1.Lots of new phones have a cutoff feature that stops charging once it reaches 100 percent and has a 3 stage charging method these phones are safer to charge using power banks and even if it reaches 100 percent this feature stops further entry of current and helps to prevent damage to the battery.
2.If you own a device that has this feature then you should worry less though it would be recommended to plug off the charger chord once it reaches 100 percent due to two reasons. One -you cannot be sure that your device has this feature (especially if it isn't a smartphone). Two- The circuit that implements this can get faulty and malfunction causing your device to let in current if it malfunctions and this can be even made worse if power bank voltage is higher than 5.4V though 5.45V will be ok but is still risky.
3.If your power bank voltage is lesser than 5.1V never use it with your device as it will end up drawing power from your device and not charging your device or will end up drawing very little power.if your power bank usb voltage is greater than 5.4V one thing you can do is try to charge your mobile or tablet till the battery percentage reaches 80-85.
4.when charging batteries reach almost 80 percent the battery charging circuit senses this and reduces current flow to the battery as above this battery percentage level if too much current enters it causes damages by gassing and heating wasting a small percentage of the energy and can result in swollen batteries and destroy your device over time.
5.If you charge batteries till 80 percent it might last a decade one of the reasons for this is given above. Even though damages by gassing is negligible below 80 percent it does happen from 60 percent to 80 percent but is more or less negligible and moreover you are going to change your phone in 10 years time so nothing much to worry about.
When charging with your power bank either make sure that the usb cable that you use to charge your device is the one that it came with and not a cheap and not good cable. The cable should be as short as possible and as fat as possible(as it offers less resistance) otherwise the cable won't be offering any charge current due to its high resistance.