In Unto Zeor, Forever, they were used to supply Selyn power to lights, factories and other devices that required power.
In Easy as Hop, Skip and Jump, Selyn batteries were used for space travel. i.e. in "warp" type travel.
At the level of sophistication of battery technology in the space age, It is possible for a channel to take selyn from a battery for personal Need, but it was considered to be a repulsive experience, and could not sustain a Sime for an entire month, or be repeated. Some channels are able to endure battery splices and shunts for longer than others, but it is a stress which shortens lifespan because the selyn that has been stored in a battery may have quantity but not quality -- it is sensed by the Sime as "dead" selyn. Think of direct current as opposed to alternating current - put direct current through a circuit built for alternating current and it'll fry. Selyn from a battery does not "pulse with life" and thus is seriously dangerous to a living organism.