City-Data Forum How do you test the alternator in a car with a dead battery? (auto, idle)
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08-19-2011, 01:52 PM
 3,392 posts, read 6,758,323 times Reputation: 3919

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In this scenario my car battery measures only 11v and won't start the car. I need to know if the low reading was caused by the alternator not charging the battery. Can I jump start the car and test alternator afterwards? In other words, as long as the car is running, the condition of the battery shouldn't have any bearing on the testing of the alternator, right?

Thanks for your help.

08-19-2011, 02:05 PM
 Status: "C'est la Vie. C'est la Guerre." (set 10 days ago) Location: The Triad (NC) 25,116 posts, read 54,258,144 times Reputation: 25988
How old is the battery?

1) if 3 years or more... buy a new one.
1a) if less than 3 years go to step 2
2) check, clean and tighten EVERY electrical connection.
3) With the car running (new battery installed or jumped) then check the charging SYSTEM

go from there.

08-19-2011, 02:05 PM
 Location: Eastern Washington 12,715 posts, read 40,934,857 times Reputation: 10295
Well according to Hoyle you should test the alternator with a fully charged battery. But yeah, if you jump-start the car, if you have a voltmeter and apparently you do, you can do the following:

First, check the voltage on the battery before you put on cables or start the car - let's go with 11V - OK jump start the car, take the jumper cables off, check voltage again. If it's more than 11V, and goes up to some reasonable level like 12.5, 13 V when you rev the engine a bit, then levels off - the alternator and regulator (which is probably part of the alternator on your Honda) is working at least to some extent.

Usually if an alternator quits charging suddenly, usually, it's just the brushes have worn out, all you need to do is replace the brushes or take it to a good auto electric shop where they can do it. Some alternators for example some Bosch for VW cars, the brushes are part of the regulator and you can change this without even taking the alternator off the car. Some Ford alternators you have to about completely dis-assemble to do the brushes, and these I take to a shop.

Assuming your alternator flunks the test - If you have a good auto electric shop available consider buying a used alternator from a yard, take it to the shop for a check-out, this is usually cheap or even free. If it needs brushes or whatever get that done, put this in your car. Then take the old alternator to the shop and let them fix it too, put it on the shelf as a spare.

That assumes you plan to own this Honda for a long time.

If you get a "rebuilt" from FLAPS, usually you have to take the pulley and fan off your old unit and put them on the new unit, which is easy if you have an air wrench handy, hard if you don't. If you get a used alternator from a yard, it will have the pulley and fan already on it.

Just sayin'.

08-19-2011, 02:13 PM
 Location: Eastern Washington 12,715 posts, read 40,934,857 times Reputation: 10295
Quote:
 Originally Posted by MrRational 1) How old is the battery? If 3 years or more... buy a new one. 1a) check, clean and tighten EVERY electrical connection. 2) With a new battery installed then check the charging SYSTEM go from there.
Let me guess, you sell batteries as part of your job, or maybe you own a lot of Exide stock?

I just changed a battery that went 10 years (it was an 84 month battery)

You do have somewhat of a point if the battery is well beyond it's warrenty period - maybe. But a battery can be tested and/or charged as well.

OP, another tact would be to take the battery and alternator to AutoZone or similar, where they can test them independently off the car, frequently they will charge a battery for free. Although, realize that compared to my friendly local auto electric specialist shop, where the guy testing the alternator has been rebuilding alternators for 20 or 30 years and has "been through fire, water, and a copper pipe" so to speak, at AutoZone the guy doing the testing just plugs the alternator into a machine, pushes the "go" button, and reads the lights. AutoZone guy tested the alternator I have on my Scirocco right now and pronounced it dead in all 3 areas the machine tests. Specialist shop recognized it just needed brushes, did that, bada bing, bada boom.

To really test a battery properly you charge it up and measure the specific gravity of each cell, a floating ball type hydrometer will give a rough idea, but one with a graduated scale and a thermometer on it so you can compensate for temperature is the tool to get the dead nuts facts.

08-19-2011, 02:26 PM
 10,630 posts, read 39,408,004 times Reputation: 13342
Quote:
 Originally Posted by davidt1 In this scenario my car battery measures only 11v and won't start the car. I need to know if the low reading was caused by the alternator not charging the battery. Can I jump start the car and test alternator afterwards? In other words, as long as the car is running, the condition of the battery shouldn't have any bearing on the testing of the alternator, right? Thanks for your help.
The quick down and dirty answer is that if you can get the car started and the alternator/regulator system comes up quickly to a normal charging voltage of mid-to high 13 v range (or even up to around 14.2 v), than it's an indicator that the charging system may be OK.

BUT:

A low battery charge (or defective battery) will affect your alternator output readings. So if you only test system voltage with your voltmeter, you are not getting all the information you need because all you are measuring is the system voltage under the load of the discharged battery and electrical consumers at that time. You may, in fact, have a good (or bad) system but not be able to verify it with the first simple test.

Much better to test the alternator output (voltage as well as max amp output) after fully charging the battery and verifying that it can accept and hold a charge.

Cleaning all the cable connectors is important, especially the ground circuits which tends to be ignored on many cars. If the battery accepts and holds a charge, there's simply no reason to replace it on the basis of 3 years of service ... especially in climate zones where 5-8 years is a more typical service life.

