Help with controller selection.

I am looking for some help finding a controller to do what I need it to in what I call a unique situation. I have a solar operated water pump that is in a well that I use to fill a trough for livestock. The pump runs great during the day on sunny days and slowly on cloudy days on these days it does not keep up.  So during the next day I run out of water and have a hard time catching back up.  I have installed 2 - 12 volt batteries wired in series that I can hook up and run the pump thru the night to help keep up with water.  I have been charging the batteries using a gas powered generator 12 volt system.  So I am looking for a controller that will feed my pump control during the day but yet charge the batteries too but at night or at times of low voltage draw from the batteries to feed the pump control.  The output from panel is 36 volt the pump control reduces that down to 30 volts max to protect the pump.  Maybe there is no such thing but any ideas would be apreciated. Thank you.


  • EstragonEstragon Registered Users Posts: 1,616 ✭✭✭✭
    If I'm understanding the issue correctly, the basic problem is not enough sun.

    You now have two types of storage; the livestock water trough, and the batteries. Of these, the trough will be the most efficient, taking less sun per gallon of water. As a result, adding the batteries actually makes the not enough sun problem worse.

    There are 24v controllers that can charge your batteries and supply extra current for pumping, but the power to both has to be available.

    Making a few assumptions / wild guesses:
    - 2x100 amp hour 12v batteries
    - 1x250w solar panel
    - pump averages 8a@24vdc for 5hrs = 40ah(960wh)/day.

    This time of year, the panel might put out 150w for 2-3 full sun equivalent hours per day, depending a lot on tilt, wire losses, climate, etc. That provides maybe 50% of the load to get/keep the troughs full.

    Adding batteries into the mix to supply the other half is 480wh/24v nominal or 20ah (~20% of our 100ah@24v bank), but charging batteries isn't 100% efficient. Depending on battery type, condition, and charging regime, we might need ~20% more to account for this, so ~600wh.

    So our power requirement is now 600 + 480 = 1080wh/day. One panel is giving us 300-450wh/day, so the bottom line is we need a couple more panels to be able to fill the trough and charge the batteries with the sun available at this time of year, or run the generator, or some combination of both.

    These are all made-up, but plausible numbers. You could get more accurate average solar data for your location at and use actual specs for you pump, batteries, and panel to get more than the wild guess I've made.

    This time of year at higher latitudes is tough for solar. As well as short days and low sun angle, many areas also tend to be cloudier around now. This means some combination of more panel and more generator run time. Often it comes down to enough solar to cover all but the worst 2-3 months of the year (now in the northern hemisphere), and planning to run the genny to make up the deficit in the worst months.

    In terms of storage, it will be more efficient, and maybe cheaper in the long run, to add water storage rather than battery storage. Depending on your pump capacity, with more panel you may be able to fill a bigger storage tank as well as the trough, so instead of running off batteries on cloudy days, you draw down the tank. May not be practical if freezing is a concern though.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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