Your phone actually has many protections internally to protect from damage during charging. So even in the rare occurrence that a power bank is outputting too much / too little power, your phone can typically handle it. For example, most Apple devices will stop charging completely until reconnected if the power output is too low (as many people experience with a solar charger on a partly cloudy day).
Some additional information: https://www.quora.com/Will-charging-with-a-portable-power-bank-damage-cellphones-battery-life