If your power bank is charging slowly, it's always best to first try using a different cable and wall/car charger to see if this fixes the issue Naturally, power banks with large battery sizes are going to take longer to charge. If your power bank is charging slowly it could be due to using a wall charger with lower AMPs/power. Let me explain. Wall chargers come with different AMP levels, and usually this is printed on the charger itself.1AMP wall chargers are the slowest wall chargers available - it can take over 24 hours to charge a power bank 20,000mAh or...