I suspect that my battery does not receive enough power from the generator. How do I verify that? I know that involves a voltmeter, but what are the exact things I'm measuring and how?

I have a Skoda Felicia 97 1.3L, battery is standard and 1.5yo. Idle RPM seems to be a little low, but that's a separate question once I have proper data from the question above.


First thing is to check the Battery, see my post here.

Provided your battery is good, the following will let you know about the alternator.

Start the vehicle and let it idle. The voltage should be about 2 volts above open circuit voltage.

You can also do a loaded voltage output test. Turn on the high beam headlamps, AC on high, rear defogger etc. Bring the engine RPM up to 2000 RPM and check the voltage at the battery. It should be at least 0.5 volts above open circuit voltage.

You also need to check for AC ripple to make sure the diode rectifiers are still good. Put you meter on AC volts put the black lead on the battery ground and the red lead on the output lead of the alternator. You shouldn't have more than 0.5 AC volts, if you do the alternator is bad.

Most auto parts stores in the USA will check you battery and alternator for free. No idea what's available in Russia but it's worth asking.

  • Output lead of the alternator is that thick red wire, right? (googled pic: h-a.d-cd.net/a9a60a8s-960.jpg)
    – Kromster
    Nov 17 '14 at 20:04
  • @KromStern Yep, just move the black boot and the terminal should be exposed. Nov 17 '14 at 20:08
  • Is that correct, that "Voltage should be about 2 volts above open circuit voltage" meaning 13.89..14.66?
    – Kromster
    Nov 20 '14 at 10:47

If you'd like to see if your alternator is working, first you need to know what your battery is reading with the engine off. Put your multi-meter on vdc reading and test your battery. It should be about 12.6vdc if in good shape. Next start your engine. Your reading at the battery should be somewhere around 13.1-13.6vdc. It may even be a bit higher than that right after start-up (say in the 14vdc range), then settle down into this area. It may fluctuate a little bit (like .1-.2vdc), but nothing erratic. Most vehicles have what is called an "idiot light" in the instrument cluster which will come on if the alternator is not producing the amount of voltage it needs to sustain the battery and keep the engine/accessories running. It's really the difference of voltages, off vs running, which you're looking for to assure your battery is charging. If you put your multi-meter onto the battery and it is discharging (voltage constantly dropping), you need to look at your alternator as a source of mischief. If your battery is not charging up to snuff (around the 13.1-13.6vdc) arena, it could also be a source or problems.


You're measuring voltage. Idle could be related to the battery as well, since the battery might be lacking. Post results once you've found out. If it's not (from memory) around 12 or higher, then it's slowly degrading. Charging it does fix it temporarily, but it will die out eventually.

  • I'd like to get some instructions please. What do I measure, where, how, what values should I expect, etc. Dealing with the results is a separate topic.
    – Kromster
    Nov 17 '14 at 13:45
  • I don't know what voltmeter you have but, turn it on and set it to V which stands for voltage (NOT ohms). You need to place the red needle(if it has one), (under the batteries plastic rubbery bits which are red and black) place the red and black onto the appropriate terminals (red for positive/+ , black for negative- . This should now give you a reading.
    – CarlJB
    Nov 17 '14 at 13:48
  • Refers to this image, you should get a similiar reading if your battery is good shutterstock.com/pic-59384866/…
    – CarlJB
    Nov 17 '14 at 13:50
  • Okay, I know how to use voltmeter. What's next? Do I have to measure it on idle RPM? What values mean what? Do I need to compare with an engine turned off or at higher RPMs? Please add more info :)
    – Kromster
    Nov 17 '14 at 13:50
  • Only when the is engine on, what numbers came up?
    – CarlJB
    Nov 17 '14 at 13:51

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