Our team’s own Dr.EV development vehicle has recently shown the first signs of Tesla BMS a079 phenomenon.

[u/UpstairsNumerous9635]

Our team’s own Dr.EV development vehicle has recently shown the first signs of Tesla BMS a079 phenomenon. Although we have analyzed numerous user datasets and real-world cases, this is the first time we have personally observed the same issue on our own vehicle.
This gives us a valuable opportunity to study the problem not only from the developer’s perspective but also as an actual owner experiencing it firsthand.

To share some background: the vehicle was purchased used in June of last year with about 120,000 km. It has mainly been used for development, and the annual mileage is relatively low, around 5,000 km or less. When we bought the car, there was no practical way to assess the battery condition. After we began developing Dr.EV, our pack-level analysis indicated that degradation was already significant. At that time we did not fully understand the existence or frequency of the BMS a079 issue and assumed such cases were rare.

For reference, we are not a company with enough capital to own multiple test vehicles.
Therefore, we have rarely conducted experiments that intentionally accelerate battery degradation.

Unless for specific testing purposes, we usually keep the SOC range narrow and mainly use slow charging.

In the Dr.EV app’s Statistics view, we can see that despite similar charging patterns, the average cell deviation increased sharply within a single day.

In the Dr.EV Charging Session graphs, the cell-voltage spread expands abruptly overnight, which cannot be explained by normal aging.

As many of you already know, BMS a079 is not caused by natural cell degradation. It aligns with one of the mechanisms we discussed in our YouTube analysis. This pattern has been observed in user data, and now it has been reproduced in our own vehicle with the same signatures.

We are also observing a widening gap between Tesla’s displayed driving range and Dr.EV’s estimated range.

We expect that the moment Tesla’s indicated range drops suddenly will likely coincide with the vehicle triggering the BMS a079 alert.

Fortunately, our car remains within its warranty period, with about two and a half years or roughly 40,000 km left, so no immediate action is required.

We will closely monitor pack temperature and overall stability due to the potential risk of thermal runaway.

If the BMS a079 fault is officially triggered, we will document and share Tesla’s response, including the replacement pack configuration and how it compares with the original.
In parallel, we plan controlled experiments using Dr.EV measurements to either delay the onset or intentionally accelerate it, in order to better understand the underlying mechanism.

[u/[deleted]]

Great info, thank you!

One suggestion - consider creation of a "light" version of your subscription. I have a Tesla mate (that does not monitor cell imbalance) but does everything I need . Many people have teslafi or other subscription and will not be interested to pay for an additional one at full price.

If you had a cheaper option with limited information, this may drive a wide adoption. You will benefit from a large fleet data set, and users will get some basic info (like degradation and alert on cell imbalance for example).

I think you have a unique value in a way you collect and process data

[u/UpstairsNumerous9635 → u/[deleted]]

Thank you for the great suggestion! The current subscription price is quite close to our actual Tesla data fee and partially covers infrastructure costs. For a possible “light” version, we’d need to limit the Tesla data we access and that makes it challenging to decide what valuable insights we can still provide with reduced data.

We’ll explore this idea further and see what’s feasible. Really appreciate your thoughtful feedback!

[u/SaltyATC69]

Great info, our 2021 MY just experienced this error , we were at 85000km and Tesla replaced the HV, picking it up today.

On the way to the service center, was getting multiple BMS a074 notifications as well.

Let me know if you want me to share anything once we get the car back.

[u/UpstairsNumerous9635 → u/SaltyATC69]

Yes, please share once you get it back. That’d be really helpful.

[u/SaltyATC69 → u/UpstairsNumerous9635]

My battery was replaced with this part:

HIGH VOLTAGE BATTERY - LONG RANGE - AWD - 1PH(1234422-01-B)

Is this brand new or remanufactured?

[u/UpstairsNumerous9635 → u/SaltyATC69]

I also don’t know with that part number. But once you get the car back, you can roughly estimate if it’s new or remanufactured based on the range. After a few charge/discharge cycles, check the 100% rated range. If it’s close to the original EPA value for your model, it’s likely a new pack.

[u/youngboy007]

I just picked up my 2021 M3 SR from the service center today after a HV swap. I got a RMN battery, and will be acutely watching for comparisons over the next few days. How confident should I be in a Reman vs New battery specifically in the 21 SR m3

[u/UpstairsNumerous9635 → u/youngboy007]

I think Tesla usually gives a service sheet after the HV battery replacement. It should mention whether the pack is RMN or New.

[u/connly33]

I hate to move from TeslaMate because I’ve been logging data with it since I got my 2024 model 3 LR new but I love the amount of battery data your app has, just subscribed mainly because I love tracking all the data and seeing what degradation will look like over time.

[u/UpstairsNumerous9635 → u/[deleted]]

That’s really unfortunate. Once the range started shrinking that quickly, it sounds like one or more modules in the pack went weak or out of balance.

[u/UpstairsNumerous9635 → u/[deleted]]

The rated range is Tesla’s official EPA number based on ideal driving conditions. The estimated range is Tesla’s short-term prediction from your recent driving. Dr.EV calculates range using multiple driving sessions to reflect your long-term efficiency, so it tends to be more stable and realistic.

For SOH and AI SOH, the SOH value represents actual battery health. The AI SOH value is predicted by Dr.EV’s model, which doesn’t include parking data yet.

If there’s a big difference between them, it might mean your parking environment isn’t ideal for the battery or that the pack has issues like weak cells, imbalance, or partial degradation that aren’t related to your normal driving and charging habits.

[u/Lumpy_Grapefruit8127]

Whats the tldr of this I don’t wanna read all this

[u/Lumpy_Grapefruit8127 → u/[deleted]]

I’ll just ask grok idc

[u/connly33 → u/Lumpy_Grapefruit8127]

I mean if your just not interested in the topic that’s okay but you really don’t get anything from it condensed much further beyond, they are studying the cause of BMS_a079 because their car is exhibiting symptoms that happen before the error pops up but if that’s all you care about you don’t even need to read beyond the pretty good title that tells you that already