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  1. #1
    RC poster
    Join Date
    Oct 2013

    Question How does 8.4v = full charge for 7.4 battery, and 7.6v = 50% charge?


    I have had R/C cars before, but have been out of action with them for a bunch of years. I had a nitro Thunder Tiger Mirage buggy for a while, and before that I just had cheaper cars that ran on NiMH batteries with very simple chargers.

    I just bought a Summit, 1 pair of Traxxas #2843 25C 7.4v 5800mAh batteries, and a HiTec #44167 X4 AC Plus charger with some extra leads to charge multiple batteries.

    I am learning all about LiPo batteries. I've learned about what causes them to burn or explode, what causes them to degrade quickly, the storage voltage thing, etc. I think I have a good handle on that.

    However, I am confused about what I think is a key points, and was hoping for a bit of clarification:

    The battery says 7.4v on it. But, the charger says it is going to charge it up to 8.4v. How can it be a 7.4v battery, and be charged to 8.4v? Once charged, my multi-meter confirmed it. This does not make sense to me.

    Similarly, I put the charger in Storage mode, and it says it is going to bring the battery to 7.6v, which is supposedly 50% charged. This does not make sense to me either.

    Will someone please explain to me why 8.4 = 100% charge and 7.6v = 50% charge on a 7.4v battery?

    Thank you for your time.

  2. #2
    RC Champion
    Join Date
    Nov 2010
    United Kingdom of England land.
    7.4V is the nominal voltage, All batteries have nominal voltage on packs. It's used as a description for the battery.

    ie. 7.2V = six cell nickel battery,
    7.4V = 2s lipo,
    8.4V = 7cell nickel battery.

    Fully charged a single cell nickel battery is about 1.4V-1.5V x number of cells (x6) = 8.4V-9V
    A single cell lipo charged is 4.2V x number of cells (x2) = 8.4V. Storage is about 3.8V per cell (x2) = 7.6V
    Guide books aren't part of the packaging.

  3. #3
    Marshal ksb51rl's Avatar
    Join Date
    Nov 2006
    Northern NJ
    Actually, even 4.2 to 3.5 Volts per cell is a very wide usable voltage range.

    Alkaline cells are full at about 1.6 Volts and pretty much dead at 1.4 Volts. Yet they are nominal 1.5 Volts.

    Current NiMH cells are full at about 1.45 Volts (t d m & I agree) and drained at about 1.2 Volts. Yest they are nominal 1.2 Volts (which is wrong IMO, but that is a conversation for another time.)

    Quote Originally Posted by Dave H @
    A typical resting voltage vs capacity table, based on discharging at 1C to 3V. Of course this will vary some by cell but most tests I have seen over the years are fairly close.

    4.20v = 100%
    4.03v = 76%
    3.86v = 52%
    3.83v = 42%
    3.79v = 30%
    3.70v = 11%
    3.6?v = 0%
    Note that this is RESTING voltage. Voltage level is not static. It sags under load. It sags more as amp draw increases, but less with higher rated (C-rating) batteries. On certain trucks, my LVA is set for 3.5V/cell, but after it sounds and I remove the pack(s) from the vehicle, they read about 3.8V/cell. This is normal and a fortunate occurrence since I can simply store them after a run without having to charge or discharge to storage level.
    Alt-248 on the number pad =

  4. #4
    RC poster
    Join Date
    Oct 2013
    OK, I guess I just need to quit looking at the 7.4v as a specification, and more as a "type" label, and accept that it's weird.
    Like how a 2x4 at the lumber yard is 1.5x3.5.

    Thanks for the explanations.

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