Quick update on this one…
There were a few problems:
- Some cells were “weak” and have been replaced with new ones. The cells I removed seem to be holding a charge ok so I may repurpose them for lower current duties elsewhere in a 12V pack.
- Top balance - it took 2-3 weeks of balancing but I finally managed to get all cells to 3.50V and they now appear to be staying balanced or balancing easily/quickly (an hour or so). I had to help the initial balance by adding some extra charge to some cells, and manually burning off charge from other cells using an automotive light globe.
- Poor connections - the original cells were provided with 4mm scr ew holes which are way undersized for the application and cannot support the torque required to simply hold the busbar in place. Most of these I have drilled/tapped to 5mm and inserted grub screws as terminal posts. A small number have even been tapped to 6mm. I also found a lot of the terminal connections about 1/8 of a turn off full torque. They’ve now been cleaned and torqued properly. Note that all terminals use a washer/spring washer combination or a serrated nut so they shouldn’t be working loose. I’ve also liberally coated each connection with vaseline - found this to be much cleaner and just as effective as the no-ox grease.
Under load now I’m seeing a much more even voltage drop across all cells. A few cells did show a larger voltage drop which I resolved by a clean/torque of the terminals or swapping out the cell.
A steady load for this application can be 20-30A (0.2 - 0.3 C) and I have run them at 100A (1.0C) for a few minutes without any problems. Yes they show a large voltage drop at that load, but they recover well to a more normal voltage when the load is reduced or removed.
I do wish I had 280A cells instead of 105A, both for runtime and to support a larger load, but with over one hundred of them the cost difference becomes significant. Interestingly the cost of 280A cells is almost down to what I paid for 105A cells 12 months ago…
Thank you to everyone that got involved and helped out; after all the dramas it is still a fun learning experience.
Cheers,
Jon