Current Draw for Lights and Accessories at Idle, Measurements taken on my car today Options

[pbanders]

Some of you know I recently converted my car to all LED's, to reduce the parasitic drag on the engine from the alternator at idle, when the lights and other accessories are turned on. I did some measurements with the car off, went out and got it fully warmed up (it's 95 deg. F here today, didn't take that long), then I took a series of measurements of the current supplied to the battery (from alternator B+) and the supply to the car from the battery, using my digital loop ammeter. Did some calculations, and here are the results. Battery was fully charged before staring this test (12.75 V with the engine off), idle steady at 1000 rpm, and battery voltage at idle was 13.7 to 13.8 V. Each accessory was turned on individually and the current for the accessory was calculated by subtracting the total current with the accessory off at idle. Current draw for various combinations of accessories was calculated by adding the accessory currents to the total current with all accessories off at idle. Note the table below is the current supplied to the lights and accessories (as well as the ECU, AAR, Fuel Pump, and ignition), and NOT the current coming from the alternator B+, I'll get to that later. You get the idea, if you have any questions, just ask. I'll explain the abbreviations in the table in a minute...



OK, here are what the abbreviations mean.

Rows
------
Lights (lo) = lights on, low beam
FA(3) = Fresh air blower, maximum setting
Heat = Heater blower
Wipers(1) = Wipers on 1st speed setting, flipped up, not dragging on windshield
Radio(20) = My radio on volume setting 20, quite loud, unit max is 22W x 4

Columns
----------
w/LED's = my current setup, all LED lights, including headlights and driving lights
w/LED's + hi + dl = same as w/LED's column, plus current for LED hi beam and DL's
w/Inc (lo) = as if I hadn't done the LED's, w/standard H4 halogen low beams
w/Inc (hi) + dl = as if I hadn't done the LED's, w/H4 halogen hi beams + H3 DL's

FYI, my old H4 halogen bulbs were 55/55W

Hope that makes sense.

First thing to note is the current with all the lights and accessories off. This is the basic current requirement for the car to run, supplying the ECU, the AAR heater, the Fuel Pump, and the ignition (I have a Crane XR3000 with one of their hot coils). This current is quite low, only 5.0 A. I also measured the B+ charging current from the alternator under these conditions, it was varying between 5 and 7 V as the voltage regulator was modulating the field current in the alternator, to keep the voltage in the system stable. I might note that I have one of the transistorized Bosch voltage regulators, instead of the electromechanical stock Bosch voltage regulator.

As you can see, even when I'm running full LED's and am just on low beam, if I turn on all the accessories, I'm pulling a lot of current at idle, as much as 26 A. However, this is still reasonably OK for the alternator, as it's spinning at 2X idle (2000 rpm) and at 80% duty cycle can put out about this much (I'll post some alternator data to this topic later about it's output at speed).

Try the same thing with H4 halogen low beams and you would pull as much as 43 A. According to Porsche, the max output of the alternator is 50 A, but I think Bosch says 55 A. Regardless, 43 A at an alternator speed of 2000 would be insufficient to charge the battery and would run the alternator at 100% duty cycle, not advisable. If you were dumb enough to be running your H4 halogen high beams and H3 fog/driving lights while at idle with everything on, that 59 A draw would cook your alternator in no time!

Prior to doing the LED conversion, what I saw was that the idle began to be affected when I just turned on the H4 halogen low beams (55W). As you can see, this corresponds to a current of about 25A, which with my LED's is only approached when I turn everything on, so the reduction of current by going to LED's lets me use my lights and accessories without any noticeable idle drop, for virtually all combos. And, that's what I've seen in actual driving, too.

Note that even if you weren't at idle, and you had on all the accessories, if you had standard lighting and H4's, the stock alternator at max output would barely keep ahead of the current draw, and you'd likely be pulling down the battery. Note that all of the currents in the table above are what's being provided to the lights and accessories, as well as the ECU, AAR, and ignition. I also measured the current coming from the alternator (the B+ lead), and as you might expect, it was about 2 to 3 A higher, because it was also charging the battery, so the actual requirements on the alternator are 2 to 3 A more. If your battery is in crummy shape or just flat from not being run, this could go as much as 10 A.

Anyway, I've never seen this data before so I wanted to collect it and understand what it means. I think you can see that for reasonable combinations of incandescent lights and accessories, that the stock alternator can be strained to keep up with requirements, and as a result, put a lot of drag on the motor as well as barely keep the battery charged. By going to all LED's you can keep the stock alternator and significantly reduce stress on it, making it last longer and improving the idle at the same time.

[914_teener]

Brad,

This brings to mind an older thread as I am still contemplating the whole LED upgrade to my car as well.

If memory serves, didn't the stock configuration of the car prevent the high beam and fogs from coming on at the same time?

So wouldn't this change the theoretical max load to avoid the issue if you were still using all incandescent in the front?

I seem also to remember that the engineers knew this and if you had incandescent light or upgraded AND you did the J-west upgrade as I call it...you would also exceed the load of the alternator and possibly fry it.

This makes me think that if we wanted to upgrade to LED's one would want to do all of them...at least for the front of the car.

Thoughts?

[pbanders]

Brad,

This brings to mind an older thread as I am still contemplating the whole LED upgrade to my car as well.

If memory serves, didn't the stock configuration of the car prevent the high beam and fogs from coming on at the same time?

So wouldn't this change the theoretical max load to avoid the issue if you were still using all incandescent in the front?

I seem also to remember that the engineers knew this and if you had incandescent light or upgraded AND you did the J-west upgrade as I call it...you would also exceed the load of the alternator and possibly fry it.

This makes me think that if we wanted to upgrade to LED's one would want to do all of them...at least for the front of the car.

Thoughts?

I did some rewiring and added a couple of relays a few years ago to allow me to flash my fog/driving lights by pulling on the turn signal stalk when the lights are down and off, and also to allow me to turn on the fog/driving lights when I switch on the high beams. I've also got a small toggle switch I put on the fuse box to turn the simultaneous high + fog/driving light off, too. J-West came up with a similar mod the next year, I'd have bought that if it were available.

Basic automotive engineering tells you that at least with modern cars, you should design the charging/battery system such that when you're at idle, you can turn on every single accessory (at least all of the ones that run constantly), and not exceed the alternator's capability to supply the required current, while simultaneously providing sufficient current to charge the battery. Additionally, the active idle control should keep the idle stable at the setpoint, regardless of alternator loading and other mechanical loading (e.g. a/c compressor, P/S pump, etc.).

Unfortunately, that's not how the 914 was designed. From what I can tell, and I think my experience is shared with others, even with just the headlights on at idle, there's significant idle drop due to alternator loading, and because there's no active idle stabilization, the lower idle means the alternator is putting out even less current. Add in any additional loads (e.g. heater, fresh air blower, radio, etc.) and it gets progressively worse.

Going to LED's "fixes" this problem somewhat, but it's really a band-aid. The best solution is a combination of LED's, a higher capacity alternator, and active idle stabilization. Two of those three are available now, and I hope we can figure out a way to add idle stabilization in time to the 914.