On volt meter use DV voltage setting... On cable that plugs into the wall you should have 18 volts out the adapter that connects to battery testing with volt meter, if you don't have 18 volts remove the adapter and test out the barrel, one lead on inside one on outside, this will tell you if the charger is bad or the charging adapter is bad.......
If cable tests good then measure voltage on the battery after 12 hour charge, you should have between 13.5 and 15 volts give or take a few tenths of a volt, If that tests good you can connect battery to excal but don't connect the screw on connector that locks cable on battery, turn on the excal and measure voltage running, it shouldn't drop much more that .5 to 1.0 volts when excal is turned on and running, if battery drops several volts then you have a battery going bad or problem on detector drawing too much voltage.