Be aware that refurbished outside Apple's own store is a scam: Nobody other than Apple themselves is allowed access to original replacement parts, so at best they are frankenstein'ing together working devices from used parts, which might not restore full functionality such as the water resistance rating. Apple refurbished iPhones come with new housings and new batteries, these certainly do not.
It's also a scam because they do not actually refurbish anything - as explained above they can't, at least nowhere near as good as Apple. Instead they just sell you a used device at a premium. It's no better than if you buy one used. In fact it's better because you can be relatively certain that the typical iPhone user doesn't go around opening up his device.
I bought such an iPhone once, last year, an iPhone X, off Amazon, because I needed a temporary device for a couple days. I wasn't planning on keeping it, though that was out of the question once I actually received it. It looked fine but the bottom left corner where the caps lock key is had a completely dead touch spot. It was impossible to even login with the Apple ID without switching into landscape orientation so I can type upper case...
So that's how great they "refurbish" things. It's just so they can extract more money from you.
That could be 81% at which point the battery most definitely has exceeded the 500 charge cycles it is rated for and needs to be replaced regardless. At 80% the battery is done, full stop. Sometimes it still works well enough, I have an iPhone that works with 78%, but outside in the sun with display at high brightness it dies quickly, basically no longer usable throughout the day without a charger. Even if it's somewhere around 90% the daily battery life might still be significantly reduced already. The percentage is just an estimate after all.
