EC2 hits 85% usage limit without being used

You get 750 hours of free EC2 every month. And 85% of 750 hours is 637.5 hours.

So what the email means is that you have used 637.5 hours worth of EC2 this billing period (which is almost certainly this calendar month). Even if the instance isn't doing anything useful and is just sitting idle, the clock is still ticking.

You can stop the instance and the clock stops ticking. Although because today is about 85% through the month of July then, as long as it's only one EC2 instance and it is within free tier, then it's very likely you're not going to incur any charges even if it continues to run.

Also be aware that EC2 (compute) is different to EBS (storage) even though it all comes under the EC2 category. For EBS storage you're allowed 30GB/month, which basically means you can provision 30GB on the 1st of the month and keep it there for the whole month you'll never be charged (if you provision 60GB on the 1st of the month then you have used up free tier by the 15th of the month, and so on).

This is a really useful document about how to avoid unexpected charges in free tier https://repost.aws/knowledge-center/free-tier-charges Be sure to check the links at the foot of it which go onto other areas which may be of particular interest.