I posted recently about my struggle with a broken BMS from an Enginer PHEV kit. I initially tried to fix the BMS myself but without success.
During the process, I did manually top balance all my cells one by one using a LiPro balance charger Imax B6.
That was before I decided to go without BMS, and that is unfortunate because now I need to go the opposite way: discharge the whole pack to about 2.7V per cell.... it is now fully charged at 3.5V per cell and we are talking about 90AH cells.
The LiPro small charger is able to discharge a cell, but it will do it at 2AMP. Quick calculation means it would take 45hours per cell, that is too long.
So I am looking for a safe way to discharge my pack, cheap if possible. The challenge is to have something that could be unnatended if possible and that automatically disconnect.
I have some ideasusing some diode. Maybe by placing a couple of power diode in series i can make a device that dischare a single cell and naturally stop discharging at around 2.7V (2.7 divided by .7 per diode, that seems legit but maybe too good to be true).
Annyway there are many experienced EV builder out there who bottom ballance regularily, so I am looking for your advice.
Thanks in advance
JLD 404 setup with a switch and contactor connected to a motor to discharge the cells. Set up to discharge to like 2.7 volts. So when you have your meter setup correctly you can run a motor at high amps with a single cell and when that cell reaches 2.7 volts it will disconnect and when the battery bounces back up it will again energize the system and the motor will again begin to drain the cell and when it reaches 2.7 volts again it will again shut off. It will continue to do this until the cell can no longer bounce above 2.7 volts and at that point your cell is done. Do this for each cell until done. They will all be balanced perfectly.
I set mine so if they no longer trigger after an hour of rest then I consider it done enough. It may over a few days rise enough to trigger but there is so little to drain it once triggered it might as well not have done so.
So if you forget to check your cell will never go below 2.7 volts as long as all your setup is working properly. Works great and with a high current discharge like you get from a big motor then you are golden.
If you need a motor you can actually use the motor in your vehicle if it happens to be a DC motor. Other wise you will want to find another source to connect your cell up to for discharge. I just happen to have a few extra DC motors hanging around here that are perfect for the setup.
Don't do this with out the setup to shut it all down. I tried that and if your voltage is getting close to the end voltage you can't walk away. I did and lost a cell in a matter of 4 minutes when I went to pour and fix up a quick cup of coffee.
But with the JLD and all that I never lost a cell and all cells are well balanced.
Once I have the cells at 2.7 volts or for my case 2.6 volts I set my low voltage cut off on my controller to 2.8 volts. That way I will never go below my balance voltage on the low end. EVER. I have my current set so it will never go below that set voltage.
Pretty simple. Works and you can walk away without worry if it is all working correctly.
Hope this helps.
Might be time to revisit this topic.