at what battery health should i get my battery changed?

Hello ~ It is recommended at 80% and take a look here:

iPhone Battery & Power Repair - Apple Support

Optimizing iOS Battery Usage

~Katana-San~

Hello ~ Is there a technical question we can help you with? You should get you battery changed at 80% and read this great tip:

https://discussions.apple.com/docs/DOC-9732

~Katana-San~

It's recommended to change the battery at 80% or close to that.

I had this question as well and what happens if you don’t change your battery

The phone will eventually fail to power up.

i did that and when my health got to 50% it acted like 50% was 0% on my phone and my phone was super slow