hope we are able to gather some good information on this thread.

    Thanks!

  2. #2
    Hi,
    The main thing I see missing from your list is current draw. Have you verified that your powertrain doesn't sometimes draw over the maximum your cells are rated for?'
    Apart from that, seems like a good list. I would emphasize that testing your BMS is crucial, so you can trust it to protect your accumulator - accuracy of voltage and temperature readings
    (specifically temperature as that's sensitive to placement of sensors)

    What issues are you having with the LTC6804, out of curiosity?

    regards
    Tim

  3. #3
    One more thing, if you don't buy all the cells at once, make sure when you combine old and new cells that the pack stays balanced. If you've already put the cells through some cycles this may require you to cycle the new cells as well.

  4. #4
    Senior Member
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    Thanks for that Tim.

    For a while there I thought ... The Next Dark Ages had finally arrived! (Watch the movie Idiocracy and see that along with wall-to-wall idiots there are also wall-to-wall advertisements. The last week+ on this forum has been all advertisements.)
    ~o0o~

    Mukul,

    I know next to nothing about LiPo batteries, but I do know that they don't like mechanical stresses.

    So DO NOT HAMMER A NAIL INTO THEM!

    But also avoid inappropriate touching, fondling, or groping.

    (I think the Samsung issue was to do with "pinched' cells?)
    ~o0o~

    Some more technical insights from some of you very clever students would be nice. Better than the boring ads.

    Z

  5. #5
    Hi,

    Z, we will take care to keep the hammer and nails away from our battery pack.

    Thanks Tim for you advice
    Our peak current draw is limited by the power consumption according to the rulebook and it does not rise above ~ 230 A (also confirmed from energy data)
    Max continuous discharge according to cell datasheet is 330 A (15C). So the current draw is fairly under the limits.
    Would love to hear your views on this. It looks like we can use a cell with lower continous discharge current as that usually results in higher energy density and hence a lighter battery pack.
    Thanks for pointing out the matching process. We buy all the cells at once, but if some cell in the pack behaves oddly (high internal resistance or low capacity) we replace it with a new cell. We will take care from this time onwards to do the matching correctly before replacing a cell.


    Some recommendations we got from our cell manufacturer

    "The cut off is different for differnent discharge rates and temperature conditions, testing should be done to figure out when the cut off needs to be done. Capacity at end of the use must be minimum 20%, not lower or you will reduce the lifetime of the battery.

    When you have finished to drive, you absolutely need to charge the cells at the storage voltage that is 3,85/3,86V by cell. This is the voltage condition to get the best life and performance.

    If you donít use them for months, you should perform a slow discharge to 3,4V and charge again to storage voltage at 3,85/3,86V by cell twice a month. Storing the cells 4/5 months at 3,6V with no discharge charge is not good and can damage the cells.
    "


    Mukul
    IIT Bombay Racing

  6. #6
    Hello Mukul,

    Few teams have suffered from safety related issues with cells from the same manufacturer as you. It is wise of you of being concerned.

    Your first priority should be to have a reliable BMS. It's been since 2011 that Poly eRacing uses LTC BMS chips without reliability issues. I would recommend to build their own BMS. All source files (Eagle files, Gerbers, C source) are available for free on their website, in the blog section.
    http://eracing.polymtl.ca/blog/

    Also, I started a wiki on battery safety, including as many advice I could think of. I invite you to read, and all community members (including yourself) to share knowledge and experience in the field of battery safety on this platform.
    https://batterysafetytech.miraheze.org/

    Hugues Marceau
    Formula North organisation
    Poly eRacing founder

  7. #7
    Mukul,

    Hey mate sorry to hear about the dramas you have been having with your battery packs.

    I have customers in Formula E also I've been flying RC electric aircraft for over 20 years so I have some rough rules of thumb for you.
    *When charging your LiPos never exceed 4.2V/cell. What chargers do you use? Reading between the lines part of me wonders whether your charging process is giving you grief. Also are you balance charging?
    *In terms of discharge never go below 3.3 V/cell. Never ever and I mean never ever go to 100% discharge on a LiPo.
    *Make sure the pack is getting plenty of cooling. I have a friend of mine who designed an unlimited high speed aerobatic aircraft where the fuselage looks like a glorified box but it goes like stink because his cooling of the motor and packs is spot on.

    Also a word on the cells. In the RC aviation community the cells of choice right now are Pulse, Hobbyking Nanotechs and revolectrix. They are 65C cells and they can withstand quite a bit of abuse. This is worth checking out.

    My two cents worth and good luck.

    All the Best

    Danny Nowlan
    Director
    ChassisSim Technologies

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