PowerBank

Recently purchased a better powerbank (at least better than my mom's). This is a 20k mAh powerbank. It can charge my Iphone 6plus 3 to 4times. It is however a pain to recharge this bad boy. I think it took me an entire day to recharge it. I know it shouldnt take this long but it did. Did i get a broken powerbank? It charges my device without problem though.