Also looking to get a phat external battery to extend the life of the iPad's internal battery. No way that I want to take the iPad apart, nor buy a new one.
The life of the battery is determined by how many recharge cycles you have. I believe I’ve read that after 1,000 charge cycles that the percentage of capacity left should be 80%
Now, there is one sure fire method to prevent the battery from wearing out to even that minimal extent - don’t get to 1,000 recharge cycles. And how do you do that? Simply ... by running it as much as possible from the plug ... :-) ...
And, by the way, a charge cycle is a full cycle, and not a partial one. So, if you use 20% and then charge it up, do that again and charge it up - and do that five times in a row ... that is only ONE FULL CHARGE CYCLE.