To drift the thread a little, I was headed north east on an arctic adventure in one of my planes. I'd given it a pretty thorough inspection before I left, and the battery was just three years old, and had always been in my care. About 600 miles into my trip, and just beginning to leave civilization to the north, I began to have transient electrical anomalies. To say they were problems would be too strong a term, but I was noticing them. The airplane always started just fine, which was my main interest, as this plane cannot be hand propped. I flew the entire trip of a week around the near arctic, with these transient problems, occasional low voltage (digital volt/ammeter) and the odd radio problem. When I got home, I investigated more ('cause now I could risk finalizing whatever problem might be lurking).
I found that the battery would never hold a voltage of more than 10.8, it had been the alternator bumping the bus voltage up in flight. It's a smaller alternator, so when consuming lots of electricity, the voltage would pull down, and my avionics suffered a brown out, and behaved accordingly. But the starter always worked! It turns out that the normal operation of the starter draws so much, that it pulls the battery voltage down to 10 or so volts anyway, so my low voltage battery was not really reducing starting capacity much. After some inquiry, I determined that the battery had had a shorted cell for some time, and would never again produce 12+ volts. I bought a new battery.
If you're just flying day VFR circuits, you can afford to take some chances with a suspect battery. If your adventures take you further from home, or into not so VFR conditions, having a dependable battery is cheap insurance. Whether stopping fill up along the way, keeping my emergency kit up to date, wearing my life jacket over water, or paying the cost to always have a dependable battery, I find the cost of good maintenance to be less than the possible cost of inconvenience, delay, and risk.