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Old 7th March 2008, 05:50 PM   #1
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Question Time for a new alternator?

I was testing a SMPS in my car today and was a bit shocked at the results.

No load - in 14.4v / out 98v
10R load - in 11.3v / out 64v

64v^ / 10 = 409.6

So, with a ~410W load my charging system dropped to 11.3v. Does this sound normal for a stock ~70A alternator at idle?

I'm leaning toward the alt is OK, just underpowered. Either way I'm going to buy a new alt; I'm just wondering if it's worth breaking the bank for a 160A unit.
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Old 7th March 2008, 05:56 PM   #2
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At what engine RPM? You may find at idle the output collapses a lot more easily than say at 2000 RPM and above.
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Old 7th March 2008, 06:11 PM   #3
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~700RPM idle. I'm going back out to the car to see what the voltage drops to with the engine held at 2K.

I think generally an alternator will put out 40-50% of its rated current at idle.

EDIT: Done. The results were pretty much the same, even at 3K. Also, two 10W 20R resistors in parallel cannot survive for more than 4 seconds at 64V. Time to find a new load.
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Old 7th March 2008, 06:23 PM   #4
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That is bad...but is your alternator/batter what is wrong, or wires going to smps?
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Old 7th March 2008, 06:28 PM   #5
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The battery is only a few months old, but the alt looks original and possibly on it's last leg. The wiring is 10ft of 8 gauge.
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Old 7th March 2008, 06:30 PM   #6
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Where did you measure that voltage? On BAT? Is any bad grounding possible?
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Old 7th March 2008, 06:57 PM   #7
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I was checking the voltage at the supply input. I'm going to check the voltage at the battery as soon as I find something to wrap this 10ft if nichrome wire around.
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Old 7th March 2008, 07:04 PM   #8
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Even at supply input there shouldn't be that low voltage
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Old 7th March 2008, 07:33 PM   #9
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Ahha, definitely a wire problem. The voltage at the battery is 12.6v while the voltage at the supply is 11.15v. This is with the new load I made that is just under 10R.

I would like the try the supply connected directly at the battery, but there is a bit of a snow storm out right now.

Does 12.6v sound more reasonable for a 410w load?
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Old 7th March 2008, 07:51 PM   #10
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Lool snow storm, I have luck, nothing over here
Engine on @ 12.6v on battery = not good. Even my small clio can do better and I am sure you have bigger car...

You can check wire voltage drop, to see if you can get only by using bigger wire more out to smps.
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