Lithium Swap 2 - Balancing Cells

Once I had the truck back in my home community, I decided that I should probably determine whether the battery pack was any good before making up a schedule for swapping it into my car. Since the truck was currently registered, and it ran and drove, I added it to my insurance policy with the intent of putting some miles on it to see how well the batteries performed. I installed a suitable 240 volt receptacle at the recycle yard so I could charge up during testing. If things checked out alright, I'd drive the truck the 12 miles up the river to my house and begin the project.

The truck was a bit reluctant to run after sitting for more than a day or two, the 12 volt accessory battery under the hood wasn't keeping a charge, so it required jump starting most every time I tried to run the truck. Out in traffic, the truck was sluggish, and my unfamiliarity with the feel of the clutch and transmission didn't make shifting any smoother. The instruments seemed to be indicating that the batteries were holding up alright, though.

Charging at the yard worked, but the BMS would cut off the charger before I expected, and the charger was incapable of running on 120 volts, which it's supposed to be able to operate from. Overall, it was a pretty unedifying experience, and after a couple of weeks of on-again-off-again testing, I had Scott bring the truck to my house on the wrecker so I could get deeper into it all.

One of my first concerns was that several of the BMS modules (every one of the 38 cells has one) were defective, while five others had been replaced with a similar, but different design module. Added to this, many of the modules had corrosion growing on them, and everything was covered with a heavy layer of road dirt, dead bugs, etc. When changes in the atmosphere caused condensation, the cells, the BMS modules, and all of the other electronic bits and pieces would get beads of water building up on them. It was obvious that the entire setup needed to be disassembled, cleaned, repaired, and housed in a more weather-resistant enclosure.

I removed several of the BMS modules, and though testing, culled out a few that seemed to meet the original design specifications. After removing all of the jumper straps from the cells, I began doing a balancing charge on them, two at a time, installing a jumper strap, and a functional BMS module on each cell. Then I set up a regular 12 volt car battery charger and a variac (variable transformer) to allow me to set the charge rate. The two BMS modules were linked to the BMS control head, which controlled the relay that fed AC power to the variac, so if either of the cells being charged at any one time came up to the high voltage limit, it would interrupt the charge cycle. Usually, one of the two cells under charge would come up full, while the other needed more time. I would clip-lead around the full cell, adjust the variac, and continue charging. it actually took a couple of weeks to fully charge the entire pack in this manner, playing hide-and-seek with rain clouds, as the truck was sitting out in my driveway with the cells still installed. I thought that maybe I'd put the cell jumpers back on afterwards and drive it a bit more.

The goal of the balancing charge was to bring every cell up to completely full, something that can only be done by a couple of methods, and charging individually (or in pair, as I was doing) was the best option for my situation. Some of the cells, the ones with the different BMS modules, were already completely full from charging at the yard, but most of the rest of them were down some, anywhere from 2 to 10 ampere-hours, for the most part. A couple of cells were down 20+ ampere hours, and one seemed to be down 60 - 100 ampere-hours, it took a ~very~ long time to bring it to full. I made note of the cell numbers of the worst cells, intent on keeping track of this in case it was a sign of defect on those particular cells.