I have a Flight max 3S 1300 maH that had a little abuse last summer, had to open it up and resolder one of the tabs between the cells. It worked fine the rest of the summer and worked well for about 6 flights this spring until last week. Charged normally and was showing about 4.18 per cell before I hooked it up. Took off and flew about 2 minutes or so when I lost all control and nosed in. Was not too far up so no damage other than broken prop, but had nothing on the receiver or UBEC when I got to the plane. Checked the cells when I got to my field box, two cells were around 3.9 and one cell was around 3, so I figured I had one cell going bad, but I was a little surprised that there was not enough power to keep the UBEC powered up and give power to the flight controls. When I got home I hooked up to a different battery. The Receiver came on, UBEC came on, ESC armed; everything looked OK.
Replaced the prop today and thought I would check out the suspect battery for grins. The cells were still pretty much where they were before 4,4 and 3 for a total of 11 v or so. I left the voltage checker hooked up to the balance tap and plugged the battery into the ESC. Nothing out of the plane electronics and the display on the battery checker went out. Hooked up a different battery and ran up the motor with the new prop. Battery voltages were stable and all looked good.
For the cost of a new battery, I decided the suspect battery did not owe me anything so I got my discharge circuit ( 2 wires and a 12 v light bulb) and hooked it up to kill the battery before disposing of it. The bulb did not light up as I have been used to seeing in the past. Thought this over for a few seconds, well less than a minute and disconnected. Checked the cell volts and had one cell showing 5.2,
both others were around 3.6 or so.
My best guess is something happened so that two of the cells are discharging onto the 3rd when the main power leads are hooked up. The battery is not puffed and did not get warm for the time it was hooked up.
I am now looking for ideas on how to discharge the battery other than hooking up a load and letting it die.
Maybe the old salt water bucket is the best idea for this. It is currently sitting outside where it should be limited in what happens if it does something funky.