If by degradation, you mean loss of range, that's an invalid way to determine degradation, particularly right after downgrading your firmware.
The battery capacity estimation is performed by the battery management system, and that would likely have to be completely reset for a firmware downgrade to work correctly. So it will probably take many, many complete charge cycles before it actually knows the battery's true capacity.
Also, the loss of capacity in miles is
not a proper measurement of battery degradation anyway. That's just a rough estimation of capacity, which likely is just a combination of the initial capacity loss (caused by a reaction between the electrolyte and the anode when a battery is brand new), and current loss caused by cells failing.
But Lithium ion cells fail in two other ways:
- Dendrite formation — whiskers of elemental lithium that create shorts in parts of the battery.
- Internal resistance increases over time.
Both of these can be responsible for batteries suddenly failing to produce the expected amount of current when a battery gets below a certain state of charge, resulting in sudden shutdowns.
More importantly, both of these can be detected by sudden changes in the battery's voltage, but neither can necessarily be detected on a continuous basis. You'll only see the sudden voltage change when the battery is at just the right state of charge, and not every time.
By blowing out the BMS's memory, your car now has no idea what potentially dangerous events have occurred during the battery's life.
It also has no idea how low the battery can safely get before it suddenly loses power. So you could get down to 50 miles and your car could just plain stop on the side of the road because of either of the problems mentioned above.
So again, I am firmly of the opinion that you should take the car and its battery to a
real repair shop that knows what they are doing, and let them pull out the battery pack and run each subpack through multiple charge-discharge cycles to exercise the cells and determine whether they are, in fact, performing as expected or, as I suspect, one subpack is in a state that could potentially be dangerous, either because of fire risk or sudden shutdown risk.
Ignoring warnings from the battery management system is a very, very bad idea, particularly if those warnings are telling you that charging capacity is reduced. It is possible that the problem could have been a software bug, but unlikely. And it is possible that a full charge cycle could clear such an error, but again, unlikely. More than likely, you've made a dangerous situation even worse by allowing the car to charge to a maximum charge voltage that was arguably unsafe to begin with (hence batterygate) while clearing out the BMS memory that would have allowed it to remember which subpack is faulty and avoid excessively stressing it.
Whatever you do, don't charge your car in your garage or carport going forwards. If your insurance company finds out you downgraded the car's firmware to avoid a BMS warning about the safety of the battery, they may not pay out if your car catches fire. I'm just being blunt here. I worry about having 50 watt-hour laptop batteries in that state, much less an 85 kWh battery.
