1. bigger capacity. since iPad pro has ~10k mah, and usual battery pack has conversion rate of somewhat 70%. so you need at least 14k - 15k mah battery to fully charge your iPad pro once.
2. high output is a MUST. ALWAYS go for 2.4amp output ones. otherwise your iPad will consume battery faster than it charges. if you see 1Amp, or even 1.5Amp, just close the page and look for others.
3. although iPad does not support quick charge, but if the battery support quickchage to itself, definitely a plus. with >15000mah capacity, it takes forever to recharge the battery. quick charge would be nice
4. passthrough charging is another plus. so in the nights you just plug the battery into wall charger, and then plug iPad to the battery, so they get charged at the same time.
0. usual inter-continental flights are 12-15 hours, if you sleep a little bit, have three meals, an iPad should last the whole flight.
00. look for flights that provides USB ports on the seat, therefore you don't need battery at all
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oh yeah, anker is a good brand to go to. top class quality, and not expensive at all.
mophie is another good brand, apple sells it in their store too. but just definitely on the expensive side.