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Over the years, loads of people have asked about how to now how much current is running from their battery. I normally advise to get a wattmeter, but it takes weeks for one to arrive from overseas and the UK ones are a bit expensive, so here's a quick and very cheap way to do it. No self-respecting ebiker should be without a voltmeter, so, if you haven't got one, go down to Maplin and get one (or Ebay).
First, you make a shunt. I did this one with Deans connectors and 14g wire. The thickmess of the wire is important. You can use different wire, but then you have to calculate a different length - see my spreadsheet in the link at the end. You don't even need connectors. You only have to solder two wires 100mm apart (for 14g) on your battery negative wire, or you can add a piece that goes between whatever connectors you have. Here's a photo of mine:
The thin black wire goes to the battery side, the white goes to the controller side, and you'll not that I've added a third red wire to the positive battery wire. This is so that I can use it as a remote shunt for one of those cheap Wattmeters. The length of 14g wire needs to be exactly 100mm.
All you need to do now is put this arrangement in line and connect the white and black wires to a voltmeter set to measure mV (normally 200mv scale). Here's the results compared to my wattmeter. They're the same at 1 amp and about 10% out at 9 amps. Ideally, I should make another one 11mm longer to get it exact at 10 amps assuming that the wattmeter is correct. The wire is probably a bit thicker than standard 14g, but the result is close enough. The number of mV on the voltmeter is equal to the number of amps.
Links:
http://endless-sphere.com/forums/viewtopic.php?f=2&t=55094&p=831298&hilit=current+measurement#p831298
http://www.ebay.co.uk/itm/DIGITAL-LCD-VOLTMETER-MULTIMETER-CIRCUIT-TESTER-AMMETER-AC-DC-METER-OHM-/190774031117?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item2c6b04b30d
http://www.maplin.co.uk/p/ut-30b-digital-compact-multimeter-n15by
Incidently, that's Bezzer's battery on test.
First, you make a shunt. I did this one with Deans connectors and 14g wire. The thickmess of the wire is important. You can use different wire, but then you have to calculate a different length - see my spreadsheet in the link at the end. You don't even need connectors. You only have to solder two wires 100mm apart (for 14g) on your battery negative wire, or you can add a piece that goes between whatever connectors you have. Here's a photo of mine:
The thin black wire goes to the battery side, the white goes to the controller side, and you'll not that I've added a third red wire to the positive battery wire. This is so that I can use it as a remote shunt for one of those cheap Wattmeters. The length of 14g wire needs to be exactly 100mm.
All you need to do now is put this arrangement in line and connect the white and black wires to a voltmeter set to measure mV (normally 200mv scale). Here's the results compared to my wattmeter. They're the same at 1 amp and about 10% out at 9 amps. Ideally, I should make another one 11mm longer to get it exact at 10 amps assuming that the wattmeter is correct. The wire is probably a bit thicker than standard 14g, but the result is close enough. The number of mV on the voltmeter is equal to the number of amps.
Links:
http://endless-sphere.com/forums/viewtopic.php?f=2&t=55094&p=831298&hilit=current+measurement#p831298
http://www.ebay.co.uk/itm/DIGITAL-LCD-VOLTMETER-MULTIMETER-CIRCUIT-TESTER-AMMETER-AC-DC-METER-OHM-/190774031117?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item2c6b04b30d
http://www.maplin.co.uk/p/ut-30b-digital-compact-multimeter-n15by
Incidently, that's Bezzer's battery on test.
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