I'd try another battery - usually using Duracell has never surprised me -.
If the problem occurs again with another one, the preamp could have some issue. I had many times troubles with the moving/not fixed well circuits onboard on other active instruments, but not my Sado so far.
If you tested fully charged, not surely will be full plugged in after longer period. I have a same symptom with my car's key.
My wife couldn't open the car with button, I took out and tested the battery and it was OK. One month later (without using it, it is the second/spare key) the key just didn't work again. Then I throw out that battery and changed for a fresh one. It has to be changed because the factory alarm cannot disengaged without the remote control key...