I also was part of the "internal resistance" camp but with my 2018, I am no longer confident of anything regarding the measurement. My ahr, SOH drops or stays the same EVERY day (not a single rise...not one) since day one. But my Hx is currently in the 116% range having been as high at 117½%. It fluctuates up and down a lot with the "ups" obviously more common.
2011 SL; 44,598 miles. 2013 S; 44,840 miles.2016 S30 deceased
. 29,413 miles. 2018 S40; 11,987 miles, 485 GIDs, 37.6 kwh 110.89 Ahr , SOH 96.00, Hx 115.22
My Blog; http://daveinolywa.blogspot.com
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