Measuring DC Current

[kittysmitty]

I'm looking for an easy inexpensive (cheap)method to find out how my amps my lawnmover/105 amp auto alternator is putting in the battery bank. It's not used often,and accuracy is not important, say plus or minus 10%.I have a DMM, So, I determine the resistance in the cable from the battery to the alternator, by using a known load, ie 50 watt bulb, measuring the voltage drop in the cable, and using ohm's law to determine the resistance of the cable. I can then measure the voltage drop across the cable and now since I know the resistance, using ohm's law again, determine the current. I did some testing on another cable and found that the resistance of the cable was .0004054 ohms.Can it be this simple? or am I missing something? Thanks

[Flux]

Yes that will work. A bigger load than 50W would give you more accuracy but it will do.

Make sure you measure the cable volt drop and not include various connections.

Flux

[SparWeb]

Sounds like what I'd do.

One last step:  When you take the lamp out of the circuit, its resistance is removed from the circuit.  Since your test conditions aren't the same as the normal conditions, you could take that into account in one last calculation.  There's just a little bit more current going into the batteries in the normal condition than there was in the test.  The difference could be negligible, of course.

[kittysmitty]

Thanks for the replys. Somewhere else I read that you could use the voltage drop across the main fuse, in my case 200 amp. Would this also work?

[Flux]

Yes but it may not be very accurate, fuses run hot and I have no idea what the fuse element would be, it may have a fair temperature coefficient.

Flux

[Ungrounded Lightning Rod]

Easier - and maybe more accurate than the lamp hack - is to look up the resistance-per-length of that size wire in a table.  Manufacturing tolerances for the wire will probably be better than those for the bulb and you won't be sensitive to your line voltage and other pathologies.

Remember to connect the meter taps to two points on a continuous piece of the wire rather than fastening them to a connector.  The connection resistance can swamp the wire's resistance.

[gizmo]

Adding to what Flux said, you also need to consider what happens to the amp meter if the fuse blows.

[gizmo]

I put a page together about petrol driven alternators on my site, it might be helpfull.

http://www.thebackshed.com/Windmill/articles/PortableCharger.asp

This is the circuit I used.

Glenn