LCD displays have a lot of internal power supplies. you have the inverter for the backlight, the inverter for the LCD voltage, regulators for the cpu and screen controller. if the monitor has an audio amp, it will probably have a regulator for that too.... with a lot of regulators, inverters and such, the chances that just one of those devices has a high dropout voltage is very good. if the CPU detects a monitored voltage drop out, it will shut everything down. also if one of those batteries has a slightly high internal resistance, chances are that one of the inverters is going to run marginally (maybe even with a "jumpy" output), and the display cpu will trigger a shutdown for stuff like that as well...