I've a 7th generation iPod Nano. And it's working fine.
I usually used to charge it either by connecting it to a wall charger or through my iMac. And every time it got charged just fine. The battery symbol was changing to full in the end.
Recently, I connected my iPod Nano to my MBA which was being charged as well. When I woke up the other day, I found out that my iPod Nano didn't had the full charged symbol. I realised that my iPod Nano was near full charge when I unplugged it which was weird because my iPod Nano got connected to my MBA which also got connected to the wall charger all night long.
Anyway, I tried charging my iPod Nano one more time through my MBA and this time it didn't get charged nearly as well as the first time that the charge level of my iPod Nano was nowhere near full.
And still, my iPod Nano gets perfectly charged when it gets connected to my iMac or any wall charger.
Another weirdness is that my MBA can fully charge my iPhone 5 or iPad Air perfectly without any hassle.
If my MBA can charge an iPhone 5 or even an iPad Air which has even more current running in it, why can't it charge a plain old iPod Nano with much less current flowing in it?