You need to ensure that an extennal battery can output at least 10 W @ 2.1 A current. Then you need to ensure that it has sufficient amp-hours, depending on how much of a recharge you want. If I recall correctly, the new iPad has a 42 Watt Hour battery (I'd have to look this up), so to get at least a full recharge, you need that much or more, preferably more, since the efficiency is typically less than 100%. So you need a 42WH battery or more to fully recharge. But even if you had less, you can still get some recharge in to extend the time you can work on the iPad. If you have 54000 mAH and it outputs at least 2.1A, then I would say you should be good for several recharges. If it provides less current than 2.1A, then the charging will probably be too slow to be useful. And if you are outside and have the screen turned up to 100% brightness, then you may have trouble charging it altogether, depending on what else you are doing. You probably want to have your ipad plugged into the battery as much as possible rather than letting it run down, because if you run it down it will take longer to recharge it.
What external battery do you have? For a full recharge, you will need an external battery which provides about 11350 mAH or more. But even with a 5000 mAH external battery, you can get half a charge (assuming it can deliever 2.1A), so you may not need to replace your external battery.