First Hello, I'm new to this group and this is my first post.
My native language is not English, so apologize in advance for typos etc.
My bike is not a MTB but an ordinary city/tour e-bike (Gazelle Orange C8 HMS) with a E6100 motor and 504 Wh, BT-E6001 rear rack mount battery.
I've been reading this battery life thread with great interest, as I see some of the same issues with the BT-E6001, even if I only bike on asphalt and sometimes light gravel roads.
We also do not have steep hills here around Roskilde, Denmark (~30 km west of Copenhagen)- No more than 20 - 30 meters, I think.
Here are the resuls for my battery:
1) # of charging cycles = 11
2) remaining capacity = 90%
3) charger = E6000 (fast charger)
4) age = ~3 months
My battery also suddenly jumped from 100% to 90% when charging cycle went from 10 to 11.
Battery has never been below 20% and normally charged up to 80 - 90 % and 100 just prior to a longer run.
Always kept and charged indoor at room temperature.
(I'm retired electronic engineer, so I know how to handle Li-Ion batteries)
I mostly do short trips around 10 - 20 km and to local market only 2 - 3 km, so it's hard to tell if range have been degraded.
Longest trip done so far is 45+ km.
As I've always been measuring things due to my former work in the calibration world, I've made an Excel chart of the calculated range as a function of "(kmTravelled * 100)/ (ChargeBeforeRide - ChargeAfterRide)".
This chart is attached and it show a slight decrease in the linear trend (dottet curve) from app. 82 km to app. 75 km/full charge.
The range decrease can be caused by colder weather, more windy and/or me using higher assist level as ohters have mentioned in earlier posts. If it flattens out in the spring, I'll be pleased. If the indicated battery health decreases again after 20 charge cycles I'll make a claim to the bicycle dealer.
After I saw the sudden decrease in capacity I've measured the energy (Wh) going into the battery during charge with a cheap energy meter and will follow this closely in my spreadsheet too. It is not a precise figure as there are several uncertainties (loss in charger / battery, phase error etc), but it gives a good relative picture if the battery charges up with less Wh over time.
Sorry for the logn post...

Regards Ole in Denmark.