I am afraid you are having the problem of old or weak batteries. Old batteries at times tend to show such problems and do not show charge while charging properly. For this purpose, you can use a reliable Nicad Battery Repair Guide through which you can recharge the batteries without any problem and make them work like the new ones within no time, without anyone's help and you can do this all by sitting at home! I tried it and now I never buy new nicad batteries (http://www.nicadbatteryrepair.com/).

If this is an older unit, the battery may be going bad. I think this unit may use nicad or nickel metal hydride batteries. If you are somewhat proficient at repairing things and have a volt meter, you can open the unit and check each battery cell for a dead one. If you find a dead cell, (0 volts) replace all of the batteries or you may have others go bad on you. A good source for rechargeable batteries is: http://www.batteries.com/