The three bars are supposed to represent the percent of battery power available to accelerate with - 3 is 100%, 2 is 66.67%, 1 is 33.33%.
Theoretically when you have two bars you should only be able go up to 66% on the % of Power gauge when accelerating and 33% when it is at one. If it's none you probably aren't going anywhere.
Since I rarely ever exceed 25% I've never noticed a difference in the performance of my car, but others who like to hit the pedal hard can tell you if it actually has an effect.
The cold has a big effect on this. In the summer I have to be down around 10% on the SOC gauge to go down to 2 bars. For me this winter it can be 30% or more and drop to 2 bars when in years past it was 20% in winter. One thing different for me is that I had the 2016 BMS update done in 2020.
If it's not the BMS update, it may be the indicator of a weakened battery pack or just that the SOC gauge is overly optimistic and you really don't have as much power as it says. I have noticed that the SOC does seem to lose more suddenly now sometimes for some trips that I take - as in losing 30% for a trip that usually only takes 20%.
I am keeping an eye on mine and will see if the effect continues into the summer or is just in the winter. I didn't see any issues in the summer or fall after the BMS update was done late spring (if my memory is correct).
