I have watched my transmitter voltage drop from 10.8 down to 9.5 in a matter of 30 seconds. I just happened to catch it as I was about to taxi out with my corsair. That was also an old battery. I do no more than 3 years on my batteries. Lost too many planes to battery failures and I accept that as pilot error. It pays to do regular battery maintenance. Out of 5 this year, I had two cycle at less than %70, out they went.
Edwin