Q: I have a 2003 BMW Z4. The battery died twice without warning because of old age. I've been advised to check the battery regularly, especially after one year. I bought a $50 battery tester so I can check it myself every other week. Is this is a good plan in terms of battery reliability?
A: I admire your willingness to test the battery on a regular basis, but how is testing the battery going to prolong its life? It's not, but it may give you an idea of when the battery is starting to fail.
Battery life is somewhat unpredictable. Most automotive batteries seem to survive three to five years -- nicely in line with their warranties. I don't think a car owner can extend the life of a battery, but I do believe a car owner can shorten battery's life.
The major factors in battery life seem to be environmental/ambient temperatures, state of charge, level of electrolyte, quality of terminal connections and security of battery mount. Car owners can't keep it from getting cold outside, but they can influence the other factors.
If you absolutely do not want to deal with a dead battery, there's a relatively simple answer: Replace the battery every two or three years. Is this potentially wasteful? Yes, but unless you accidentally discharge the battery by leaving the lights on or something similar, the car will always start.
Install a new battery. A fully charged automotive battery at room temperature should develop 12.6 volts, give or take a tenth or two. Double-check the battery voltage at the battery itself with a digital voltmeter. If it confirms the 12.1-volt reading, the battery is weak. I suspect you'll see a bit more voltage at the battery, but if it's not above 12.3, the battery is suspect.
You can have both the battery and alternator load tested to confirm their performance, but the voltage readings you see with the engine running indicate that the alternator is performing normally.
Subject: Tester Won't Give A Battery Any Extra Life 
