How do I figure out the amps/hour usage of a DC air pump?

[Ben Brewcat]

I have this Frabill aerator for my minnow bucket, and while you can put a couple D-cells in there, I often camp for a few days at a time. I'd like to run the thing off my camper's battery, but want to get a ballpark of how much current it draws. It has a 3 volt DC plug adapter for the 12V cigarette-style plug. Any guesses?

[Ben Brewcat]

OK, now I'm really confused . I remembered I have a voltmeter (for the same battery) but I don't really know how to use it. The directions helpfully say "set the rotary switch as desired DCA position" but the face has NO DC amps written on it. I get a reading of 26.9 in what I think is the AC voltage part of the dial, and nothing in the section I think is DC (on the right, 20 milliamp through 10amp, 2 o;clock to 5 o'clock).

OK, can't seem to upload images. Grr. Anyway shouldn't I be able to measure the draw by putting the leads on the alligator clips and using the DC current setting on the voltmeter?

[allniter]

BEN-------watts div by volts = amps
volts x amps = watts

[Ryan McEachern]

Looking on-line, it says the model 1420 portable aerator will run 30 hours on 2 "D" batteries, not sure if this is your model or not.

each D battery has a capacity of 12000mAh, so if your aerator has 2 batteries, then its 24000mAh. Divide by their stated run time of 30, = 800mA draw. or .8 amps. Thats at 3 volts, so the unit draws somewhere in the area of 2.4 watts.

If you run a 12v to 3v adapter, then you will cut the amperage in 1/4 (but not really because of the ineffeciencies of the transformer). The pump will still be drawing the 2.4watts, but at 12 volts, the amperage will only be .2 amps.

Transformers are not 100% effecient, some power gets lost as heat. Assuming 66% effeciency would give you an amp draw of .3 per hour out of your 12 volt camper batteries.

If your battery is a 120 Ah reserve capacity, and you only want to draw it down 75% before charging it up (that is prudent) then you should be able to run your aerator for 300 hours. (12 days straight) with no problems.

If you have a smaller battery, or it isn't brand-new, then you will want to down-grade this estimate.

[Rader Sidetrack]

If you expect to measure current, the current must pass through the meter. That means the meter must be in series with the aerator. If measuring voltage, the meter is in parallel with the battery or aerator.

So to measure current, one lead of the meter goes to the battery plus, the other meter lead goes to the aerator wire that would normally connect directly to the battery plus post.

Most cheap meters that I am familiar with have a separate socket on the meter to measure amps, so you need to check that. Can't help with the settings/display without a picture.

[MichaelShantz]

BEN-------watts div by volts = amps
volts x amps = amps

typo: volts x amps = watts of power
watts x hours = watthours of energy

[Ben Brewcat]

Great information y'all, thanks! On the formulas, I'm no algebra whiz but since I don't know watts or amps I don't think I can solve the equation with only one variable (lacking Ryan's wizard formula ). Exactly what I was needing. I do run some other doodads on these batteries, which is why I wanted to capture the amp-hours of the pump for sizing my battery bank.

[David LaFerney]

The non technical method - Try it at home first