There is such a wide range of quality, temperatures, individual usage details, and vehicle demands that it is very difficult to pin down a estimated time of death. What I have noticed with todays FULLY computerized cars is that now you
absolutely need the correct 12v+ battery voltage to get the car to fire. Gone are the days of slow cranking or bump starting...
The Miata batteries, in both of the 1992's I had were gel cells or AGM batteries (Absorbed Glass Mat). They get 10yrs service life out of them fairly regularly, but the downfall is that they are NOT SERVICABLE.
The traditional flooded wet cell batteries, and even most of the "Maintenance Free" batteries will have caps or lids, sometimes hidden under decals, where the electrolyte CAN BE SERVICED by refilling 1 or more of the 6 cells. In this pic we can see the cell on the right is low.
While working in the auto parts stores, I would try and save folks some money by recommending that they add water and charge it. However some customers just did not have the time to mess with it, they had just been stranded and wanted a NEW battery. We also had a very shrewd customer, who would come in every week to inspect the old customer CORE batteries. More often than not, after looking through all of the cores, he would find himself a battery that had an electrolyte level inspection window that showed one or more cells low on water. He would then pay us the $5 core charge to buy the "bad" battery, take the battery home, add water, charge it, and he would resell them, and install into cars at his used car lot.
