1. Check the output: those with 0.5amp will not charge your tablets (1amp-2.1amp) unless you have a custom-made cable. However, I always charge my iphone with those 2.1amp chargers...quicker to get fully charged.
2. Check if you can charge the battery bank with your phone connected to it together. (It should charge both)
3. Higher storage(5000amp etc) more hour to get fully charged.
My recommendation, get a Samsung Galaxy Note 10.1 caused it got 7000amp... ;)