I just got a 20000 mAh power bank and I'm wondering how many hours should I charge it so that its life would last longer.
20000 mAh is 20 Ah. That could be a small gel lead-acid battery or quite a large lithium cell battery. The answer in either case is to connect a battery charger of the proper specification, switch on and leave it until the charger indicator shows that it is fully charged. Don't use any old charger unless you are sure of the specification.
