The test procedure is quite simple. Set the meter to read DC volts, and set the correct range if needed. Some meters will select the correct range automatically. Nearly all meters will come with instructions on how to set them to read DC volts. Next, connect the meter across the vehicle battery, positive (+) to positive (+) and negative (-) to negative (-). With the engine running at a moderate idle - say 1200 to 1500 RPM, the meter should read approximately 13.8 to 14.8 volts and be steady. If the reading is substantially outside these figures and all other items mentioned have been tested and verified good, then the alternator is likely defective.
Car Battery Lifespan and What Causes Batteries to Fail. The average life expectancy of a car battery is three years or so, but there are several factors that can have an impact on that estimate. Even under the most ideal conditions, chemical reactions cause batteries to break down, and your vehicle will likely need a new battery within a few years. Though car battery problems are often associated with cold weather, Consumer Reports magazine says heat is a bigger enemy of car batteries and will take a bigger toll on performance and reserve capacity.
Of course, there are plenty of wiring problems an engine can experience and they can be very difficult to locate unless the mechanic is experienced. Analysing electrical problems requires using special equipment combined with mechanic expertise. Today's engines are extremely sophisticated combinations of electronics, electrical wiring, and moving parts and seem like they are half-computer and half-mechanical.
Use a multi-meter to confirm that the switch is getting 12V of power. Attach the leads of the multi-meter to where the electrical wires come into the switch and set the meter to DC volts. It should read 12 volts. Trace the wiring from the switch to the motor. This is the path that a signal will travel to move your window when you press the switch. Any breaks or loose connections in this path will prevent your motor from functioning properly. Use the multi-meter to confirm that the motor is getting 12V of power.