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.
Extender:
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.
Cheers
Mike
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.
Extender:
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.
Cheers
Mike