a 500mAh charger will charge your phone, but extremely slow. This all depend on which battery you have. I have a 1350mAh, therefore a 500mAh is not good enough for me.
I can charge the battery outside of the phone with a 500mAh, but when it's in the phone, I can't simply charge it, because the phone is using the battery faster than it charge.
850mAh is good enough.... but I'm looking for something at least 1A
I'm thinking of buying this one on
eBay
it's only $9 and it's 12V 1A with screen display