“Tesla's BMS is actually quite advanced at detecting irregularities in the pack. In a normal unmodified pack, if a module or cell group started to no longer match capacity (SoC/CAC) with the rest of the pack, the BMS was designed to detect this and fail as gracefully and safely as possible (usually by limiting range, and eventually disallowing driving).”
“Cutting a cell fuse (Gruber’s method) or replacing a module directly causes discrepancies the BMS was designed to detect. While the BMS was designed to attempt proactive balancing of the string as a whole to keep everything in line, it was designed to be able to handle normal variations in factory-new modules used together as a pack, not 14 modules 8 years old and 2 oddball modules from who knows where. That's way outside the BMS's ability to keep things in line.”
Like the Electrified Garage and Gruber Motors, Hughes has a company for Tesla repairs called 057 Technology. He learned about this issue while trying to do precisely what these other companies are now willing to offer.
“Originally, I was in the camp that Tesla designed the system to be able to handle a module replacement (prior to my full reverse engineering efforts) and tried this on a vehicle with a physically damaged module. Matched as closely as possible at the time, the car worked perfectly once all closed up. Success! Except after about five charge cycles, the car started losing range. After about a month, the car had about 20% of its original range usable and finally threw a visible error that the battery needed service. A few cycles later, and it was no longer allowing driving.”
“I tried this several more times, thinking if we could just get a close enough matched replacement module that it might just work. In the last case, I waited for months to find battery modules that matched the characteristics of the pack we wanted to try and replace a module in. We lucked out and found one that was built the same month and was only a few hundred serial numbers apart. External testing showed the match should have been as close to perfect as you'd ever expect. We did the repair, fixed up the car, and all was well! Success, finally. I eventually sold the car to a couple about 600 miles away.”
As you must have already guessed, it just briefly seemed to work as it should.
“After almost a year, the buyer reached out about a loss of range that had been accumulating. I still run a small private network of Tesla vehicles and could see that over the course of a year of normal use, this car started showing an SoC imbalance with that replacement module. The car had already restricted use of about 35 miles of range.”
“They decided to drive the car back to us so we could replace the pack. On the final supercharge stop, the fast charging pushed the imbalance past a tipping point, and they were stuck. I had to drive up about 175 miles with a trailer to pick them and the car up.”
This is why the Tesla Hacker started warning buyers that this was not something they could do and expect it to work for good. Hughes found an alternative solution for these cases.
“At this point, I just explain to customers that this isn't a viable fix. Instead, we just sell them a full replacement pack and give them credit/discount off the work equal to the value of the usable percentage of components in their existing pack. So if two modules are bad, they basically pay for two modules plus labor and get a full replacement pack, not a franken-pack.”
commented on Twitter that reman (remanufactured) battery packs from Tesla already come with a lousy imbalance. Hughes clarified that there’s a difference between voltage imbalance and CAC/SoC imbalance. Tesla’s BMS would constantly correct the former, but not the latter.
The Tesla Hacker said on Twitter that he gives these other companies the benefit of the doubt, but that they “don’t have all of the information.” That said, he felt it was his duty to warn about how ephemerous such repairs may be. In a way, that's also helpful for the companies offering this sort of service with no idea of what can happen in the future.
“The problem with these fixes is that they do make the car work again. It's the long term that's the problem. It could be days, could be a year... but they all eventually fall back due to the SoC imbalance. So when a company like Gruber or guys on YouTube are like: ‘Look what we did to save these people a ton of money!’ people are in awe and never get the follow-up story.”Gruber Motor and the Electrified Garage
With what Hughes told us, we talked to Gruber Motor and got in touch with the Electrified Garage. They both have presented their methods not only as money-saving but also as safe ways to put these Tesla vehicles back to work.
Gruber Motor’s CEO, Pete Gruber, did not hold back talking to us or publicly.
“It’s simple: we figured out how to do it, he hasn’t. We have been putting Teslas back on the road, Roadsters, and now Model Ss, for years with no issues. How does he think Tesla created reman battery packs in Lathrop, California?”
“We have a method for isolating resistive and parasitic cells that is lengthy and proprietary. We also find a fair share of BMS board problems, all repaired down to component level, and some water ingress issues. We carefully match CAC values on modules to eliminate the incompatibility issue Jason talks about.”
With a demand of two of such repairs per week, Gruber told autoevolution that his company had only one Roadster and one Model S return due to new parasitic cell issues. He offers a one-year workmanship warranty, which primarily covers installation errors.
“We have done over 50 Roadsters so far and a dozen Model S vehicles. The Model S uses the same mass cell 18650 cells with a BMS. We are finding them to be sufficiently similar and getting the same results, which is years with Roadsters, and a year or so with the Model S since they are now coming off warranty.”
responded to the Tesla Hacker on Twitter.
Benoit first mentioned how much he admires Hughes’ and Gruber’s work. They would have helped him put Dolores – his first rebuilt Tesla – back together through the TMC (Tesla Motors Club) forum. He also mentioned more than once that Tesla also works with remanufactured battery packs.
To try to clarify the situation, Benoit said the Electrified Garage performs both the replacement of the entire battery pack and modules. He then said that replacing battery packs is always better than just the modules but that this sort of repair intends to give customers a choice. For a car worth $25,000, replacing a component for $22,500 does not make financial sense.
Electrified Garage partner, they have already performed both kinds of repairs, the oldest one being two years old already. However, Benoit did not mention if this oldest fix was replacing a battery pack or a module. He also did not inform in which vehicle it was performed. Steven Salowsky, the Rich Rebuilds YouTube manager, told autoevolution that the repair Benoit mentioned was made in a Model S and it was just a module replacement that went really well. Unlike Hughes' experience with them, it is still going strong after two years, according to Salowsky.
Hughes shared on Twitter that the Model 3 has a slightly better balancing ability and that it has 4 battery modules instead of 16, which may make it easier to replace them. However, he thinks that this EV may also present the issue.
Hopefully, these guys will join forces instead of arguing with each other and offer all their clients repairs that will last for a long time, regardless of how much they cost. What everybody wants is to see these EVs on the roads instead of in junkyards and Tesla is not the only option to make that happen.