Balance at the cell low voltage limit

So the end result for the subject of this thread is that you do not want to "balance at the bottom" if you have any of the following
1. Mottcells with overcharge protection.
2. Any balancer, either DB8s or a BMS16C
3. CellLogs that do not have the 8 cell power draw fix.
Additionally you need to be very carefull using the newer charger with "balance at the bottom".


Hi Dan –


Some good info here, but I have an additional comment about your conclusions. I just finished adding an additional 2KWh to my original 2KWh system. I charged up the original cells to top where the BMS-16 shut off the charger. I removed those cells and made buddy cells out of the old 16 cells pairing the highest with the lowest as has been previously described in PC. That was probably in one of your previous threads. Next, I made buddies out of the new cells and installed them as cells 9-16 in the new 4KWh system. These cells were fully charged by Enginer. I used the Enginer charger to complete the charging process. To my surprise, when the BMS 16 shut down the charger in the new 4KWh system, the original cells in 1-8 were considerably under charged compared to the new cells. I drove the Prius to get to shut down by the BMS-16 and observed that the old cells were causing the system to shut down early. (no surprise here!)


I used a variable voltage power supply to add charge to the original 16 cells. The power supply has both voltage and current limiting. The current limiter and will only allow 4 amps to be drawn at 27 volts. The 1 hour of charging brought the old cells up closer to the new cells. It is difficult to tell how well balanced the cells are because after the power supply is removed, the cells will sag back to a lower voltage than when the charger is on. After using up the charge by driving the Prius until the red light on the dash came on, I found that the old cells were still shutting down the system. This time I added only 45 minutes of charge not wanting to make these higher than the new cells. So, I find myself doing manual balancing even with the BMS-16. I’m uncertain as to why the original cells were so much lower. However, slowly adding charge seems to be working to even out the bottom voltage of the old cells after the red Enginer dashboard light comes on.

Paul

 

All cells, both new and old, are Mottcell pre 09/12/10. I used the voltage as read from the BMS-16 to make the pairing decisions. I have no means of measuring actual capacity, but I didn't realize that voltage and capacity aren't related. I will keep adding charge in small amounts to the old cells until I am satisfied with the balance between the old cells and new cells. Then will let the BMS-16 do its job. Perhaps the next version of the BMS will take care of this type of problem by doing the balancing automatically.

The added capacilty of 2KWh is really great. My round trip commute is so short (13 miles) that I only need to charge every other day unless I make a long trip somewhere.

Cheers,
Paul

 

Dan -

Here are the numbers. 16 cells at 100202005 and 16 at 100302005. This took a while to get to you since I've been on a business trip during our recent PC conversations. I got back late Sat. night and could not get to the battery pack which was in the airport parking lot.

BTW, on Sunday I did another day charging at the bottm to equalize the cells and I am still not happy with all of them. However, I am now happy with 12 of the 16 buddy pairs, which is up from 8. I think I will start to balance at the top for a while. I'm still not clear on which method is the better, but all 16 cell pairs will be balanced when I'm done with this!!!

Thanks for your interest, as you can tell I am new at this.

Paul

 

I agree with Dan on the balance at the bottom warning. It is a rather tedious process and very dangerous if not monitored closely. I had to balance 64 cells in my 8kWh pack and that took quite some time at 10 amps. Fortunately I have not had to do that process since Feb this year and after about 150 cycles I am still within 0.010V margin between my quad buddy cells.

I am about to get my converter upgrade and BMS16C so I will be experimenting with the new BMS later. I may just keep it there for now and leave my setup alone after I install the 5000W converter.

 

Thanks Dan. I have an extra string of 16 cells of the motto batteries i purchased from Jack as a emergency replacement. I will run some test to see how they work with the BMS16C since I have an extra converter in the short term. I was planning to test in my Hybrid Altima to see how the MPG would be affected. Just need an extra charger to create another system.