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Old 09-29-2008, 04:42 PM   #20 (permalink)
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Join Date: Aug 2008
Location: The Wet Coast, Kanuckistan.
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The Golden Egg - '93 Toyota Previa DX
90 day: 31.91 mpg (US)

Chewie - '03 Toyota Prius
90 day: 57 mpg (US)

The Spaceship - '00 Honda Insight
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If you have an ammeter in your car you can estimate your battery life by the capacity rating. Example is a Trojan 12v 27tmx with a 175 minute capacity. This battery can provide 25 amps of power for 175 minutes in ideal conditions and down to 100% discharge. You should really only discharge to 50% (12.06V) max for deep cycle batteries. So half that figure is about an hour and a half.

Watts / volts = amps. Amps x volts = watts.

Low beams are typically 55 W each X2 = 105 / 12 = 8.75 amps. The heater fan depends on the setting and the vehicle. Many lights can be replaced with
LEDs which only use a fraction of the power.

From another forum...Don't know the car model but would be good enough to estimate.

I tested at both voltage settings of the alt, where 14.4 is typical alt voltage, and 12.7 is similar to a fully charged battery. The idle current is subtracted out of the other settings.

Idle: 128w @ 14.4v, 89w @ 12.7v
Rev up to 3500 rpm: +35w, (not tested)
Heater fan setting 1: 55w, 38w
Setting 2: 98, 67
Setting 3: 141, 99
Setting 4: 189, 135
Marker lights: 95, 71
Full lights: 240, 179
A/C (heater fan subtracted, but including condenser fan): 163, 114
Defroster: 104, 77
Wipers 1: 48, 38
Wipers 2: 76, 64
Radiator fan: 144, 83
The more current you draw the less capacity your battery can provide.

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Last edited by orange4boy; 09-29-2008 at 04:48 PM.. Reason: clarity = added formula
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