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The phone might recognise that it's a smart battery case and not just a regular cord. Maybe it'll disable that feature just for the case.
 
On my XS Max I can confirm that my phone stays at 100% and even if I take it out of the case and let it drop below 80%. Putting back in the smart case always charges the phone back to 100%
 
I've noticed the same issue, but keep thinking that it has got something to do with my wireless charger and maybe even the hot weather right now (as crazy as that may sound, I have experienced problems charing when it's very warm). Using a regular charger, or like right now, a Mac adapter for fast charging and all, 'solves' it; it'll charge all the way up to 100%, for both the phone and Smart Battery Case (SBC).

Bit frustrating, because over the past days I keep waking up noticing my phone is at 98% and the SBC at 80%. For now I'll stop using my wireless charger, because I am confident that it is the reason it doen't fully charge.
 
I’m not having any of the same issues.

But I only charge the phone overnight in the SBC. Always wake with both at 100%
 
I was definitely hoping the feature would work with the smart battery case... logically, if it’s bad for the battery to keep it at 100% all night, it’s probably also bad to keep the battery at 100% constantly when it’s being powered by the case...
 
I was definitely hoping the feature would work with the smart battery case... logically, if it’s bad for the battery to keep it at 100% all night, it’s probably also bad to keep the battery at 100% constantly when it’s being powered by the case...

It must learn as mine now won’t go over 80% unless it’s night time.

I used my phone in the SBC a few days ago whilst using google maps. SBC dropped to 40% after 3hrs but phone was still at 100%.

Plugging into the vehicle charger and it stopped charging at 80%.

I tried on further journeys by using phone in case until SBC dropped to 53% (no particular reason it was when I took a rest stop). Then used the phone only until it reached 60%.

When I connected the SBC it charged the phone to 80%. When I plugged into the vehicle charger after that it charged and held both at 80% (was on charge when both reached 80% for nearly an hour)

So the feature does seem to work. Assume it just takes a bit to learn your usage patterns.
 
I’m not having any of the same issues.

But I only charge the phone overnight in the SBC. Always wake with both at 100%

Same. And I've charged both just the case with an Apple cord and also with the iPhone X on a wireless charger. Both charge to 100%.
 
This week I left one day with my SBC and phone at 100%. It let my phone get to 13% before the case kicked in. However it was indicating charging the entire time.
 
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