ElectroClassic EV
Classic Cars Reborn into the Electric Future

Cell Swap

orion_util_hi_resist2Since our encounter with the median on Coast Highway requiring AAA to flatbed us home, I noticed some strange drops in the pack’s state-of-charge, sometimes falling up to 10% in an instant. I called Orion to discuss the issue, and they requested some cell data logs. You generate them by driving around with a laptop running the Orion utility, connected to the BMS via the USB port, and waiting for the suspicious event to happen. A glut of data is captured for each cell on a second-by-second basis, including cell voltages and internal resistance, shown in the columns above. Notice Cell 21 registers over 5.3 milliohms internal resistance, which is at least 5 times that of a healthy CALB lithium cell. The usual is just under 1 milliohm. Orion suggested either Cell 21 was damaged, or the cable connection to that cell was presenting a high resistance.

 

new_cellsMy gut ruled out bad cabling, so I immediately went on the prowl for a replacement for the ailing Cell 21. The CALB SE180AH is among the first generation workhorse prismatic LiFePO4 cells that many EV conversions are based on. CALB has since produced a second generation grey lithium cell, but I wasn’t for mixing them with my first generation pack, so I began my search for an original blue cell. CALB’s US warehouse in Pomona did not have any remaining old stock, and the next shipment by sea from China wasn’t due for 3 months. It wasn’t happening, so I reached out to several EV parts dealers hoping that somebody would have a spare, and finally located a few at EVTV. Because they were so hard to locate, and were in good used condition, I bought all three for good measure. Three birds in hand…

 

punch_cutoffOnce a chunk of time became available, I set about extracting Cell 21 for replacement. The first step is to disengage the battery pack before touching any tools to EV components. My emergency disconnect is a Curtis-Albright high current snapper switch, which is mounted on the center console inside the cabin. Its one job is to break the series battery circuit, dropping the pack voltage to zero. I call it “punching the clown nose.”

 

bms_disconnectThe next step is to disconnect all of the cell taps and other connections to the Orion BMS. This prevents accidental short circuiting of partial pack current through the BMS, which would certainly end its life on this planet.

 

aux_batt_disconnectFollowing that, we separate the small green Anderson connector, isolating the auxiliary battery from the vehicle’s electrical system. Again, this ensures that there isn’t an accidental and unanticipated short that could torch any number of sensitive EV components.

 

celltaps_disconnectNext step is pulling the large connectors from both input and output of the DC-DC converter, which is the gray box shown above. Then all red cell tap wires and high current cables are disconnected from the cell block that’s being extracted.

 

cells_extractedThe vertical poly straps that secured the module to the battery rack were clipped, and voila! Fifty pounds of plastic, copper, and lithium lifted straight out of its resting spot in the former fuel bay. Lastly, the horizontal poly strapping was snipped, and the cells separated.

 

cell_strappingAlong the way, I decided that Cells 23 and 24 were also exhibiting internal resistances slightly higher than the rest of the pack. It was an obvious advantage to also replace them while they were accessible. Above are the “new” cells configured, numbered, strapped together, and ready to drop into the battery rack. The strapping tool above takes a few attempts to master, but ends up being the simplest, thriftiest, and dependable solution to mount these heavy cells firmly in their racks.

 

dcdc_new_standoffWhile things were apart, I also noticed the rubber standoffs for the DC-DC converter were a bit too soft for the job, and caused it to sag more than I liked. So I bought a few larger standoffs from McMaster-Carr, and replaced them while access was easy. In the shot above, I’m holding one of the older, skinny rubber mounts, with the new, thicker mounts already in place.

 

new-cells_in_placeHere is the DC-DC converter reinstalled, and the new lithium cells strapped into the battery rack. Operating the strapping tool in such a confined space was a challenge, but the poly staps became intensely tight just one ratchet click at a time.

.

noaloxBefore reconnecting the cells, the battery terminals and strap ends are brightened with a bit of sandpaper, and then schmeared lightly with Noalox to prevent corrosion and promote maximum conductivity at the joint.

 

new_cells_back_finalHere are the cells reconnected, complete with braided bus straps, cables, and cell taps. All remaining connections are made in reverse order – the DC-DC converter is plugged back in, the auxiliary battery is put back online, all connectors are restored to the BMS, and the clown nose is given a good yank. Several seconds pass without a trace of magic blue smoke, which is very good.

 

cell21_better_resistanceDriving about and capturing more cell data shows Cell 21’s resistance falling within acceptable range. It’s still slightly high, probably due to the 3 foot of 2/0 cable that connects to Cell 20, rather than the usual short braided straps. According to the gents at Ewert Energy, this is not unusual, and they discouraged me from losing sleep over a slightly elevated reading.

 

hydraulic_crimperI didn’t lose sleep, but I did have dreams about eliminating that last milliohm from Cell 21’s resistance. I didn’t think the cable was bad, but the culprit could easily be a bad connection at the terminal lug on the cable. What you see above is a gift to myself – a 16 ton hydraulic crimping tool, purchased online for about 50 bills. It really is the best solution, compared to the barbaric swedge tool that I had used for all the original cable crimps. It also includes 10 sets of different size crimp bits to accommodate various cable and terminal dimensions.

 

lugs_cut_daylightLuckily, there was enough extra length on the front pack cables to allow for new terminal lugs without needing to re-cable everything. After disconnecting all traction pack cables, plugs, and leads, I cut the terminal lugs from all four of the cell connections on the front pack modules.

 

good_hex_crimpThis is a perfect hexagonal crimp on a 2/0 AWG cable terminal. The idea is to put so much pressure on the cable inside the lug, that the copper strands are caught in a “cold weld,” fusing them together into a single lump of copper. I apologize for not including a picture of the process, as it required both hands.

 

cell_balancingFinally, we see most of the cells in the pack (in red), being discharged to bring their voltage down to the same level as Cell 23 and 24 (row 3 – column 3 and 4, in white). This is called top-balancing, where the goal is to match the voltage of all cells at their maximum state-of-charge. The EV field is divisive on this issue of top vs. bottom balancing. The arguments for bottom-balancing are very compelling, but this system was built to top-balance, and that is how it will continue to operate. Perhaps I will give the alternative a shot in the near future. In the meantime, I have already recovered about 5 miles in range, and continue to gain as the pack gradually comes into balance over repeated charge/discharge cycles.

 

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3 Responses to “Cell Swap”

  1. Very interesting!

  2. just wondering did you get rid of any additional resistance in cell 21 when you re-crimped? I have same set-up with Orion and grey 180 cells. I have a few cells in the 1.3 range.

    • Hey Mike,
      That is a great question. I’m not sure why I didn’t answer it for myself after the new crimps were installed. For that matter, it might be time for me to take the laptop out on the road to collect more operational data. Sorry I wasn’t more helpful, but thanks for reminding me it’s time for a checkup.
      Mark


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