this is a known problem from Tascam and it is repairable under Tascam's warranty
this message is per Patrick Madden

Tascam DA78HR, Problem: Battery drains rapidly (between 3 months to a year, battery is supposed to last about 5 years) Low battery Error (LO BAT).  Fix: send to an authorized service center to have a resistor replaced in the battery circuit.  In most cases Tascam will grant warranty coverage of parts and labor for this modification.  If there is a .25" blue dot over the serial number area this modification has already been performed on the unit.