Question:  I thinking of purchasing rechargeable batteries for my WBS-670 beltpacks and want to know if you recommend using Lithium Ion rechargeable AA batteries.  I also see that NiMH rechargeable AA batteries are also available, will these work as well? 
Answer:  The battery pack will operate the WBS-670 / 680 beltpack in full-duplex mode for 14 hours with a fresh set of alkaline batteries. The optional rechargeable NiMH (nickel metal hydride) batteries will give over 11 hours of continual use.
Non-rechargeable Lithium batteries are readily available in the 1.5 volt  AA form factor, they provide approximately 60 to 70 increased battery life in most wireless beltpacks (they are very popular for small digital cameras), they are about 50% more expensive than standard alkaline batteries. 
The optional WTR-BAT battery sled for the WBS beltpacks can be charged for approximately 500 charging cycles.  Batteries are rated at 1800 mAh providing over 11hrs use (NiMH, 1.2V).  Given that these specs were provided 3 years ago I would suggest that 2100 to 2600 mAh NiMH batteries would probably work just as well in the WBS-670 / WBS-680 beltpacks. 
Could Lithium-Ion rechargeable batteries be used in place of NiMH batteries in our products?  Lithium Ion 1.5V AAs exist in this form factor but these batteries are usually 3.6 volts or higher so that would rule them out for use in WBS and CellCom beltpacks. 
See the link for more information:   http://www.metaefficient.com/rechargeable-batteries/rechargeable-litihiumion-aa-batteries-exist.html