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36V lifepo4 charging voltage

Discussion in 'Electric Bicycles' started by patpatbut, Nov 12, 2012.

  1.  
    patpatbut

    patpatbut Pedelecer

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    Hi again everyone

    I am a bit confused about the charging current for my 36v 12ah battery from BMSbattery.

    This is my battery

    36V12Ah LiFePO4 Alloy 03-CASE EBike Battery With a Carrier Rack - BMSBATTERY

    the EMC-120 charger shows 42.6v for the charging voltage

    and this one below, 36V 20AH lifepo4 battery

    36V 20Ah LiFePO4 Electric Bicycle Battery Pack - BMSBATTERY

    the EMC-180 charger shows 43.6 for the charging voltage

    Why are they different from the charging voltage? I thought they should be the same no matter the capacity is...

    Pat
  2.  
    morphix

    morphix Pedelecer

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    I don't think it makes much difference to be honest..there's a slight variation in the voltage..the bigger battery can take more current..1 volt shouldn't make much different though really..the batteries have a BMS circuits built into them which regulates how much voltage and current the battery draws and puts out.. so these are intelligent batteries which prevents overcharging and protects the battery cells against being run too low under load to prolong life, much like in laptops, except LiFePO4 is a different chemistry to the more common Li-Ion technology used in laptop batteries and other devices..
    Last edited: Nov 12, 2012
  3.  
    d8veh

    d8veh Pedelecer

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    My Ping 36v LiFePO4 charger is set to 45v. I read somewhere that the maximum charging voltage isn't critical on LiFePO4s. You can charge them higher, but there's hardly any extra energy goes in, and as soon as you switch on they go down to about 42.7 anyway because that's where the discharge curve really starts.
  4.  
    vhfman

    vhfman Just Joined

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    Just an observation regarding LiFePo4 chemistry batteries; using my DIY A123 LiFePo4 batteries as an example.

    Headway LiFePo4 and A123 cells are 3.25V +/- 0.05V nominal, and recommended standard charge @ 3.65V +/- 0.05V.

    I usually bulk charge my 12s3p batteries (which have no built in BMS) using a CCCV supply to 3.65V per series cell, so a pack HVC of 43.8V. I check the parallel cells with Cellog8s monitors to make sure individual cells don’t go above 3.7v. I notice over time the individual parallel cells vary in their apparent full charge voltage. E.g. each 6s3p group have end voltages between 3.51 – 3.7V. So every now and then I balance charge (using icharger 106b+) the batteries in 6s3p groups.

    Getting to the point of my observation, the amount of current required to balance the groups is usually only ~23mA. So the end point can be as low as 3.51V per cell or as high as 3.7V. Which would suggest a 12s battery could be charged between 42.1V and 44.4V with very little significant difference to the cells actual capacity at full charge.

    It also depends on how accurate the charger has been setup in the factory. According to the BMSBattery site, both the 120W and 180W charges, are set for LiFePO4 Battery Charge Voltage = 3.55V x the number of cells in series.

    More importantly the BMS in the batteries you refer to will control the ultimate end point voltage of the charged battery.

    Chris
  5.  
    Kudoscycles

    Kudoscycles Official Trade Member

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    Patpatbut....Our experience is that the charge voltoge is critical and must be matched to the battery. If you use the lower voltage charger on a battery designed for the higher voltage charger,you will only 75% fill the battery. The charger will cut out prematurely.
    How do we know this - the hard way,by experience.
    Not sure what happens the other way,probably not good.
    Ask your battery supplier for the correct voltage charger to suit your battery.
    Dave
    Kudoscycles
  6.  
    d8veh

    d8veh Pedelecer

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    Sorry Dave, that doesn't sound right at all, and it doesn't match my experience. I've got loads of different chargers for all my batteries and I swap them around at it makes no detectable differenc4. You have to be more careful with the voltage for non-LiFePO4 batteries that are less tolerant of charge voltage. I think VHF man summed it up pretty well in his tests.
    If you look at the charge/discharge curves in this link, you can see how they kick up above 3.5v. which is why it makes very little difference.
    http://www.efirstpower.com/li.html
    Last edited: Nov 12, 2012
  7.  
    Kudoscycles

    Kudoscycles Official Trade Member

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    Dave.I step aside with your greater knowledge. The reason I made the posting was that we had a customer who had a failed charger 43.6v,we supplied a replacement charger 42.6v,thinking it made no difference-the customer reported reduced range and shorter charge time. Supplied a 'correct' 43.6v and all back to normal. From that time we are careful with the charger voltage. But if you advise that makes no sense I am happy to accept,maybe some other factor.
    Dave
    Kudoscycles
  8.  
    d8veh

    d8veh Pedelecer

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    Yes, I can think of a few other factors - e.g. the customer thinks he's been diddled out of a volt on his charger.

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