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I'm using a digital multimeter to check the voltage on my motorcycle battery - I'm attempting to confirm the proper installation of a new alternator in the charging system.

The voltmeter has 3 ranges that I'm using: 20V, 200V, and 500V.

With the engine off the battery reads ~12.8V on all three ranges. But with the engine on (and hence the charging system going), the battery reads different voltages using each range:

  • 20V range jumps around from 0-15 volts
  • 200V range holds pretty steady around 17.5-18V
  • 500V range jumps around from 40-100V

The multimeter is a brand new Equus Innova 3300: http://www.walmart.com/ip/Innova-3300-Equus-3300-Hands-free-Digital-Multimeter/14644665

I'm not expecting this community to diagnose the motorcycle issue, but does anyone have any idea what electrical behavior could cause these readings in the multimeter? Why would the readings be so different at each range?

I checked all the diodes in the rectifier (3-phase rectifier, 6 diodes), so I don't think any AC current is getting through to the battery/multimeter, which was my first thought here.

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There's going to be an AC component to almost any battery charging system. It's DC, but DC with some ripple on top of it. How much ripple depends on the charger. So that's why the readings jump around when the charger is active; the voltage is actually jumping around! You'd need an oscilloscope to see exactly what the waveform looks like. The DC reading on the meter is going to filter the ripple down, but exactly how it filters it, and to what degree, is going to vary from meter to meter and from range to range. The differences in this filtering are what's causing your different readings.

Unfortunately, I'll have to leave the details of how that filtering takes place in the meter to someone else...

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  • \$\begingroup\$ I will +1 if the phrase is not contradictory. There's no AC, but DC ripple, but your answer says the two :) \$\endgroup\$ – Diego C Nascimento Jan 5 '14 at 1:03
  • \$\begingroup\$ @DiegoCNascimento hmmmm... so a single rail op-amp circuit that accepts AC and level shifts it to be within the supply rail and 0V doesn't have any AC signal after the level shift, just DC ripple ? ... it is a point of view that to my mind is valid. As is AC with a DC level .... both are valid. So for Stephen's it's a +1 for a clear explanation is deserved in my mind. \$\endgroup\$ – Spoon Jan 5 '14 at 1:29
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    \$\begingroup\$ @Diego a signal is a signal, it does not care for our AC / DC distinction. The signal we are talking about here has both a DC and an AC component (what you call, less accurately, DC ripple). \$\endgroup\$ – Wouter van Ooijen Jan 5 '14 at 11:26
  • \$\begingroup\$ @DiegoCNascimento The concept I think you're missing is superposition of signals. If you add a DC signal and an AC signal, you get a signal with a DC component and an AC component. This can also be described as a DC signal with ripple. \$\endgroup\$ – Stephen Collings Jan 5 '14 at 16:49
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    \$\begingroup\$ What I think we're all saying is that these are two different ways of naming the same thing. Current does not reverse directions, it's true, because the signal is more DC than AC. But you can extract a component of the signal, and in that component current does reverse directions. So it has an AC component, even if the entire signal is not AC. \$\endgroup\$ – Stephen Collings Jan 6 '14 at 14:12

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