Hi Roger.
That's a bit odd. One bar would indicate that the batteries are undercharged. Be sure to try DIFFERENT batteries.
The instruction manual says the X-Terra 70 dislikes overcharged batteries too.
The X-70 works VERY nicely with standard Energizer Batteries. I've used nothing else but those and they hold a VERY long charge.
In theory your batteries should work although clearly, if the machine shuts down whilst indicating a LOW batter charge ("one bar on the battery LCD icon), then I would try a new set of batteries or test your old ones to see if they are fully charged. Good batteries only have a usable shelf life of three to five years as long as they are not exposed to heat. Lithium batteries can remain on a shelf for 15 years with no measurable loss of charge. But usually Lithiums can often carry an overvolt so I only use them in digital cameras etc.
Try a new set of 'different brand' batteries. If the problem persists, it might be an issue with the detector or the metal springs in the battery compartment. If your coil is NOT connected properly, the same problem can occur. Be sure the threaded metal retainer ring (attached to the end of your coil cable) is threaded on correctly to allow proper connection of the cable to the control box (a very common cause of power errors)