Fluke AC/DC diff. voltmeter malfunction

Picked up a cheap Fluke AC/DC differential voltmeter, model 887AB/AN,
at a hamfest this past weekend that's acting a little flakey. When
it's switched to battery operation, the meter deflects full scale and
remains that way until switched off. Any ideas? Is this a typical
failure with these? I haven't attempted to connect a power cord to it
yet since it has a few disconnected wires coming from the area where
the transformer, AC plug, etc. is and I haven't obtained a schematic,
calibration info., etc. for it yet.

The battery pack is said to be good, and I was told that it should be
charged once every week or two... not sure for how long though; will
the batteries be overcharged if charged for too long?

 

Pegging of the needle on a meter is often due to an open shunt
resistor. I am not familiar with this unit, but that is the first thing
I would look for.

 

Disturbed wiring in the power supply area could mean any of several
troubles... in the power supply area. And that's where your battery pack
connects. You need to get a manual, or at least, the schematic for the unit
and start troubleshooting.
The normal battery pack in those meters were NiCads, and the normal charging
period for those instruments was 16 hours. Don't leave it on charge for
much longer than 16 hours.. it will definitely damage the batteries or
seriously shorten their lives. As I remember, there was no end-of-charge
detection in those old meters.. just a resistor to limit charging current.