Looking for a cheap, small, low voltage, battery operated, low volume, low power pump for water. It only has to lift 6" and deliver less than 2 to 4 oz/min. It will not be immersed. Water will be at about 40F.
Maybe a car windshield washer pump, $15 to $30 at the local car parts store. 12 V, probably 1 A or so - too much for a single cell, but probably OK with a small 12 V sealed lead-acid battery, or maybe a few cells (NiMH or some kind of lithium) with a switching regulator. These will be designed for intermittent duty - on the order of 30 seconds on, a few minutes off - not continuous.
A "universal fit" one may be easier for you to use - these tend to have small-diameter inlets and outlets (0.25"/6mm or so). Some are designed to mount directly to the side of the reservoir and have a huge inlet - like 0.75"/19mm or more - which may be harder to connect to.
Live well pumps (for a fishing boat) are designed for continuous duty, but do hundreds of gallons per hour and need a few amps at 12 V - probably not what you want.
If you just need one pump, look for a used or broken Keurig single-cup coffee maker that you can take apart. These have a couple of small DC- powered pumps that may fit your needs.
This pump runs on 12 V at about 0.25 A and might do what you want. I don't have one of these pumps; I just Googled it up.
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I found a small pump in the sprayer of a bottle of insecticide. It is powered by 6 Volts DC supplied by 4 Alkaline AA cells. The insecticide is "Ortho Home Defense", 1.33 Gallon/5 Liter size.
I would not use the pump for drinking water, but it is too good to throw away when the insecticide runs out.
"DC 3-12V Water Pumping Electric Micro Pump Motor RS-360SH ... Price: $5.52 & FREE Shipping on orders over $35. ... Sold by uxcell and Fulfilled by Amazon."
"You gave most of the requirements, but you might get more creative answers if you told what you are trying to do"
Hahah.. no secret. This will circulate ice water in a special, joke beer cooler. It would be nice to have it run on a couple of D cells and run for a couple days. I've seen air pumps in portable live tanks that run for days on D cells.
The flow I've seen in small water falls and things is more than is necessary.
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