Using a clamp-on DC ammeter

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I just got a clamp-on multimeter that can measure DC current. I'd been looking for a reasonably-priced one for a while; most can only measure AC current and all of the ones that can measure DC current are over $100. Well, I found one for $50, not much more than an AC-only one. These are nice to have for troubleshooting starting/charging system problems.

I decided to take some current measurements at the battery of my 2006 Saab 93. I used the ground wire since it was the easiest to get to:

With the key in the off position, stereo on: A draw of about 8 amps.

After starting the engine: Battery initially charges at about 35 amps and tapers off to about 5 amps after a few minutes. I didn't wait to see if it would drop lower than that but I expect it would. This was at idle by the way.

Then I decided to take some current measurements at the wire which connects the alternator to the battery (this is the thick wire between the battery and the fuse box). From the battery it connects to the fuse box so this wire basically supplies all of the power from the alternator to the entire electrical system, making it a good place to measure how much current is being drawn from the alternator.

Several minutes after starting, cabin fan on low, stereo on: About 30 amps.
AC turned on, engine cooling fans running, cabin fan on high, stereo on: 65 amps.
The above plus the high-beams, seat heaters (on high), and rear defroster: 100 amps (the voltage had dipped below 13V at this point).

All of the above was tested at idle so I decided to rev the engine to 2000RPM, now it was pulling about 125 amps from the alternator, which is the highest I was able to get it.

100 amps is impressive output at idle, particularly for one rated at 120 amps. I think the 65-amp alternator that was on my Mustang might have been good for 30 amps at idle.
 
Yes, I've long wanted a clamp-on DC ammeter. Now I can test alternators (and batteries) without removing them from the car. Apparently, a bad battery (presumably with a shorted cell) can pull large amounts of current and never taper off like it's supposed to..a weak sulfated battery won't pull enough current.

I haven't tried getting readings of starter current draw except on my Mustang and that read 88 amps. That seems sort of low but I guess it's possible since the engine was fully warmed up.
 
I bought a Craftsman 600-amp AC/DC clamp meter off Ebay a few years ago. Works great. I use it to check alternators, check starters on smaller engines (doesn't work so well on class 8 trucks that draw 800 amps), and trace wiring (activate the circuit, then follow the amps).

I've checked it against my 10-amp in-line amp-meter, and it's very accurate down to two or three amps.

Good times.
 
Most alternators on GMs are rated for 160 or 180 Amps, and that measurement is typically at 2000 or 2500rpm depending on the model alternator. So 100 Amps at idle isn't unheard of, but now you see why alternators can wear out :)

Just for the record I'd like to add this for energy nuts:
100 Amps * 14V = 1400W (1.4kW) = 1.88HP

I'm sure an alternator isn't 100% efficient so it's probably using around 2-5hp to turn that alternator to keep your fans on, seats warm, battery charged, stereo on, etc. Just something to think about :)
 
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