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Old 08-30-2018, 07:52 AM   #1
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Default Measuring alternator charging current

After another "110A/high output" took a dump just 4 months out of warranty, I decided I needed to find a way to measure charging current to characterize my FC's charging needs as various loads are switched on & off. I found this little gem on Amazon for about $17 shipped that does the trick:


Basically it's a hall effect current sensor ring that you slip over the B+ alternator charging wire. They have 100A and 400A versions; I opted for the 100A version. You wire power to the little circuit board, either via a 9V battery or hard wire it to the car's 12V switched supply. Since I didn't want to permanently install it, I went the 9V battery route.

The resolution on this sensor is rather course, about +/- 0.2A per spec for my 100A version, which is useless if you want to use it to help find parasitic loads/shorts when the key is off, but was fine for my purposes - charging current characterization where +/- 1A resolution will do.

After installing my new alternator (more on that later), I installed the sensor ring, and temporarily mounted the display/battery to the dash and went for a drive. Here's a few observations:

- For the 1st few minutes after starting the car, current draw is much higher than what I was seeing as "steady state". Steady state for my car (no additional loads, but engine running) was around 18~20A. Same loads but within < 5 min after start, and I'd see about 50A. I attribute this to the fact that on start, the battery discharges a bit, and the alternator is trying to bring it to a fully charged state ASAP. During this higher current draw period voltage will read 14.5V, and will drop to about ~13.6 as the current draw drops to its steady state 18~20A figure.

- Whenever my main e-fan kicked on, I saw a brief 20A inrush (added to whatever the steady state was), which dropped to about 10A while fan is running. Oddly enough, just the brake lights impose a similar 10A load, with much less of an initial inrush current. Think it's time to replace all those halogen bulbs with LED equivalents!

- Full boat current draw on my car at idle, with AC on max, main E-fan & stock Aux E-fan running, headlights, fog lights, brake lights + rear defroster on was 95A. So it would seem my new upgraded 110A alternator is keeping up, but there's little headroom there, which explains the short life. Bottom line lesson learned is that I need to (a) reduce electrical loads where possible and (b) move up to a 150A rated or more alternator.

Regarding my alternator, I figured I'd call the vendor I purchased the dead one from, tell him about its early demise and see if they would be willing to do anything for me. Since it was 4 months out of warranty, I figured best case I might get a 10% discount on a new one. Instead they asked me to just ship the dead alternator back, and as soon as I could send them a tracking #, they had a new replacement alternator on its way to me for free. The shop is Ace Alternator & Starter motors in Whittier, CA, and they sell on Ebay. Shipped the dead alternator out to them on a Friday and I had a new one in my car the next Monday

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