No mention made of if the unit does auto saving? This was a big pain in the ass on the 680, of you lost power during recording you lost the file. Recovery was iffy at best and Tascam did not help with any kind of support for recovery. Dos not seem like a big deal unit you lose power ehther with batteries going dead or cord unplugged .
Direct answer from Tascam regarding your concern:
After the DR-680, we fixed all our devices to be tuned to be
more sensitive to the battery voltage, and the recording is
automatically stopped when the battery goes too low.
If you keep AAs in, and power from a USB power pack, you
get a built in UPS functionality as well - it seamlessly
switches over.