Typically battery backups are SLA (sealed lead acid) which is most likely why they don't get packaged with NAS systems - they're heavy, they're a dangerous good by transportation and shipping rules, they expand and can release gasses when under load and charged. The upside? They have way more capacity than that of a lithium-ion as shown in this little unit, they (probably) cost much less, and they last longer (5+ years vs 2-3). In terms of a NAS though, the capacity isn't really an issue if the only thing the NAS is doing is shutting down to avoid data loss/corruption.