I'm traveling from China to Ghana. Can I take a power bank with 20000maH capacity in hand luggage?

According to the rule, one is allowed to take no more than 2 power banks. If the power is less than 100Wh, you can surely take it and there won't be problems. But for power banks of 100Wh~160Wh, you can only take it with the approval of the airline.Or, it will be confiscated.
So, you'd better make sure the output voltage of your power bank is less than 8V. Remember not to use it during the flight!