Yes. The US Department of Transportation (DOT) regulations on passengers traveling with lithium batteries can be found at safetravel.dot.gov. Per the regulations, passengers may not carry spare lithium batteries in checked luggage unless they are within a portable electronic device. However, any number of spare lithium batteries are allowed in carry-on baggage if they do not individually exceed 8 grams (~100 Watt hours) of equivalent lithium content.
All Advanced Bionics lithium-ion rechargeable batteries (PowerCel™ Slim, PowerCel Plus and PSP/S-Series™ batteries) have equivalent lithium content weighing less than 1g each, which falls well within the 8g limit.
When traveling, protect the battery terminals by using the blue plastic PSP/S-Series battery cover or the Harmony PowerCel battery pouches. For Naída CI PowerCel batteries, use PowerCel plastic covers or a sealable plastic bag.