Rsdio tipped us to an app note from Maxim describing how to protect your Lithium Ion batteries from reverse insertion into the charger. The circuit is a Li-Ion battery charger with an added analog comparator designed to detect when then battery is inserted the wrong way and disconnect it from the charger.
Combining a linear-mode single-cell lithium-ion battery charger (MAX1551) with a comparator (MAX9001) and n-channel FET adds a layer of reverse-battery protection that protects a single cell lithium-ion battery charger and battery from damage due to backwards insertion