Electronic – Different voltage readings when using different ranges on a multimeter

multimeterrectifiervoltage

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.

Best Answer

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...