Twice the operation time
Two standard SE400 batteries are attached to special contact plates mounted on the shoulder straps of your back pack. This means perfect balance, and no added weight to the fan unit at the back. Freedom of arm movement is not affected. The two batteries are connected to an attachment that fits on the SE400 just like a standard battery. No modification needs to be done to the fan unit.
It is impossible to give advice as to the life span of a battery or when a battery will lose its usefulness. There are simply too many variables to give such advice, such as mode of use, storage, work load, number of recharges, frequency of recharges, normal chemical changes within the battery cells, and many more. The warranty on a battery covers manufacturing faults of components and workmanship; it does not cover the actual performance or output of the battery over time.