|
echofrancis, I had already measured the current used by the blower motor at different speeds. (There's a post in this thread with all that data laid out.) I then simply applied Ohm's Law to work out the rest.
In my case (VW Mk4 Golf) that works out like this...
Current draw of blower motor...
Speed 4/4... 17.5A
Speed 3/4... 12.4A
Speed 2/4.... 8.5A
Speed 1/4.... 6.0A
At full speed (4/4) there is no resistor in series so I can calculate the equivalent resistance of the blower motor from Ohm's Law (R=V/I) 12.4V / 17.5A = 0.7ohms.
So then for a 6.0A current passing, there would be a 4.2v drop across that 0.7ohm 'resistor' (Ohm's Law again: V=IR). Therefore the remainder (8.4v) of the total 12.4v is seen across the resistor, and at 6.0A that is 6.0A x 8.4v = 53.76w dissipated (wasted) by the resistor and only 25.2w used by the blower.
There are other ways of calculating but it's basically Ohm's Law.
I find it quite shocking how much energy I'm wasting by having the blower on all the time, and yet, in the winter especially, I can't do without it, so that's a constant 50w being totally wasted whether at night or in daylight.
Now I'm trying to maximise mpg and I've measured the difference in mpg (at 30mpg) at different levels of electrical load. At that speed I'm getting around 90mpg (Imperial) and the difference made by using an extra 1A of electrical power is just under 0.5mpg at 30mph. So that 50w, being equivalent to roughly 4A, is robbing me of just under 2mpg. Your mileage may vary, but the absolute amount of fuel wasted by having 4A of unnecessary electrical load is going to be about the same no matter what vehicle you have. (Well, an efficient diesel will waste about the same as mine and a less effiecient petrol engine will waste much more to produce those 4A.)
(I can't believe car manufacturers still use this old technology. Well, I guess it took them long enough to start using LED's. I well remember cursing them for using tiny incandescant light bulbs in the dash illumination when I had to take the whole dash apart to change a blown bulb (!) This was years ago but LED illumination was already well-established technology for low level lighting. LED's would have been so so easy to incorporate onto the instrument circuit board and would have lasted for the life of the vehicle.)
So, the short answer is, I just used standard Ohm's Law. But I would never have known how much was being wasted there if I hadn't measured the actual current being used on my car. I really recommend doing that. Borrow a DC clamp meter if you haven't got one. That makes it really easy, and you only have to do the measurement once. Once you know what each item of equipment is using you can refer to the list and you'll know how many amps you're using depending on what equipment you have on at any one time.
Oh, and by the way, I've been calculating and measuring based on 12.5v, because I'm planning to drive around without the alternator, but if your alternator is running and producing 14.5v then the power being wasted will be even higher.
And just for the record, I know I measured the effect on mpg of electrical load with the alternator running (of course!) so the voltage will have been 14.4v and not 12.4v. That will skew the figures a bit.
|