First, when you step on the plug on an integrated cable, you throw the power bank out. With a separate cable, you buy a cable for a few bucks. Use a caribiner to keep the cable and power bank together.
Second - how large a power bank do you want - there can be a huge difference in price between, say, 5,000 mAh and 20,000 mAh power banks. (The G5 has a 2,800 mAh battery, so a 5,000 mAh power pack will get you almost 2 full recharges. A 20,000 mAh power bank gets you over 7.)
(As with all lithium batteries, never discharge the phone or the power bank below 40%.)