The current PD converters are designed to be usable without a battery. I believe the manual/literature mentions this somewhere. Depending on what or how much 12V stuff you're powering, it may be useful to put in at least a small battery for high instantaneous current demands like motors starting up. It wouldn't need to be a particularly big or fancy battery for this usage since you aren't going to be running in a disconnected state in general. (It would also be mighty handy if you're someplace where power failures are somewhat common.)
