Need Guidance charging cell by cell on LFP40's
My LFP40 cell voltages on my XM3500 are drifting apart, as expected without a BMS.
The highest cell is 3.89v after charger cutoff, and the lowest cell is at 3.29v, and I want to bring the low ones up using a CV/CC supply, but have never done this, so thought I'd ask for detailed guidance to avoid smoke. The manual is pretty sparse and not much help for me.
The supply is a Mastech 0-60V, 0-6A
From the manual:
"For constant voltage mode turn Current controls full CW(Max), switch on power supply and adjust to desired Voltage, Connect load to output terminals"
OK, easy enough...
For a battery at 3.3V for instance, what voltage do I set the Power Supply to? 100mv over battery? 50mv? Then just keep bumping it up until I reach my goal of 3.7V?
Or do I set output to my goal voltage of 3.7V and limit current to X Amps?
I'm building my version of Jans Darlington/LED simple BMS today for basic cell protection, but want to get all my cells above 3.7V before connecting it up, as that seems to be where the BMS takes cells with voltages higher than this.
I suppose these low cells will just drop below 3.7v again, but that's another issue, and maybe not if I always keep SOC higher than I have.
Turn the current adjustment all the way up to the maximum value. Then, with the power supply disconnected from the cell, set the voltage to the desired value (3.70 volts is ok). Connect the power supply to the cell, making very sure the polarity is correct. The voltage will rise as it charges at full current to 3.70 volts (CC stage) then it will hold at 3.7 volts while the the current automatically decreases (CV stage). You can consider the cell fully charged when the current is down to 0.30 amps.
I'm building my version of Jans Darlington/LED simple BMS today for basic cell protection
Let me know how Jan's circuit works. I have reseravtions on how well it balances given part-part parameter differences and also how hot it will get. I precharged my lower cells just like PJD mentions (I had my supply limited to 2A max as this is all it would handle) and then have used my BMS design ever since and I am getting +/-0.02 volt balance (better than I need but it works great).
Can you point me to a link with full info on "Jans BMS"?
actually, the charger is with BMS and balancing function. There are two wires in the charger to control every single cell. you can call it a smart charger
Hi milo - I went to your link and looked but did not find anything on your charger, just the LFP.
yeah, two wires
As an aside. last weekend at the EV fair in Palo Alto there were some guys selling a LiFePo PHEV Prius conversion kit(80xTS LFP40's) and they had a BMS which I asked if they sold separately. Yes; $20 per cell, $100 for controller, and $80 for LCD display.
So, for an XM-3500; $500 w/o display, $580 with.
As I recall, the large display rotated through the pack showing cell#, voltage...that's all I remember, probably more too.
They use cell-top boards on each cell and the boards link serially to one another only. It's probably someone on this board, but I don't know. I've been emailing with Richard today to get more details about their system. I took photos but should ask permission before posting them.
From that description I'm pretty sure you had to have been talking with Richard Hatfield.
His BMS design is mounted on a circuit board the exact size of the end of a thundersky cell. You bolt it directly to the battery terminals. On the board is some logic and a 5W resistor (for the shunt load). They can also be strung together in a bus for being read by a centralized controller.
Hey Ross - nice picture! My only concern with their setup would be the mechanical stress on the 5W resistors from riding around (they are just hanging on their leads and this will not survive long term; having leads break on these will cause issues) and also there serial connecting cable as the resistors will get hot and the wires are too close (yes, routing can solve that but the connector placements could be better). Otherwise good idea.
I am curious; what does their controller/display monitor? I have purchased the Paktrakr which can handle up to 24 cells and it monitors a lot and I love it, but it put me back $350...but overall I am at $450 total for my 21 cell BMS/balancer/monitor so slightly less than what it would cast for the system you are showing (but not a lot).
P.S. One thing I like about my BMS is I unplug it when done charging so don't have to worry about its mechanical robustness (and it does nothing when you are not charging anyway which would be the same for their balancing part). The Paktrakr is mounted permanently and has low cell alerts so this is there all the time.