LiFePo 12v charge info

ws9876ws9876 Solar Expert Posts: 435 ✭✭✭
I put an SOK 12v 100 ah LiFePo bat on a solar charge. The bulk was set at 14.6 and float at 13.8 per manufacturer,SOK.
On a KID 30 amp controller. It is new and was about 13.4v. It immediately went into float mode and is staying at 13.8v but the 
status shows 11 amps...?? If it is floating why is it taking in that much? Shouldn't it be resting at a small current flow?
And why didn't it go into bulk and go to 14.6 v..???? Of course it has its own BMS.....what is going on with this new battery?

Comments

  • RCinFLARCinFLA Solar Expert Posts: 1,475 ✭✭✭✭
    edited September 24 #2
    You jumped to float prematurely. Just because overall battery is showing 13.4v does not mean all four cells are at 13.4v/4 or 3.35 vdc.

    Likely cause was an interaction between BMS actions and charge controller seeing the action as absorb complete. There are several ways this can happen all because cells in SOK are not well balanced in their state of charge.

    If it is taking 11 amps let it continue until battery current drops off in current.  Then reinitiate a full absorb cycle again.  You might catch it dropping to zero charge current several times if one cell exceeds 3.65v-3.7v trigging an overvoltage cell shutdown by BMS.  It will eventually bleed the cell down and will reset the BMS so it charges again.  Eventually, all cells will level out.  It might take several hours or several days.

    You need to get to 14.2-14.4v with charge current tapering down.  The BMS will not bleed balance a cell until it gets above 3.4v so to ensure full balancing you need to get above 14.2v and enough soak time to allow all cells to get into a BMS balancing bleed on all cells.

    You may see several BMS cell overvoltage shut downs along the way. Sounds like your charge controller will drop back to float voltage each time this happens.

    The resistor bleed balancing current in BMS may only be 50-100 mA so it can take a long time to balance a few AH's of state of charge difference between cells.  LFP cell self leakage is about 1.5 to 3% of cell capacity per month so if a new battery was sitting around for months the difference in self leakage rate between cells can drive an AH or two difference in SOC between cells.
  • ws9876ws9876 Solar Expert Posts: 435 ✭✭✭
    edited September 24 #3
    thanks for info. I changed the panels and it went into bulk and got over 14 ,I didn't see if it got to 14.6v.
      How long should I keep it at 14.6 which is the factory recommend..?
    2 minutes? 30 minutes?
  • RCinFLARCinFLA Solar Expert Posts: 1,475 ✭✭✭✭
    edited October 2 #4
    That depends on how misbalanced cells are.  The bleed current on BMS is likely 50-100 mA.  1% of 100 AH battery is 10-20 hours to bleed off 1 AH state of charge difference.  

    >1% misbalanced cells can cause a premature BMS overvoltage shutdown.

    Bleed does not happen until a cell gets above 3.4v on most BMS's.

    If you keep tripping BMS overvoltage shutdown, lower charge current to get a lower ratio between charge current and bleed current.
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