We've had an first gen iPad since about 2010, just after they came out. I replaced the battery last year with a cheaper aftermarket batter from a large online book seller. It only lasted till about 10 months before it would have a hard time staying up for more than a couple of hours. This go around, I splurged on the iFixit battery and lo and behold, it's lasting about 14 hours of my son streaming youtube. Well done.
Second time around went smoother and using the iFixit guide helped a lot.
If you keep your ipad in one of those heavy duty cases like and Otterbox or Griffin, don't bother with the screen clips. Just makes it harder to take apart the next time. These are devices with a planned obsolescence. My repairing them is a solid middle finger to those who would have me just go out and buy a new device, instead of repairing what I already own.