PowerBoost X1 2600mAh Premium Portable iPhone Charger Rechargeable Battery With USB To 8-pin Connector Cable (Works with iPhones 5 / 5s / 6) And With USB To Micro USB Cable (Works With Most Android Phones) from Duroplus
It is compact with a metal casing and has worked great for when I need an extra boost but can not conveniently charge.  Some have said it will not work with iPad and other larger devices that need more power, but I was able to charge my iPad 2 and give myself some extra time on it.  I haven't tried it on a newer one.  It took a while, but I got this for when I am in a pinch -- an extra 60 minutes is better than 0 minutes.