Rebalancing my LFP battery bank to add a 17th cell.

Raj174Raj174 Posts: 639Solar Expert ✭✭✭✭
edited February 17 in New Battery Technologies #1
 
Well I just spent the last two days rebalancing my LFP battery bank, and what a chore it was. I had to rebalance because I added a 17th cell, which I originally bought as a spare, and balancing the bank a few weeks ago didn't go as well as I would have liked. I think I was just in too much of a hurry, and to do it properly takes time. Anyway, this time, I first brought the 16 195AH cells slowly up to an average of 3.8 volts while bleeding off voltage from specific cells with resistors as they started rising faster than the others. I did this with a 58.4 volt 10 amp charger and two 3.3 ohm 10 watt resistors soldered to wire with large alligator clips ( a trick @Simon told me about). This took about 8 hours.

The next day I charged the 17th cell up to 3.8 volts with a 3.65 volt 10 amp charger then disassembled the bank, wired all 17 cells in parallel and connected the 3.65 volt 10 amp charger to the parallel bank and began charging. I had to move the charger connections around about every 15 to 20 minutes to keep voltage of the connected cells from rising too fast. All the cells slowly increased in voltage until they reached 3.6 volts. At this point this 3.2 volt nominal 3315AH battery was drawing a little less than one amp. I stopped charging and left them connected in parallel for another 4 hours. The bank is now top balanced at 3.6 volts.  This took about 10 hours all together.

I disassembled the parallel connections, reassembled them in series, connected the bank and brought the system online.
This was tedious and I am glad this only needs to be done every two or three years, or maybe not at all if the possibility of using resistors to adjust cell voltages works to maintain balance. In any case, I certainly think that a two day investment of time and effort that gets me a trouble free no maintenance battery is well worth it. I just thought I'd share my two days of fun with you guys and anyone interested in the process. 

Rick
3600W PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 54.4V 207AH LiFePO4 no BMS, 4500W genset.

Comments

  • Dave AngeliniDave Angelini Posts: 3,955Solar Expert ✭✭✭✭
    How would the resistors help Rick from an offgrid slant? You would still have to lose power or have a secondary battery or generator running the house right?

    Does Winston make a way to automate the BMS ?

    Sounds like you are ready for hurricane season :*
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • Raj174Raj174 Posts: 639Solar Expert ✭✭✭✭
    How would the resistors help Rick from an offgrid slant? You would still have to lose power or have a secondary battery or generator running the house right?

    Does Winston make a way to automate the BMS ?

    Sounds like you are ready for hurricane season :*
    You are correct Dave. In an off grid situation, a secondary battery bank or a generator would be necessary to accomplish the task. As far as the resistor goes, it is just a tool for keeping the voltages of the cells close together while charging the bank. I am new to this method and probably could have used larger ones to reduce the voltage faster, saving time. 

    Might have been easier to take the bank voltage up till the highest cell reached 3.5V, then put them in parallel for a few hours, then back in series for a second charge and then back in parallel for the final charge. May try that next time. 

    I'm not sure if Winston makes a BMS, but I'd guess probably not.

    Yes, God forbid we get another hurricane this year, but I am as ready as anyone can be. If only we could have had this system in Miami when hurricane Andrew hit. No power for over two weeks.  :o  

    Rick
    3600W PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 54.4V 207AH LiFePO4 no BMS, 4500W genset.
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