Apple Watch Ultra stops charging when at 75%
My Apple Watch Ultra recently stopped charging once it hits 75%. This is a recent issue (two days now) and I have not changed any settings.
Apple Watch Ultra, watchOS 9
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The Ultra watch has a new battery management system, called Optimized Charge Limit. When you start wearing the Ultra watch, the watch tracks your usage over the first 30 days, then based on how you use your watch, limits the charge to a level which can range from the mid 70% range up to 80%. It uses machine learning to charge your battery to the right level for your needs. Optimized Charge Limit changes dynamically depending on how you use your Apple Watch Ultra. Based on your usage patterns, your watch might charge to 100% on some days, and less on others.
It takes some getting use to, but your watch is doing what it was designed to do. You can turn it off if you'd prefer your watch to be charged to 100% when you get up. I would recommend not doing that as you spent a lot of $$ on this watch, and using the Charge Limit will extend the life of the battery.
This link offers more on the subject --> About Optimized Battery Charging on your Apple Watch - Apple Support
Read the section on Optimized Charge Limit for Apple Watch Ultra.