E-Crosskart: Battery Balancing & Testing

I started balancing the battery modules yesterday to prepare for installation. I'm using the EBC-A40L from ZKETECH. It comes with software that has loads of features for doing all kinds of battery testing and data logging.

​As some of you know, I made a change to the module wiring since the battery build video. Previously, I had both positive and negative cables attached the same, exiting the module at the top. Because electricity tends to take the path of least resistance, this wiring would have forced the top cells to handle most of the load when charging and discharging while the other cells slowly try to equalize on their own, and eventually the top cells would need to be replaced sooner than the bottom cells. To fix this, I spun the negative cables around so that they exit the module at the bottom cells to force current to flow from top to bottom. Two of the pics below show the old wiring (left module) and new (right module).

​Atm I'm topping up each of the cells individually and putting them through a couple of full discharge and charge cycles to test capacity. Then I'll connect them in parallel and cycle them a couple more times before disconnecting them and bottom balancing individually. Basically, that involves discharging the cells to around 2.7V, stopping the discharge and waiting 24 hrs for the voltage to rise, then repeat until the resting voltage stays at 2.7 24 hrs after discharging. I'll do this with each cell and then they'll be balanced and ready to go in the kart and connect with the Daly BMS, which will take care of the balancing from then on.

The chart below shows the discharge and charge curves for one of the battery modules. The modules are discharging at least 139 Ah (445 Wh) from ~3.45V down to 2.65V, a lot more than what I was expecting. I purchased the 32650 cells from Battery Hookup as 5000 mAh, but with 139 Ah from a 24P module, that means I'm getting around 5700-5800 mAh per cell instead. That's an extra ~2 kW more than I was hoping for, so needless to say I'm pretty happy about that.

Discharge/charge curves for Battery Hookup 5000 mAh 32650 LiFePo4 cells (24P)

battery_1_capacity_data.csv
File Size:	716 kb
File Type:	csv

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The EB tester software allows the user to set up a schedule to execute different tasks automatically and log the data as shown above. To cycle and test the capacity of the modules, I set my tester up to do a constant voltage charge with 30A to top up the modules to 3.65V, cutting off at 2A, and then wait for 60 min for the voltage to rest to around 3.4-3.5 (typical resting voltage for LiFePo4 chemistry) before automatically switching to constant current discharge with a 30A load down to 2.65V, then switching back again to constant voltage charge with 30A to 3.65V, cutting off at 2A. Then repeat another discharge and charge, cutting off at 3.65V and 0.1A. 

The tester is a bit pricey, but I think it's a good investment for anyone who wants to do something like this (ie: build an ev). I can't imagine how long it would take to balance the modules for a battery this size using one of my little 5-10A power supplies. It would be months, at least.  ​