I experience a bad batt in my 30cc Citabria ( one batt ) I discover it by doing what I do every time before TO I run the engine at full throttle and move all of the sticks to full deflection, doing that the engine was misfiring, the batt had been charged the night before, it also was two years old, I now use two switches with one batt just in case.

I believe that by keeping track of your batts and checking them before each flight plus the little exercise​ before TO can keep you from loosing an airplane because of a bad batt, I saved my 19 years old CG Piper Cub when it was around 4 years old, I discover it had a bad batt before the first flight, the batt had been charged the night before ( it was old also )

When I say keeping track it means checking them constantly, I checked the capacity on my 27% Gee Bee batts ( two batts ) one fail the moment I began to discharge it, one cell dropped below 3v right away and this is right after been balance charged, that batt also didn't peak at the usual 7.2v but around 6.8v, that was a given that the pack had to go after been used for nearly three years, I got two new 1800 LIFE packs on the way which I will charge to full then discharge downtown 6v only, note time and mah then recharge and note time and mah, that will tell me if the pack is good and how many flights I can fly with the pack then cut that in half, with two of them I should be able to fly at least 8 flights without sweating it.