Ran update iOS 15.7.3 last night on my iPhone 6 now the battery drops fast

An iPhone 6 cannot run iOS 15.7.3, or anything higher than iOS 12.5.7.

Maybe you have an iPhone 6s or iPhone 6s Plus?

The battery really has to work hard after any update to index everything. Give things a few days to get back to normal.

When was the last time that you replaced the battery on the phone? Check Settings > Battery Health / Maximum Capacity. What is the number that you see?

Same here, but with iPhone 7.

My battery health is 80%, and all was fine until I upgraded to IOS 15.7*?

I was on IOS 14 before and the battery would last about 5 days!

Now, it's draining within 12-16 hours. And it will suddenly go down to 1%, then I do a soft or hard restart and it jumps up to 88% (on low power), and then I can literally watch it go down in 1-2% increments every few seconds. I have nothing running the background etc.