It's also a good idea to use an o'scope on an alternator output to see the condition of the output diodes. Many alternators can produce all or close to all of their rated output amperage with a defective diode ... which can cause a non-running battery discharge, or damage to electronics and the battery when the car is running. Other failed diodes can cause a discharge, too ... which can run the battery down. What you can have is a situation where the alternator can do an adequate job of recharging the battery, but the alternator may also be the cause for the battery to run down between charging. At some point, the charge/discharge cycles will destroy a battery.

08-19-2011, 02:44 PM
 Location: Keosauqua, Iowa 8,646 posts, read 14,795,285 times Reputation: 10745
In the old days you could pull the negative battery cable with the car running. If it died, the problem was in the alternator. If it kept running, it was the battery.

I don't know how good that would be for a modern computer-controlled electrical system, though, so I haven't used that method for a few years.

08-19-2011, 04:27 PM
 Location: Eastern Washington 12,715 posts, read 40,934,857 times Reputation: 10295
Quote:
 Originally Posted by duster1979 In the old days you could pull the negative battery cable with the car running. If it died, the problem was in the alternator. If it kept running, it was the battery. I don't know how good that would be for a modern computer-controlled electrical system, though, so I haven't used that method for a few years.
That's strictly for generators, not alternators.

Of course, if you do this test with an alternator, it's quite likely to blow one or more diodes, so if your alternator was good, now it's bad for sure. Probably along with some other electronic components.

Thanks to Sunsprit for giving the full correct answer. I forgot about a single diode possibly being bad. Worn brushes have been more common in my experience, but then again my experience is relatively limited.

I'm thinking that a decent DIY test for a bad diode might be to disconnect say the ground connection to the battery with the car OFF, engine OFF, key turned OFF, and connect a test light between the ground terminal of the battery and the main ground wire. If the diodes are good, the light should stay off - if one or more is bad, the light should at least glow dimly.

I'm speculating based on theory, have not proved this out in actual practice, nor did I see it as a recommended test in any manual.

08-19-2011, 04:39 PM
 29,184 posts, read 22,289,394 times Reputation: 16516
as m3 and sunsprit indicated, if you can get the engine running, you can test the charging system output with a volt meter. you should get between 13.5 and 14.5 volts. more than that and your system is overcharging the battery, and that will cause problems with electrolyte boiling away and showing a low battery voltage. less than the 13.5 volts, and you either have an alternator going south, or a regulator that is dying.

my advice is to take the battery to your local auto parts store that can do battery testing on a load tester, and see what the battery is doing.

as m3 indicated a bad alternator is usually a worn out set of brushes, or as sunsprit indicated a bad diode or two, sometime the whole bridge burns out.

and while an alternator can be easily rebuilt, understand that some rebuilders take short cuts and dont replace all the parts needed to properly rebuild an alternator. they only replace the dead parts.

08-19-2011, 08:54 PM
 3,392 posts, read 6,758,323 times Reputation: 3919
Quote:
 Originally Posted by duster1979 In the old days you could pull the negative battery cable with the car running. If it died, the problem was in the alternator. If it kept running, it was the battery. I don't know how good that would be for a modern computer-controlled electrical system, though, so I haven't used that method for a few years.
A few months ago, a DIY mechanic in my neighborhood did this with my car. The car kept running. He said that's the way it's supposed to be if everything is OK. Now reading what M3 Mitch said about this method would damage the alternator scares me a little.

To the best of my limited knowledge the alternator tested OK. When the engine is running, the voltage is around 14.5V. When the alternator is put under load (A/C, headlights, wipers on), the voltage drops to 13V.

08-19-2011, 09:31 PM
 10,630 posts, read 39,408,004 times Reputation: 13342
[quote=davidt1;20531264]A few months ago, a DIY mechanic in my neighborhood did this with my car. The car kept running. He said that's the way it's supposed to be if everything is OK. Now reading what M3 Mitch said about this method would damage the alternator scares me a little.

To the best of my limited knowledge the alternator tested OK. When the engine is running, the voltage is around 14.5V. When the alternator is put under load (A/C, headlights, wipers on), the voltage drops to 13V.[/QUOTE]

That voltage drop under those loads indicates a serious shortage of amperage output from the alternator if you are getting this result at an engine RPM off idle, say at normal cruise RPM's ... especially in light of a 14.5 v regulated charge voltage with only the ignition and fuel systems powered up at a similar RPM. At 13v, you might even notice that the headlights are dimmer than normal while driving at night, and there would likely be a discharging battery cycle in progress which could shorten the lifecycle of the battery. You could get a visual check on this by switching the A/C on/off while driving at night and watch the headlights dim/brighten slightly. In any event, I'd suggest you get the alternator/regulator bench tested by a competent automotive electrical shop; there may also be an issue with grounds under the higher load and the regulator is responding to what voltage it "sees" at the lightly loaded compared to max loaded electrical demands.

The alternator disconnected from the battery test isn't a good idea ... the alternator depends upon having the battery power source/load for it's regulation as well as stable voltage field excitation power supply. Solid state electronics don't react well to the voltage surge that results when the battery is disconnected while they are on-line. You could do this test safely with a generator because the field excitation power could be supplied from the residual magnetism of the generator and the voltage regulators were electro-mechanical, not solid-state electronics devices. The generator delivers it's power directly through to the battery (via mechanical brushes and commutator) without the need for solid state rectification as an alternator requires to deliver it's pulsed DC output.
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