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Why can't I charge My battery fully.

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    Why can't I charge My battery fully.

    We get this question a couple time every week but instead of just explaining that there are many factors and it's totally normal to be off a little I will try to get in to more details.

    I wanted to write about that for more than a year but needed some real data and lack time to record the data.. Well this week when I came across and article written by a trusted neutral testing blog called I knew that site for a very long time but I missed out on one particular article...

    Example: "I bought a new 52V pack and charger and the charger outputs 58.8V, when I charge it fully and unplug it it shows 58,4V (or so)",

    This is the question that we get:
    • Is my battery used?
    • Is my battery defective?
    • Is my charger defective?
    • My BMS will never balance cells!
    • I'm loosing capacity help me!?
    • I think I need a new battery and charger!!

    Now the first thing you have to understand is this:

    If you did the recommended 4-5 charges to 100% and the battery works fine and does not cut when charging or discharging well it probably means that there's nothing wrong with it, your cells are fine! The BMS will balance the cells if one or a few went off balance and will stop the charger if there is a problem.

    SIDE NOTE ON BMS BALANCING: True the BMS only balance at 4.2V (+-,025V) but that is for a single group... NOT all the series cell group must be at 4,2V .. So if there is a single cell group that is weaker in your pack it will go down faster than the others but it will also charge faster... That group will reach 4.2V before the others and the BMS will start to bleed that high group while the other are charging and will continue to do this after too... There is so little energy in last 0,5V per cell I'm sure you wont see it..So stop worrying about balancing and go ride!

    You want to be sure that you have the full capacity? Well get a Watt-meter and measure the capacity... Just just make sure that you understand that a rated capacity in a pack is from 4,2V down to 2,5V per cell and obviously you probably will not be able to discharge below 3V per cell, So you will probably not get the exact numbers.

    Quick example of real capacity: I made a 14S5P GA pack last week and I did a fast continuous discharge test from 58,78 down to 41,80V (22,5amps AVG load) and got 16,7ah out of it. It's not the 17.5ah but it's still better than the manufacturer discharge specs, according to Panasonic I should have gotten 15,5ah if I stopped at 3V per cell (42V) and used a 20amp constant load. I'm getting sidetracked here!

    OK so we ruled out that capacity, the BMS is working fine, the charger is outputting the correct voltage and the battery is new

    So is my charger bad?

    If it's outputting the correct voltage (4,2V per cell, so 58,8V on a 52V system) and it's charging normally then it's alright. See the charger job is to charge to 58.8V and once at that voltage reached on the battery it will drop the current to a minimum and than it will stop.. IT does not say anywhere that the battery voltage after the charge will be 58,8V it's a charge voltage not a battery voltage. Also your charger can act differently with another battery, it can go all the way to 100% with a smaller pack and stop a little short with another larger battery.

    Take a look at a battery spec sheet: it says normally that charge voltage should be 4.2V and charge current should be X amount... but it's often missing the cutout amperage the amperage were the charger is cutting out and were the LED turn to green! And that cutout specs is often lower than the actual charger cutout.

    Why they do that? Well it takes a long time to to really fill it completely at a low current and there are so little energy being put in that cell that it doe not make much sense to do that? Would you wait another hour to get a fraction of 1% capacity? NO!

    Ok here is a real life test done a new cell by Lygte.
    The spec of the cell was to cutout the current at 60ma and the charger used had a cutout at 100ma. As you can see quite a drop after an hour.

    Click image for larger version  Name:	Efest IMR18650 2600mAh (Purple) 2016 A-charge-1.0-Zoom.png Views:	1 Size:	43.6 KB ID:	60935

    To extrapolate these numbers on a 52V pack: Well as soon as the LED turns green the cell starts to loose voltage so by the time you unplug it and measure the voltage it can now be at 4,195V per cell (that is 58,7V on your 52V pack) and a hour later it can be sitting at 4,182V (58,55V). That would be perfectly normal and I can say I had similar results. Also you charger is not a calibrated power supply that cost 1000$ or more so there are a margin of error, it's not intelligent and cannot adapt the cutoff based on the manufacturer specs or every cells there is...

    With time the battery capacity will fade a little and while your pack is still good the effect will worsen, so the next year or X amount or cycles you done on it the battery will probably be sitting at 58,5V right after a charge and at 57,8V after a few hour and again it's normal!!!

    My 2 cents: Chargers are not calibrated power supplies where the output is digitally monitored and adjusted so you can expect your cheaper Li-ion charger to output 58.80V but DO NOT expect to fill all the different chemistry and Ah of batteries you have at 58.800V once unplugged. If you wish to do this, please buy a lab calibrated power supply and manually disconnect it once the desired threshold is reached and you will be able to get 58.8V if the cell is new.

    Graphs are from :
    Last edited by Sebz; 04-04-2018, 06:39 AM.