Time for a new alternator?

theAnonymous1 · 7th March 2008, 05:50 PM

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.

richie00boy · 7th March 2008, 05:56 PM

At what engine RPM? You may find at idle the output collapses a lot more easily than say at 2000 RPM and above.

theAnonymous1 · 7th March 2008, 06:11 PM

~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.

luka · 7th March 2008, 06:23 PM

Hi

That is bad...but is your alternator/batter what is wrong, or wires going to smps?

theAnonymous1 · 7th March 2008, 06:28 PM

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.

luka · 7th March 2008, 06:30 PM

Hi

Where did you measure that voltage? On BAT? Is any bad grounding possible?

theAnonymous1 · 7th March 2008, 06:57 PM

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.

luka · 7th March 2008, 07:04 PM

Hi

Even at supply input there shouldn't be that low voltage

theAnonymous1 · 7th March 2008, 07:33 PM

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?

luka · 7th March 2008, 07:51 PM

Hi

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.

7th March 2008, 05:50 PM	#1
theAnonymous1 diyAudio Member Join Date: Feb 2004 Location: Anonymityville	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. __________________ "If you don't like funerals don't kick sand in Ninja's face." - Ninja

7th March 2008, 05:56 PM	#2
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	At what engine RPM? You may find at idle the output collapses a lot more easily than say at 2000 RPM and above. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

7th March 2008, 06:11 PM	#3
theAnonymous1 diyAudio Member Join Date: Feb 2004 Location: Anonymityville	~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. __________________ "If you don't like funerals don't kick sand in Ninja's face." - Ninja

7th March 2008, 06:23 PM	#4
luka diyAudio Member Join Date: Sep 2005 Location: in Slovenia :)	Hi That is bad...but is your alternator/batter what is wrong, or wires going to smps? __________________ home page @ http://www.classdaudio.co.cc @ 24/7 all year long I FEEL SLOVENIA

7th March 2008, 06:28 PM	#5
theAnonymous1 diyAudio Member Join Date: Feb 2004 Location: Anonymityville	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. __________________ "If you don't like funerals don't kick sand in Ninja's face." - Ninja


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7th March 2008, 06:30 PM	#6
luka diyAudio Member Join Date: Sep 2005 Location: in Slovenia :)	Hi Where did you measure that voltage? On BAT? Is any bad grounding possible? __________________ home page @ http://www.classdaudio.co.cc @ 24/7 all year long I FEEL SLOVENIA

7th March 2008, 06:57 PM	#7
theAnonymous1 diyAudio Member Join Date: Feb 2004 Location: Anonymityville	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. __________________ "If you don't like funerals don't kick sand in Ninja's face." - Ninja

7th March 2008, 07:04 PM	#8
luka diyAudio Member Join Date: Sep 2005 Location: in Slovenia :)	Hi Even at supply input there shouldn't be that low voltage __________________ home page @ http://www.classdaudio.co.cc @ 24/7 all year long I FEEL SLOVENIA

7th March 2008, 07:33 PM	#9
theAnonymous1 diyAudio Member Join Date: Feb 2004 Location: Anonymityville	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? __________________ "If you don't like funerals don't kick sand in Ninja's face." - Ninja

7th March 2008, 07:51 PM	#10
luka diyAudio Member Join Date: Sep 2005 Location: in Slovenia :)	Hi 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. __________________ home page @ http://www.classdaudio.co.cc @ 24/7 all year long I FEEL SLOVENIA

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