Battery Management System for Large Lifepo4 Cells

Solar Mike

New Member
I have a collection of 40AH Lifepo4 cells removed from a 100V bank, some of the cells are at the end of their life and start to swell whenever they are charged, many are ok with approx 50% capacity but definitely on the way out. So I am going to build up a number of 28V (8 cell) banks and use for various projects, because they are so mismatched a BMS that can be set to limit each cell to 3.47v is really required.

I also have a 400AH 100V bank that also requires a management solution, so to keep costs down have designed my own. A main controller board with 8 cell sensing that is expandable in groups of 8, using a Picaxe 20M2 as the CPU. A common floating switchable voltmeter is used, using Optimos dual switches to select any individual cell in the bank. The measured voltage is isolated by a ACPL-C87A optically isolated voltage sensor, filtered, then read by the CPU, having a common voltmeter for all cells makes for easier calibration and only a single CPU to program.

The big 400AH cells have charge currents approaching 100 amps, so currents near 5 - 15 amps are not uncommon as the cells approach their sharp voltage rise around the 3.45 volt mark; means for balancing a load between 1 - 5 amps are required for short periods. This design uses large ceramic power resistors that can be switched across each cell as required. Some heat there but in a stationary pack not really an issue.

Added relays for external processes to tell that balancing has started, over\under voltage etc.

Here are the PCB's for controller and extenders.



The 20X2 CPU sits on a small plugin pcb, so I can experiment with different CPU's in the future; there is no schematic dia for this at the moment as I rushed this design out to include with some other boards to save on shipping costs; but I will create a schematic or parts of it and publish here once I have got it going.