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What are safe voltages for Lithium Ion batteries

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    What are safe voltages for Lithium Ion batteries

    For Lithium ion batteries I've seen many blogs and ebike vendors call out nominal voltage as 3.6V, max volt of 4.2 and 'safe' minimum volt 3.1. So for a pack with 14 batteries in series I get (rounding off) 50, 59 & 43 V.

    So for an 80% charge I get 47 V = 0.8 * 59, but my charger stops at about 55V = 59* 0.93. What am I missing here?

    Also I recently saw several posts calling out 46V as safe minimum which is about 77% of max charge or 92% of nominal voltage. What would be case for 46V??


    #2
    Depending on which cell you have, my 52 volt pack is the Sanyo GA cell. They have a minimum capacity of 3350Mah, but are typicaly 3450mah. Charging voltage for this cell is 4,2 volt. Minimum end voltage is 2.5 volt.

    To calculate the maximum and minimum voltage of this cell multiply the cells in series by the maximum and minium voltage

    14s by 4.2 volt = 58.8
    14s X 2.5 volt = 35 volt.

    Most BMS will cut out long before this though. Around 3.3 to 3.5 volt. I haven't tested mine but it has run down to around 44 volt.
    These cells are rated at 10 amps . which equates to 40 amps in 4P configuration. The harder you push this pack clser to 30 amps on the BBSHD then the more sag in the battery. Then this will trip the BMS. Best to run the battery in pass level 1 or 2 when the battery's under 48 volt.. As you can see by the discharge sheet the cell discharges quickly from 3.3 volt.
    Click image for larger version

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      #3
      It is "possible" to run Lithium batteries between 4.2V per cell, and 3.0V per cell (or lower). I recommend 3.6V to 4.1V per cell for maximum life. Read what Tesla and ebikes.ca have written about lithium battery oongevity. If using the 3.6V to 4.1V protocol that I recommend, that may cut out ten percent of your possible range, but it may double or triple the life of the pack.

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        #4
        Originally posted by ebikefreak View Post
        For Lithium ion batteries I've seen many blogs and ebike vendors call out nominal voltage as 3.6V, max volt of 4.2 and 'safe' minimum volt 3.1. So for a pack with 14 batteries in series I get (rounding off) 50, 59 & 43 V.

        So for an 80% charge I get 47 V = 0.8 * 59, but my charger stops at about 55V = 59* 0.93. What am I missing here?

        Also I recently saw several posts calling out 46V as safe minimum which is about 77% of max charge or 92% of nominal voltage. What would be case for 46V??
        80% Energy Capacity is not 80% of pack voltage. Charge capacity and voltage are not linearly related. There is more energy capacity per volt at the top of the range, than at the bottom. 92% of pack voltage may well be only 80% of energy capacity. I didn't do the math, but the trend is correct.
        Fabrication is fun! Build something today. Show someone. Let them help. Inspire and share. Spread the desire.

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          #5
          electrical engineerings was always one of my weak points, you have sparked a remembrance that capacity non linear. For now I'll just try to stay in the 3.6-4.1 Volt/cell zone.

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