My RPi3 robot is powered by six 5000mAH Ni-MH C-cells, and a Tenergy 1025 Smart Charger.
Last weekend, when I plugged my bot in to recharge, the charger blinked "bad battery" condition. Two of the (year old) Tenergy C-cells were at 0 volts, while the other four cells read 1.1 volts.
I had some three year old EBL C-cells, never used, that showed about 0.8 volts, so I swapped two of them in for the bad cells.
After fully charging, my "safely find not-going-anywhere battery life" program reported only 6 hours of safe run time (shuts down at 4 readings under 6.5v, leaving about 10 minutes of no motors, no servos computing). I thought I might be needing $25 in new cells.
The second cycle reported a whopping 12.5 hours, even better than the 10 hours of the original six cells.
My robot and I are happy again.