I do totally agree. As I use a Wahoo ELEMNT for my SL e-bike, I am able to track each e-bike battery level independently.
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I own as many as 4 Range Extenders and have been on multiple long rides with discharging the main and extender batteries in parallel or with the RE first. That gave me a great insight into the
mechanics of the process. For instance (when you look at the level % only) it appears the main battery is discharging faster than the RE. In reality, the system just tries keeping the identical
charge for both batteries. (This is for the parallel discharging). In case of discharging the RE first, the main battery kicks in at times to meet the system power demand, especially at high assistance modes.
Assume you are discharging the RE first. The main battery is untouched for the most of the ride. However, as you are at low RE charge, the main battery starts helping the RE anyway to protect the latter!
All this is masked by the "150%" style battery level display.