In the good old days, cellphones had removable batteries that you could just pop right off whenever you wanted. That way you could always bring a juiced-up spare in case you don’t expect your battery to make it through the day. Nowadays, the vast majority of smartphones have built-in batteries, so if your device dies you have no choice but to find a way to recharge. That’s where portable chargers like the Anker PowerCore 10000 come in.