Battery or alternator?

I

iontrap

Joined
Dec 6, 2002
Messages
249
Location
Augusta, Ga
Trying to determine if I have a bad battery or alternator, 2009 Jetta 2.5 167k. First attempt to start it this morning, clicks like a bad battery and no start, the battery was at 10.1 volts. It started with a jump pack so I drove to AutoZone and had them test the battery and alternator. Their test showed battery good and charging system bad.
A few hours later I tested the battery again. The battery was at 12.6 volts, dropped to 7.8 volts when cranking and 13.8 volts idling. Voltage drops to 13.6 volts with headlights and AC turned on.
Does this sound like a bad battery or bad alternator?
 
Based on the symptoms, it certainly appears to be a bad battery. How old is it? However, alternator issues can also be intermittent.

I suggest that you take the car to another auto parts store (e.g., Advance Auto, O'Reilly, NAPA, etc.) and have it retested. Maybe go to a different AutoZone location too, if available nearby.
 
iontrap said:
Trying to determine if I have a bad battery or alternator, 2009 Jetta 2.5 167k. First attempt to start it this morning, clicks like a bad battery and no start, the battery was at 10.1 volts. It started with a jump pack so I drove to AutoZone and had them test the battery and alternator. Their test showed battery good and charging system bad.
A few hours later I tested the battery again. The battery was at 12.6 volts, dropped to 7.8 volts when cranking and 13.8 volts idling. Voltage drops to 13.6 volts with headlights and AC turned on.
Does this sound like a bad battery or bad alternator?
Click to expand...
Sounds like a bad cell or contact in the battery.
 
A quick Google search suggests a drop below 9.6-10 volts while starting is excessive and suggests a weak/failing battery. Also, a voltage of 13.6 volts at idle indicates voltage is coming from somewhere other than the battery - in other words, the alternator seems to be doing its job.

Barring some other unknown electrical issue, it's time for a battery.
 
Sounds like a shorted cell in the battery. Do you incidentally have a lot of acid on top of or around it? A bad cell when charging will make the others get too much voltage and boil out.
 
Nukeman7 said:
Based on the symptoms, it certainly appears to be a bad battery. How old is it? However, alternator issues can also be intermittent.

I suggest that you take the car to another auto parts store (e.g., Advance Auto, O'Reilly, NAPA, etc.) and have it retested. Maybe go to a different AutoZone location too, if available nearby.
Click to expand...
Battery is almost 4 years old. Advance is across the street, easy to get a second opinion.
 
fsdork said:
A quick Google search suggests a drop below 9.6-10 volts while starting is excessive and suggests a weak/failing battery. Also, a voltage of 13.6 volts at idle indicates voltage is coming from somewhere other than the battery - in other words, the alternator seems to be doing its job.

Barring some other unknown electrical issue, it's time for a battery.
Click to expand...
After my testing, I was sure the AutoZone test was wrong.
 
almost sure it’s a bad battery. 13.8-13.6v is still higher than alot of heavy machinery charges at and their batteries stay healthy.
 
If it were my car at 167K and intended to keep it for a long while, just replace the battery and the alternator. Like betting place&show on a sure thing at the horse race track.
 
^^^^
1712869744881.jpg
 
iontrap said:
Trying to determine if I have a bad battery or alternator, 2009 Jetta 2.5 167k. First attempt to start it this morning, clicks like a bad battery and no start, the battery was at 10.1 volts. It started with a jump pack so I drove to AutoZone and had them test the battery and alternator. Their test showed battery good and charging system bad.
A few hours later I tested the battery again. The battery was at 12.6 volts, dropped to 7.8 volts when cranking and 13.8 volts idling. Voltage drops to 13.6 volts with headlights and AC turned on.
Does this sound like a bad battery or bad alternator?
Click to expand...
Interested to know if the volts measured for the alternator was measured at the alternator? Should be about 14.5.

It should not be dropping much with load.

You could start the car, disconnect the battery and measure the voltage drop again, if it is gone, the battery is bad or low, if it remains, the alternator and or charging system is bad at some point. Might be good to check the main grounds while you are at it.

4 years on a daily driver battery is pushing it either way, but not worth 2 batteries.
 
iontrap said:
A few hours later I tested the battery again. The battery was at 12.6 volts, dropped to 7.8 volts when cranking and 13.8 volts idling.
Click to expand...
7.8 volts while cranking is very low, but only if that's the steady-state cranking voltage. A dip to 7 or 8 volts for a fraction of a second is normal. Steady state cranking voltage should be at least 10 volts in normal weather. It's best to monitor the voltage while cranking the engine for at least a couple of seconds. Do this with flood mode or pull some fuses to prevent the engine from starting during the test. Don't use your multimeter's MIN setting.

I would only replace the battery to start. The alternator voltage is maybe a tad on the low side, but it might be within the normal range for your car. It will still charge the battery just fine as long as you're driving often enough. The voltage might increase a bit at higher rpm as well. Look up the test procedure in a service manual if you really want to know if it's in spec.

After you install the new battery, check its resting voltage a few times over a couple of weeks to make sure it's staying charged.
 
I am suspect at both the battery and alternator tests. You should have above 14V at idle with 14.2-14.6 being the average range load dependent. What I would do is replace the battery as it is showing weak. After that check what your alternator is putting out and if it doesn't show more output with the new battery I would consider replacing that soon as well. A bad battery can strain an alternator.
 
You cant check a discharged battery and get any meaningful reading regarding its health. It should have been put on a charger before testing. Dont disconnect your battery while the engine is running unless you really want to replace the regulator which may be built into a new alternator. Your problem sounds like a bad battery but unless you can put it on a charger and check for a parasitic draw in the system you may end up with a new battery that dies in a few days leading you to replace the alternator which wont help and the cycle begins. I would pull the battery or at least disconnect it and put a charger on it overnight. After you disconnect the battery wait and hour or if you have a 12v load like an old headlight connect that for a few min. to bleed off the surface charge and remeasure. Make sure you clean and tighten the battery connectors, if they are loose or making bad contact it could prevent your alternator from completely charging the battery and cause a hard start because of a voltage drop across the connection.
 
iontrap said:
Trying to determine if I have a bad battery or alternator, 2009 Jetta 2.5 167k. First attempt to start it this morning, clicks like a bad battery and no start, the battery was at 10.1 volts. It started with a jump pack so I drove to AutoZone and had them test the battery and alternator. Their test showed battery good and charging system bad.
A few hours later I tested the battery again. The battery was at 12.6 volts, dropped to 7.8 volts when cranking and 13.8 volts idling. Voltage drops to 13.6 volts with headlights and AC turned on.
Does this sound like a bad battery or bad alternator?
Click to expand...
I've had batteries do this. Intermittent open in the battery. But a bad battery could show 12.6 but go low under a load. Min. 9.6 volts under starting load is the standard. Get a new battery and save the alternator. IMO
 
burbguy82 said:
Interested to know if the volts measured for the alternator was measured at the alternator? Should be about 14.5.

It should not be dropping much with load.

You could start the car, disconnect the battery and measure the voltage drop again, if it is gone, the battery is bad or low, if it remains, the alternator and or charging system is bad at some point. Might be good to check the main grounds while you are at it.

4 years on a daily driver battery is pushing it either way, but not worth 2 batteries.
Click to expand...
Disconnecting a battery with engine running will likely ruin the alternator...Just saying
 
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