Richard, thats not your firmware decreasing the voltage over time, it is your charger failing. Tell CS your bikes voltage is decreasing over the years and they will likely send you a new charger, very easy to install, or a dealer can do it under warranty.
@Curt and others who are not upgrading, If you have a 2017 or newer battery, there are good reasons to upgrade, I will leave it at that because its a lot of info, but the upgrade is good enough that I suggest you do it.
I know Curt has a 16, we upgraded ALL our bikes firmware to test for fixes and while I can see it is not what I would have done, it is acceptable. The only thing I would expect to be wonky is the estimated range, but the rest should be reasonable.
Thanks for the support, Cowboy, but I am not sure that a drop of one volt when the charger turns off at an SOC of 100% after a year is something that I would call a warranty issue. My bike never did finish charging at 117 V, that only happened with my 2014 S, when it was new. My 2018 S finished charging at 116 V the first time it was recharged after purchase. Now it stops at 115 V, at 100% SOC, but that is not big enough of an issue for me to make a warranty claim. I made my comment because that maximum voltage reduction happened immediately after a firmware update about 8 months ago.
Meanwhile, my old 2014 S (which originally maxed out at 117 V when it was new), that I gave my daughter, has stabilized at an initial cut off of 113 V and 92% SOC, after 5 years of usage. That still provides her will plenty of range for the riding that she does. However, if she keeps the charger plugged in for a week, it will eventually get up to 116 V and show 100% SOC on the display. The charger just turns on by itself every day and adds another 1% SOC or so to the pack before turning off again.
That why I always figured that it was a few cells going bad, rather than the charger, as that sort of thing happened to me with the Hi Power batteries that came with my two Electric Motorsport GPR-S bikes. When the individual batteries started going bad, they would fully charge more quickly than the other 24 batteries in the pack, and then start bloating up and eventually out-gas when the dumb charger and even dumber BMS would not shut off.
My experience with the EMS GPR-S bikes provided an early education into the wonders of electric motorcycles, Chinese Hi Power lithium batteries, wonky BMS circuits (one BMS caught on fire in my garage while it was charging, none of the others worked for very long) and no after-sales support, which tends to color my observations to this day.