ChugPlug External Battery Pack for MacBook Air 11″ and 13″ and MacBook Pro 13″ Portable Charger (Works with 60 and 45 Watt MagSafe Adapters): Computers & Accessories
$200 on sale for $20 (as of 12/18/2015). Great price BUT this thing is a bit too big for me to really carry it around. 1.1 pounds (500 gram)
Overall the selection is not very impressive. What i really want is this device below – but with a battery component….
this one is not a battery – but a very compact charger for laptop and extra USB ports for smaller devices:
Length: 93mm / Diameter 34mm / 100g