Check the dc bus and for you to ascertain that,you need two digital multimeter placed on the capacitor bank+ -+ - gradually monitor the dc bleeding up and see if its balance,if not it will not start on battery because the dc bus is used to push the inverter.check ac output capacitor

Please clarify your problem. is the ups on? does it report any alarm? to see this alarms press * , then scroll down using the keypad button that has an arrow in it. if there is no battery weak alarm, press the enter key. does the display read 'normal operation' and does is show load percentage? if so press # the scroll through using arrow keys untill you get 'battery capacity test' , press c then # , this should start a battery capacity test, press battery voltage key. if the batteries are okey the voltages should read 400 then they continue to drop gradually. to revert to normal operation press # then c then # this should bring back the ups to normal operation