I have a four and half year old manual transmission car with start/stop technology. The car has required jump start once after I accidentally left the lights on overnight. I have recently noticed that when I press the cluth after the engine has been stopped automatically, the starting of the engine causes the lights to momentarily dim. I don't recall seeing this phenomenon when the car was new. The engine has never failed to start (apart from the one time when lights were left on overnight). The battery is of the type designed for start/stop cars but it is not AGM.
I became interested in the condition of the battery and asked the dealership to do a battery test. Their testing device, which I believe did a cold cranking amp test and open circuit voltage test, showed that the battery is like new but all it needs is a recharge. I'm somewhat sceptical of the results, because of the phenomenon of the lights momentarily dimming when starting the engine at stoplights. I don't believe there is anything wrong with the charging system of the car, either.
How long do lead-acid starting batteries last, typically? What is the failure mode? Failure during a trip, or failure to start on a cold morning? Should I replace this battery merely based on its old age, or should I trust the dealership battery test?