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What is the voltage range of a Li-NMC battery?

  1. Dec 21, 2014 #1
    They have a nominal voltage of 3.7V or 3.6V. They are charged at 4.2V, and disconnected when the charger says 4.0V. Does that mean they are 3.6V when they are empty, or are they overchaged when they are 4.0V? How low in voltage can they go and still have plenty of current? Thanks.

    I'm designing a 36V lithium ion battery made from commercial 3.6V cells, since commercial 36V batteries are so expensive.
     
  2. jcsd
  3. Dec 21, 2014 #2
    Maybe someone in the electrical engineering forum knows the answer.

    I just know the voltage can be raised higher but it damages a lithium ion battery.
     
  4. Dec 21, 2014 #3
    I looked at some charging/discharging curves. It is difficult to know what is optimal. Looks like they agree that charging at 4.2 volts is best, and certainly don't want to go above that. Some types charge lower than that. As for what voltage the battery should have when removed, that is up in the air. The lithium ion batteries can hold more charge than rated, and perform better with the higher charge. But if you go too high, they can catch on fire. Even if they don't, they can be more dangerous if over used once charged that high. However, at higher voltage, they put out higher voltage and have a much longer range. This comes at the price of shorter number of cycles. 4.0 volts seems to be a suggested cut off charging voltage. And the
     
  5. Dec 21, 2014 #4
    Here is my particular battery:

    http://img.weiku.com/waterpicture/2...y_separator_membrane_634579634333098075_1.jpg

    Looks like when a nominal voltage of 3.6V is stated, that means that is close to dead, and is probably how it is shipped. If charged to 4.2 volts, its remaining amps are almost over at 3.7volts. If charged to 4.0 volts, its remaining amps are gone near 3.3 volts. Charging to 4.0 volts is suggested for max cycles.



    So this raises a new issue: motor life.
    Motors die when they overheat. If the voltage given to them drops, they output the same power by drawing more current, and overheat. If I'm riding an electric bike and asking for a constant 250W, I could damage the motor with the changing voltage of batteries. For this reason, corded motors probably last longer.

    Now my question is, what beginning and ending voltages should I shoot for, and how much should I stay below the 250W motor rating to extend the motor life? Or does it already include a safety margin for this?
     
  6. Dec 22, 2014 #5

    Drakkith

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    Staff: Mentor


    A "fully charged" NMC battery has to have 4.2 volts applied to it, but should deliver a "nominal voltage" of 3.6 plus or minus 0.5 volts. That just means that over the discharge cycle the voltage should be about 3.6 volts on average, but in reality it will usually be more or less depending on where in the discharge cycle the battery is at. If you only charged the battery using 3.6 volts the battery would have less capacity and output at a lower average voltage during the discharge cycle. If we think of 4.0 volts as being "fully charged" then the battery should behave approximately like the graph below (solid line).

    final2.jpg
    That's right. As the charging voltage increases it stresses the battery and lowers the lifetime. The higher the voltage the more damage. For very long life you could charge the battery to less than full, but would obviously have to suffer from reduced battery capacity.

    Yep. You have to decide on where you want the trade-off at. I don't see any real reason to charge to anything higher than about 4.0 volts.

    I'm not sure how you're getting your numbers. It appears, at least to me, that at 3.6 volts your graph shows that you have approximately half of the battery capacity left if you're discharging it at 3c. If you're discharging it at 0.2c, then you'd have less than 10% capacity left. This means that discharging at a slow rate allows you to maintain a higher output voltage compared to discharging at a faster rate. See here for an explanation of c rates: http://batteryuniversity.com/learn/article/what_is_the_c_rate

    Note that the discharge voltage itself varies with the current. A higher current decreases the output voltage because the batteries internal resistance has more of an effect when the resistance of the circuit is lower (hence why the current through it is higher).

    I'm no expert on electric motors, but I wouldn't expect a drop of 0.5 volts to affect a motor that much. However I suppose you could use some sort of voltage regulator if you're that worried. (Someone more experience with this subject may want to step in)
     
    Last edited: Dec 22, 2014
  7. Dec 22, 2014 #6
    Thank you, Drakkith, for yet another good explanation. So I will spend a little extra on ten amp hours instead of 7.5, to reduce my C number during charging and discharge and stay well above empty. I'm looking at 0.75C now instead of 1C, unless that is not enough to make a difference. I definitely want to stay far away from 3 volts since I don't have a smart charger, unless I build mine with a switch for multiple charging vultages. Either way, the motor won't like the lower voltage.
     
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