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