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Old 06-22-2009, 10:29 PM   #1 (permalink)
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Tested: power draw of Geo Metro accessories & systems (volts, current/amps, watts)

Inspired by Tim's own investigation of his Paseo ... and his constant, incessant bugging me about how much power I thought it took from the battery and/or alternator to run my Pontiac Firefly (Geo Metro)...

Some good to know basic research. I was genuinely surprised how little power it takes to run the engine (no accessories on).

Voltage measured at the battery. Amps measured in a jury-rigged circuit with a multimeter inside the car. Volts x Amps = Watts.

To start the engine for the engine-on readings without blowing the 10A fuses in the multimeter, I roll started it.

Code:
Watts ... item 

1.7   key @ ACC, ECU awake
17.8   key @ RUN, fuel pump finished priming
54      parking lights, dash lights high
59      brake lights
112     headlights, dash lights high
120     headlights high beam, dash lights high
36      key @ RUN (fuel pump running - priming)
6       radio low vol
18      radio high vol
96      radiator fan high
30      heater fan low speed (1)
49      heater fan 2
66      heater fan 3
81      heater fan high (4)
58      rear defrost
5       interior light
36      wipers low
53      wipers high
66      cigarette lighter
78      hazard lights (est)
42      horn (est.)
39      reverse lights

60      engine idling @ 880 RPM (alternator disconnected)
62      engine running @ 2500 RPM (no alt)
92      engine idling @ 880 RPM, alternator on (field current is high, topping up battery)
89      engine running @ 2500 RPM, alternator on - lower field current as battery voltage came up

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Old 06-24-2009, 01:34 PM   #2 (permalink)
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At least that incessant bugging is good for something.
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Old 06-24-2009, 02:01 PM   #3 (permalink)
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well... you can add all that up, divide by the system voltage (14v? depends on alternator), and figure out the minimum Amp draw on your alternator... sometimes it pays (a lot in my case) to find a better higher-Amp alternator as it will put less drag on the engine.
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Old 06-24-2009, 02:34 PM   #4 (permalink)
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Wait... does this mean that like... 60 watts of electrical power are significant??

Surely we can jerry rig up some method of providing such a tiny amount of power so that I could completely remove my alternator and still be self sufficient without charging?

by my rough estimation, 200 watts of juice would be more then enough to keep you afloat under most circumstances...

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Old 06-24-2009, 02:45 PM   #5 (permalink)
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well... most batteries aren't deep-cycle, so they tend to not fully recharge if run down completely.

Watts = amps * volts
watts / volts = amps
Batteries have a rating in AH (amps * hours) but most of them time they aren't "defined" unless they are specialty batteries

200/12.7 = 15.75 amps
but as voltage decreases, so theoretically does the Amps consumed to produce the same amount of work.
if the battery voltage drops to:
12v = 16.667 Amps
11.5v = 17.4
11v = 18.2

as the voltage lowers, the life on the battery charge will get lower faster and faster as components attempt to continually draw the same number of Amps...
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Old 06-24-2009, 02:47 PM   #6 (permalink)
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so... 60 watts at idle, is that just the engine running (ECU, Ignition, Fuel pump, etc) and nothing else? 5 amps is surprisingly low
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Old 06-24-2009, 07:41 PM   #7 (permalink)
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Yes, 60 watts just to run the engine, with the alternator electrically disconnected.

I was also surprised it was that low.
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Old 06-24-2009, 08:52 PM   #8 (permalink)
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The Geo has a very small engine so its got less draw. Its also an eco car from the start(or ecobox if you prefer).

A car more mainstream(read wasted design parameters) will use more juice for every one of these functions. The engine draws are likely 2x as much for most cars(4-6 cylinders obviously being about 1.75 for the 4s and 2.2ish for the 6s).

The convenient when available answer is sunlight. Downside is panels are expensive. Its about 5-600 bucks for a panel that can outrun the alt. Also if its interior the glass dampens the draw of a panel and if its outside its likely damaging the airflow and its very likely to get stolen(solar panels for alot of applications are designed to break before the mounts, our local police lose 1-2 panels a year(50,000 population)).

I'm aiming for this idea atm because I am looking at going solar for my house(instead of inverters I'm in the process of buying DC motors for virtually everything) and the panels are the dimensions of my trunk lid. I'm hoping to just be able to lift and drop it into the trunk when I get out and simply have the alt belt on a clutch(no removal no unnecessary belts or pulleys spinning and access to the alt if it gets dark or cloudy).
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Old 06-24-2009, 08:59 PM   #9 (permalink)
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Any dash light high/dash lights low measurements?
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Old 06-24-2009, 09:50 PM   #10 (permalink)
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Quote:
Originally Posted by theunchosen View Post
simply have the alt belt on a clutch(no removal no unnecessary belts or pulleys spinning and access to the alt if it gets dark or cloudy).
I have actually thought about this... install an AC clutch onto the alternator... along with some kind of "tilt" switch like the old fashioned mercury switches. Have the clutch close on downhills and open on level & uphills, and an override switch to make it charge full time.

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