I used the *.s19 files contained in the zip files within this thread.
If the file doesn't match the chip then the firmware will not load (but see below!) and the App will exit with an error.
Make sure you only use the "309" files. I have a 4S BMS and didn't pay close enough attention to the file names and loaded a 309 file instead of a 303. I guess the chip was correct even though the number of cells wasn't because the firmware loaded. The BMS "sort of started" but gave nonsense values for 8 cells. No lasting problem because I easily re-flashed once I realised that I had been stupid.
Please note that once the BMS firmware was upgraded, I needed to reboot the BMS by unplugging the cell connections for a few minutes and then I needed to apply a charging load to get the BMS to turn on properly. Just hitting the startup button only started the display but I couldn't turn on either charge or discharge.
[EDIT]: I forgot to mention, download ALL the firmware files you find in this thread, even if they appear to be duplicates. I found that the one that worked on my BMS had a file reference very similar (as in, the chip number and date) as one which did not work. It's possible that some of the uploads may have been corrupted on download or unpacking.
)
