battery and alternator upgrade for lights? -
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post #1 of 3 Old 03-24-2009, 08:34 PM Thread Starter
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1997 TJ Wrangler 
Join Date: Dec 2008
Location: Cozad, NE
Posts: 63
battery and alternator upgrade for lights?

i was wanting to know how many lights i could put on before i had to upgrade something, and what i would have to upgrade. i was thinking about running a pair of hella ff1000 driving lights and hella 500 fog lights

haymaker4life is offline  
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post #2 of 3 Old 03-24-2009, 09:07 PM
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2014 JK Wrangler 
Join Date: Apr 2008
Location: Tacoma, WA
Posts: 99
How technical do you want your answer? I'm an electrical engineer, so I'll try and keep the techno babble to a minimum.

We'll assume 14v, one, for easy math, two, because 12v would imply your motor's not running. My Yj had the same alternator as your stock one, a 90 amp. Considering that MOST Jeeps have little in the way of power goodies - power windows, rear window defogger, electric radiator fans, etc, 90 amps can go a long way for accessories.

At 14v, a 100 watt light will draw ~7.1 amps.

You didn't mention the wattage, but you'll need to run each pair on a 30amp relay of it's own. All 4 on one relay would be pushing it.

Also, keep in mind that 90 amps is the max the alternator will put out at HIGH rpm. At idle, most alternators put out about 40% of their rated current, so a ballpark would be 36 amps.

If you idle a lot, and have the heater cranked full blast, headlights on high, with your fog lights on, your voltmeter will drop to 12v, and your alternator isn't going to keep up. So if you do a lot of idling or low RPM driving, consider a 160amp or larger alternator. If you keep your RPMs over 1200 most of the time, 4 lights should be fine on the electrical system completely stock.

I'm running dual Optima's and a 160amp alternator to power a 2,000 watt inverter so I can run a Lincoln Mig welder on the trail.
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post #3 of 3 Old 03-24-2009, 09:20 PM Thread Starter
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1997 TJ Wrangler 
Join Date: Dec 2008
Location: Cozad, NE
Posts: 63
i believe they are 55w and thank you for the great answer
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