it does NOT void your warranty, if you're careful
I just made an account to help people in the same situation as myself
I recently had an iPhone that for no apparant reason suddenly no longer wanted to boot. It was stuck in an endless loop with an unknown error. What happened was; phone went black, pressed home + power off for 10-20 seconds, no reaction. Let the phone sit untouched for 2 days trying to drain the battery; no go. Tried entering recovery mode; no go. Hooked the phone up to my computer, FINALLY, reaction.. But then the odd thing started. I got no boot screen picture on phone, and itunes picked up the phone for about 2-3 seconds, and then it disconnected from the computer again. It kept looping this about every 5 minutes. Itunes picks it up, then drops it a few seconds later. No idea what was wrong with it.
Anyways, I wanted to see inside the phone if there was anything wrong with the battery, or if there was traces of moist inside it. I took off the battery cover carefully, unscrewed the little cover over battery plug, unhooked battery, inserted new battery (I had one lying), and there was no difference with the new one. Put it all back together again, made sure there was no immediate visual marks in side the phone, and then tightened and put the back cover back on.
After this I saw no other option than to send it to

for servicing, and I did so. Surely now, 3 days later, I just received an email stating that my service is finished, and that I can be expecting my new iPhone to be arriving within a day or 2
Opening the phone does NOT void your warranty, IF you do it carefully. Do not leave obvious marks inside the phone.
I do realize this is an aging thread, but it was the first result I found when googling "remove battery cover iphone woid warranty". So I would assume this post will come in handy to anyone else having issues etc.
Good luck on fixing your iPhones people

Hope my post can be of help to someone!