Q: Battery dead, car won't start

Hello. The best thing to do is have a battery shop perform a "load test" on the battery. That is very standard procedure. The battery has to be in a fully charged state (they can do that, too) before such a test can be performed. If the battery fails a load test (if it’s 6 years old, it may very well fail or be "marginally" good) obviously you have to replace it. If it "passes", be sure to ask the Tech if it marginally passed. That is, batteries don’t last forever and at 6 years (if it is 6 years) you are going to have to replace it soon anyway. If it "marginally" passed the load test, absolutely replace it now to avoid getting stuck on the road.

Regardless of the outcome of the battery test, you need to test the output of your alternator. Once you have a functioning battery, attach a voltmeter to the positive and negative terminals of the battery (don’t short circuit anything as the battery could explode). While the car is running, you should see a volt reading on the meter somewhere in the range of 13.9 to 14.5 volts, however, you must check your Factory Service Manual for the exact specification for your car (which test protocol might also require that you maintain a certain engine RPM during the test).

The range of values for voltage are simply typical and so, for example, if you are reading 12.8 volts or something well below 13.9 you possibly have a faulty alternator OR a wiring problem like corrosion or loose terminals. Even a perfectly good alternator will not charge the battery and run vehicle electronics if, for example, the connections to the battery are in poor shape. If you need assistance, YourMechanic can evaluate both your battery and charging system, during a mobile visit right to your door, and get this issue fully resolved for you while answering any further questions or concerns that you might have.