Loss of 100% battery life to 90% after install of iOS 14.6

Batteries are consumables; they lose a little capacity every time they are discharged, then recharged. On average this works out to about a 1% loss for every 25 “full charge cycles” ( some will be a little more, others a little less). As one example, if you charge the phone overnight, every night (and that is what you should do; it is a best practice), it starts the day at 100%. If it drops to 20% by the end of the day before you charge it again overnight that counts as 0.8 full charge cycles (20% to 100%), or about 24 full charge cycles per month of use. For this example your battery capacity will lose about 1% per month. Of course, if the end-of-day level is higher than 20% the capacity loss will be a little less, and if it is lower than 20%, or you charge it during the day, the capacity loss will be higher.

You were lucky with the XS; it performed better than expected. I’ll tell you a little secret about battery capacity in old phones, using an XS Max as an example. (Well, not a secret, but an observation from another forum regular, SergZak). All iPhones have a specification for the battery. For the XS Max that is 3174 milliampere-hours (MaH). So the battery monitor is calibrated for 100% at that value. But there are variations in manufacturing, so some batteries will have less capacity, and some will have more. Suppose your battery had, say, 3474 MaH capacity (10% over standard). That would still show as 100% (even though it was actually 110%), but as it aged the health would stay at 100% until it fell below 3174 MaH. This would appear to you as if your battery had fabulous life. The only way to determine this is with a 3rd party app, and then only if you check the battery capacity when the phone is new.

If you lost 100% of battery life the battery would be dead.

How long have you had the phone?