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Old 06-14-2009, 09:50 PM   #5 (permalink)
Left Lane Ecodriver
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Join Date: Aug 2008
Location: Buffalo, NY, USA
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Prius C - '12 Toyota Prius C
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I borrowed an inductive ammeter and took some readings. The car was warmed up so the O2 sensor heaters wouldn't have to run. I recorded the following:

Engine off, 12.6V:
0.3A - key off
0.5A - Acc
4.7A - key On
10A - key on, corner lights on, engine off
19.3A - headlights on

Lights out, car stationary, 13.87V:
17.4A engine on, 4000 RPM
16.0A engine on,  700 RPM
The radiator fans, blower motor, radio, wipers, etc were off during the test.

So I would say 20A for a car going down the road, +5A for corner lights (which I recommend despite the MPG hit), +15A for headlights.

Btw, I had to move the ammeter from the battery to the alternator to get any meaningful readings with the engine running.

An alternator + belt is 50% efficient, a car converts gas to torque with 30% efficiency, and gasoline has 121MJ/gal. So an alternator making 20A at 13.8V = .99MJ/hr is costing you 0.05gal/hr. At 45mph (trip average), an alternator delete would turn your 40mpg into 41.9mpg. That's worth doing if you can live with the drawbacks (limited range, have to plug in the car at night, cost of deep-cycle batteries). But don't forget to sign your house up for renewable electricity first, or all you're doing is replacing a 15% efficient source of electricty with a 30% efficient one.

Also, the energy savings are smaller if your car already has a high-efficiency alternator - 70-80% efficiency units are available.
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