Is 13.6 V output within the normal range with engine idling and no electrical loads other than the stock stereo?

2006 4 Runner

Yes, completely normal. A fully charged battery is 12.6 volts, so the charging system must be putting out higher voltage than that to keep the battery charged.

You are not actually measuring the battery.

Charging voltage: With the engine running steadily, A reading between 13.2 volts and 14.8 volts indicates a good alternator! A reading of less than 13.2 volts is a sign of a bad alternator or voltage regulator; a reading of more than 15.6 volts is a sign of a bad voltage regulator

I think he understand that he's not actually measuring battery voltage, since he asked about the voltage with the engine running. Nobody would think that a battery magically starts putting out more than 13 volts simply because the engine is running!

The point here is that the electrical system MUST exceed battery voltage in order to charge the battery. And a fully charged battery is 12.6 volts, so the system must put out more than that. Of course a voltage regulator keeps things in a safe range for the battery to avoid higher voltages that could destroy it.

Here is some more on static testing that I have collected over the years.

If the battery has one of those color indicating circles on it then it must be green. However, this green indicator only measures one cell!! If a different cell is bad, the indicator will remain green.

The battery has six cells in it with cell able to produce 2.1 volts. 2.1*6=12.6v
Therefore you could have a bad cell which is why you see something close to 9 volts all the time. However, the battery should not be able to start the engine with 9 volts

My favorite static test.
With a cold engine and not running, measure the battery voltage with a voltmeter. It should be greater than 12 volts. (Mine reads 12.4v)
Turn on the head lights for 10 seconds. Turn off head lights
Wait for 5 minutes and re-check the battery voltage. It must go up! You should now read something greater than 13 volts. (mine read 13.6v)
If the voltage goes down, then the battery has one foot in the grave

Another good test is to measure the battery voltage drop during starting.
With the voltmeter attached to the battery terminals, start the engine and observe the voltage drop. Mine momentary drops to 11.2 volts. Once the engine is running I measure 14.5 volts. If the battery drops below 9.6v during when the engine is starting, the battery should be replaced.

Voltage is a state-of-charge indicator for the battery. If you start with 12.4 volts and put a load on the battery, the voltage will eventually drop. After removing the small load that you described, the battery should recover, but it should not exceed the original 12.4 volts. Did you start the engine and then measure voltage?

Also, measuring voltage is a great way to test a battery, but once the voltage reading is used to determine that the battery is charged and there are no shorted or dead cells, the amperage is more important for actually starting the vehicle. That's why a simple load tester is the easiest tool to use for testing a battery. You can watched the voltage level reading while applying the load to ensure that it doesn't drop too far and recovers quickly.

Of course not.

It will always exceed 12.6v after applying a load to it with the engine remaining off. That is unless it is dying!

Try it. It will surprize you and it is one of the best tests to do.

Thanks for the test. I gave it a try and sure enough, just as advertised. Matter of fact, after 20-25 minutes it was up to 17 volts. Started thinking about that movie "The China Syndrome".:bugeye:

I work with deep cycle batteries at work. When they are discharging, the voltage will drop to about 12V and then go up to 12.1 or 12.2 before continuing their discharge. I think it has something to do with the temperature of the battery during discharge.

Normal voltage when engine is running 13.6 to 14.3

Normal voltage when engine is NOT running 12.5 to 12.8 (after sitting overnight)

Load test using voltage - Disable ignition so engine won't start. Crank engine no more then 15 sec, voltage should not drop below 9.6 volts.

If failed after these tests take it to any parts chain store and let them perform a battery/load test. They simulate a load and measure amps/voltage. Amp reading should be no less then 5 amps below alternator

good info. Thanks greasefingers.

Last week I had to replace the battery because it went dead with no warning. Last night the new battery was almost dead and I had to get jumped by a friendly guy at a gas station. I made it about 5 miles before it completely died. We had to push it into a parking space and it spent the night windows down.

I'm assuming it's the alternator. It did have a faint wining sound for over a week that seemed to be connected to RPMs. Before I have it towed to a repair shop, I would like to know how easy would it be to do myself? How do I know its not something else?

Thanks for any help

The 4.0 alternator is fairly easy to replace, although it is a little tight in there. Remove belt off alternator pully, couple bolts, pop it out, pop new one in, reverse.If you have a voltmeter, check the battery while the vehicle is running and it should read around 13.6 at idle +/- .2-.4. It is possible that the new battery just didn't have enough of a charge to begin with and you haven't driven around with it enough to get a full charge on it, but it does sound like your alt is on it's way out.

I could not find this info anywhere, I have an AGM and a great HO alternator from Singer that has taken my abuse well. I added a voltage booster in the summer because voltage was a little low for the AGM specs, but now in the winter I'm getting 15.2v, I'm going through Headlight bulbs much faster than I should and I assume that the voltage is the culprit? Is 15.2 too high? If that is the issue can it cause other issues?