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4s battery Fail, Low voltage alarm for New 3s Battery

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  • 15-05-2011 10:40am
    #1
    damo642
    Registered Users Posts: 40 ✭


    Hi,
    Had 4s battery and got 20 flights out of it before the charger refused to charge it as one of the cells dropped below the minimum allowed (approx 3 volts i think???)
    Now i bought a 3s battery and want to buy a low voltage alarm buzzer.

    Question 1 : can i fix the issue with the 4s battery somehow
    Question 2 : Did i do something wrong
    Question 3 : For the low voltage alarm , at what voltage should the buzzer kick in at?
    Question 4 : can anyone recommend a good buzzer suitable and loud!!

    Thanks
    Damo642


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    #2
    coolwings
    Moderators, Recreation & Hobbies Moderators Posts: 3,455 Mod ✭✭✭✭coolwings


    Can you not program the speed controller for low voltage cutoff?
    That is the simplest way to deal with avoiding battery destruction from deep discharge.

    3V will be seen during a high amp discharge, and could be a premature stopping point in a high performance model.
    But in a low power model 3V is workable - it provides a cushion to allow for inaccurate voltage measurement in components.
    Empty is 2.7 volts/cell. Permanent damage is at 2.5V

    The middle cells in a pack have no air contact and run warmer. Cheaper packs have high resistance, and run even warmer still.
    It could be temperature that damaged the cell and not low voltage.
    On cheap packs the temperature rise is dramatic, and 65 degrees (destruction) can be achieved well before the battery is emptied and low voltage tells the modeller to stop.
    If the damaged cell is a middle one, and outside cells are intact, this would be suggestive of temperature rise due to internal resistance (cheap cells).
    On the otehr hand, low voltage tends to damage more cells in one go and the whole pack tends to puff up rather than one cell in it.

    If this should be the case, a low voltage cutoff, or buzzer will not save you doing the same to other similar packs. You need to throttle down, use less amps, cool with airflow, etc, or use better cells instead.


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    #3
    T-Maxx
    Registered Users Posts: 3,142 ✭✭✭T-Maxx


    You can try and charge the bad cell separately back to life. If the voltage didn't drop too low you might just salvage it.


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    #4
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    T-Maxx wrote: »
    You can try and charge the bad cell separately back to life. If the voltage didn't drop too low you might just salvage it.

    I have often done this, charged single cells directly from a variable power supply and digital ammeter until they reach 3v or 3.3v depending on which lipo charger i will use to then balance and charge the affected battery.

    Keeping battery cells well balanced is important to stop one cell going below the minimum recommended voltage after each use. I check cell balance after every flight as the main charger i use tells you as soon as the balance lead is plugged in. If they are well balanced before each flight but one is well below the others after each flight, and the other cells are not over discharged, then a cell is bad. if however you are over discharging the battery as a whole in flight, then larger inbalances will be seen as cells will vary a little and this is made more obvious when the battery is over discharged.

    If one or more cells has puffed up, its bin time for the battery.


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