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Range drop while plugged in

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Our new-to-us MX has an interesting habit of slowly dripping away range while plugged into a Gen3 HPWC. I have it set to 90% charge so it gets up to 251 miles, but then drifts down to 248 by morning (if the doors are closed) or 241 by the next evening (if left in the garage with the trunk open). Should it trickle charge after hitting the configured threshold? Any settings that I might have wrong?
 
Once the vehicle has been connected to power and reaches the set charging goal, charging stops. The vehicle will slowly lose charge to power the onboard electronics - just like it would if it was parked without the power connection. And when it has dropped enough, the charger kicks in and brings it back to the desired charging level.

This is likely done to prevent the batteries staying at the same charging level.

Note that when parking for extended periods, the software appears to recognize extended time parking and reduces energy consumption to conserve battery charge.

We recently left our S 100D parked for two weeks disconnected from power - and still had a lot of charge left in the battery. Though you can also help reduce power consumption while parked by adjusting some of the settings (like disabling sentry mode).