Skip the auto shop - Our mechanics make house calls

Q: Car won't start after having radio playing

asked by on

While parked, waiting on a friend, I was listening to the radio with the engine was off. All electrical systems were working but my car wouldn't start. Naturally, we had to jump the car. I did some testing and left the radio on and once again, the car wouldn't start. The volt meter reads 12.8 when the engine is off, and then rises to 14.9 when I start the car, so that would mean the alternator isn't the problem. I went ahead and purchased a new battery, but the voltage reads even lower than my current battery. Can you explain what's going on?

A: When you buy a new battery, it has been sit...

When you buy a new battery, it has been sitting on the shelf for who knows how long and may start with a lower voltage.

When you start the vehicle and test the battery while the engine is running, you should be getting around the 14.5 volt range. Now turn your headlights on high beam and turn on the A/C to high blower speeds and the radio to on. Now check to see what the battery is getting. While holding the engine speed to about 1500 RPMs, you should have more than 13.2 to 13.5 volts getting to the battery.

If the voltage is below this, then you may have a problem with the alternator not putting enough power out to keep the battery charged.

If it is okay then let it idle and check the voltage and see how much the volts drop at idle. It may go to about 12.8 volts with everything on. This is normal when everything is on.

Now turn off the headlights and look at the volts, turn down A/C blower to low and check the volts. Now the volts will go up to above 13.5.

A mobile mechanic, such as one from YourMechanic can run similar tests of your battery and charging system for you at your home or office.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Save up to 30%

Get a quote

What others are asking

Q: Intermittently, all electrical systems go out

Hello It sounds like there are some electrical "gremlins" in your car. It sounds like the main issue is that the computers are losing their power sources for some reason. My first recommendation would be to check the battery cables...

Q: The car pulls to the right upon deceleration

Hi. It should be a fairly simple cause. I suggest checking your tires for uneven wear because that is the number one cause of pulling next to an alignment being out up specifications. So have the alignment checked as well....

Q: Alt and Bat duplicate request

It is not normal for the battery and alternator to go bad in such a short time unless there is a problem in the wiring connections or harness connections. You will need to have the system checked to isolate the...

Related articles

What Causes Hoses to Leak?
While the largest part of your engine is mechanical, hydraulics plays a significant role. You’ll find fluids at work in a number of different areas. Your car's fluids include: Engine oil Transmission...
How to Avoid Back Pain in a Car
If you have back problems, sitting in a car for an extended period of time can be excruciating. Even without back problems, you could experience discomfort and soreness from...
P2103 OBD-II Trouble Code: Throttle Actuator Control Motor Circuit High
P2103 means there is a fault with the throttle actuator control motor circuit, likely due to a defective electrical component or part.