Zerlolemon has a battery case for the nexus 5, it has a 3500mah built in. I ordered mine for my LG g2 but the cases and battery size are about the same, I'll post up in the LG g2 forum on how it performs. Its only$40 for mine so it should be around that on Amazon, search zerolemon nexus 5 and it should come up.
Sent from my LG G2 on the Now Network