I am baselining our XTerra's power draw. I plan on adding lights, maybe a few other toys and I need to determine the truck's DC reserve prior to adding the toys. I know that the alternator provides 90 amps. I also know that the battery will provide higher peaks for short periods of time.

I performed some simple DC current draw measurements today. I used a Fluke 36 clamp meter. I clamped around the battery negative lead. The measurements were:
Key on / everything off - 7
Key + AC only on high - 18.4 (incremental 11.4)
Key + lights only low - 23 (incremental 16)
Key + lights only high - 20 (incremental 13)
Key + radio only - 8.4 ( incremental 1.4)
Key + front wipers only - 11.5 (incremental 4.5)
Key + AC high + light low - 33.8

That being said, if I turn on the AC, low lights, wipers, and radio the truck should draw about 47.3 amps. Normally, this would probably be the max most trucks would ever require. From this I infer that there should be a reserve of about 42.7 amps. Just throwing out 5 for error, poor performing alternator, battery charging, etc, lets round to 30 amps reserve. This also assumes the alternator is turning at the optimal RPM.

Questions:
1) Is my train of thought rational?

2) If so, is my estimated reserve of 30 amps valid?

3) If not, what other asumptions need to be considered?

4) Does the detailed service manual provide this sort of current/power details?

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it's my wife's xterra; sometimes i get to drive...
accasbel@home.com

The readings don't make sense to me. For example, the reading for key on + high beams should be higher than key on + low beams. Isn't the Fluke 36 an AC clamp meter? I think you'll need one of the fancy and expensive hall-effect types to do a DC measurement.

One of the biggest and most overlooked power pigs is the rear window defogger.

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No, if i remember correctly 90AMPS are provided by alternator only on very high revs.

and we dont usualy stick around in high RPM range, so you dont have that much reserve power to play with


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Generators used to be speed sensitive, but alternators are much less so... I don't remember the specifics, it's been too long.

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01 SE 6cyl 5spd Solar Yellow

- I agree with the hi vrs low beams, but that is what I measured. Even repeated the measurement and double checked the beam setting.

- The Fluke 36 will read either AC, or DC.

- I did overlook the rear window defogger. In retrospect, that's probably OK. We live in Phoenix and only see window fogging when we travel out of town.

One thing I was not able to measure was the on-rush at device turn-on. Most electrical motors require more current to get started (overcome interia, etc) than what is required to maintain. The Fluke doesn't have a "peak" hold to capture that data.

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it's my wife's xterra; sometimes i get to drive...
accasbel@home.com

Also keep in mind the Fluke's accuracy. From their web site the Fluke Clamp 36 on DC amps:

0-200A: ±(1.9%+7)

That's 7-11 amps potential error at the values you are looking at.

An in-line meter on alternator wire would allow you to start the engine and get more accurate numbers, for what they are worth.

If you can temporarily fake the loads you are planning on adding, monitor system voltage while you drive - it will drop off 13.8-ish when the alternator is unable to keep up.

BTW, on-rush current has little or nothing to do with any of this.

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[This message has been edited by *paul (edited 06-05-2001).]

I'm running three 100-watt lights on top and two 35-watt lights on the bumper (Total ~ 30 amps at 12 volts DC). I ran all these offroad lights, fan on full, CD player, and standard lights w/fogs. Voltage (at the battery posts) was steady at a constant 13.9 volts for a three-hour period.

If you keep the external lights to below 30 amps (360 total watts) it won't be a problem.

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Snoopy , our '00 Cloud White SE, 4x4, 5SP

Between my guestimate for reserve, and that Snoopy is running 370 watts worth of light, my projected 365 should be just fine. (until Shanna wants a new stereo, electric seats, or .....)

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it's my wife's xterra; sometimes i get to drive...
accasbel@home.com

What about the winch? and the rear spots? and the ........

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"It takes blood and guts to be this cool..."

Yellowjacket
www.azxterraclub.com

I blew my wad with 300w worth of lighting in just two lights. Heh. Gotta love these 150w KC Hilites.

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01 SE 6cyl 5spd Solar Yellow