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Thread: Lithium cell tests

  1. #131

  2. #132
    From Stefan:


    I propose the following, a lot of which is already in the rules as they are:

    - On lithium batteries, cell balancing is obligatory.
    - On lithium battery packs, fuses rated at their peak maxiumum discharge current are obligatory.
    - Using lithium batteries, All power electronics (motor drives, etc.) are obliged to have a cut-off voltage to prevent over-discharging of the batterypack.

    For approval of the tech-check, the roboteer has to be able to produce the following:
    - A datasheet of the cells in the batterypack
    - A datasheet of the cell-balancer used
    - The configuration settings used on the cell balancer and a wireing diagram.
    - A datasheet of the charger used.
    - The configuration of the charger and a wireing diagram.
    - Datasheets of the powerelectronics used (motor drives, etc.)
    - The configuration settings, specifically the cut-off voltage, and a wireing diagram.

    If you are unsure that your system will be allowed on an event, gather the information you need for tech-checking and consult the event organizer.

    Because battery technology changes very quickly there is no use for stating specific number of cells or voltages. In the end its up to the tech check to determine if its safe.

    As there is always an issue with the maximum voltage 36V, it is up to the FRA to recommend a maximum number of cells for a specific technology, but only based on the maximum allowed voltage. That is the only limit that makes any sense to me.

    This would translate in the following way, for maximum cells in series (based on nominal voltage): Pb: 18 cells, NiCd/NiMh: 30 cells, LiFePo: 11 cells.

    This seems like a well thought out proposal to me. Anyone any thoughts?

  3. #133

  4. #134
    Trev, by that definition alone Nicads should be banned.

    These A123 cells are not only advertised but have proven to be safer than any other lithium based cells out there. Even LiPos have not had such a test as these cells have had. In fact I dont know of any battery type that has ever had so much data gathered on them before a decision is made.

  5. #135
    In additon:

    Are all these features stefan proposes really needed? In truth no, he went a bit overboard on them, but on the other hand there are no things in that proposal that wouldnt be advisable for other types of battery either (nicad/nimh/lipo).

  6. #136
    I also dont think these proposols are necessary for A123 M1 cells.

    Cell balancing is only necessary if you using a simple (with no individual cell monitoring) charger. If you use a cell monitoring charger (e.g. the standard DeWalt 36V charger), then it will switch off when the first cell peaks - no problem.

    However, I envisage that most people will use a simple charger at events - 1) it avoids having to fiddle around connecting all the balance leads and 2) the chargers are cheaper.

    IF youre using a simple charger, then you should periodically balance your cells, but A123 cells do not suffer from thermal runaway like LiPos when overcharged, so overcharging is not a safety issue. They presumably will eventually go pop if seriously abused, just like NiCads, but without the fireball of LiPos.

    Over-discharging is also not a big issue with A123 cells. Yes, it will damage your cells if you do it, but they will not explode. Again, not a safety issue. I have left a pack discharging on 100 Watt bulbs accidentally. Held completely flat for at least 10 mins. They recovered afterwards and were fine, but some cells in the pack are now showing a reduction in capacity, though not enough to really affect their performance in the robot yet.

    I tend to say that you should have a sufficient number of battery packs in your robot so that in a hard fight, you will use no more than around 70% of their charge. This also ensures that the current is kept to sensible levels.

    At the moment I think that the only Lithium cells of any type that should be allowed in Heavyweights are the A123 cells. If something better comes along, they can be examined separately.

    I dont believe the A123 cells require any special regulations at all. Advice on best practice in caring for them yes, but not regulations.


    (Message edited by terrorhurtz on November 20, 2007)

  7. #137
    So John, in your experience with them, would you say that with the same care as you would need to care for any other type of battery, the LiFEPo4 is safe to be used in Heavyweights?

  8. #138
    Thats already been said a thousand times.

  9. #139
    And yet, 7 months after the first test (in the UK, Australians were already using them) and multiple succesfull tests and shared results later, still nothing.

    Come on, lets get the lead out.

  10. #140

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