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Fail on first attempt :-[ ---with questions

So I bought a 45W photovoltaic kit from Harbor Freight which comes with its own charge controller ($139 on sale), a used deep cycle battery with 135 amp-hours, and a 1.6 amp DC pump that moves over 300 gallons per hour. Yeah I know I should have done my calculations ahead of time but here they are, after the fact.

1.6 amp X 12V comes out to 20W per hour times 24 hours for (roughly) 480 watt-hours of consumption per day. At 45W the solar panels would need 11 hours of full illumination/full output per day to keep up with the pump's consumption, and I'm not getting that. So starting with a fully charged battery, it goes for about three days before the battery drops to the 11V cutoff where the charge controller kicks off flow to the pump.

Not a lethal problem, since I have back-up pumps running from the grid, but I'd need to add more solar panels to keep up with my pump's consumption. I can do that (if I can find space for them)--but here is a question that is more theoretical for those who know about electrical stuff.

My pump is rated to work from 5W-13W. If I connect the pump directly to the battery rather than to the charge controller with the 11V cutoff, the pump will continue to work as the battery discharges to a lower voltage, and at this lower voltage presumably the pump's draw will be lower as well. At some point, this lower rate of consumption will be equal to the feed coming off the photovoltaics, right? Would there be a problem with this? Is it hard on a 12V battery to keep it fluctuating at a lower voltage--say between 7-10V? Will operating at a lower voltage affect the pump's expected longevity?

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Replies to This Discussion

Permalink Reply by Jeffrey Ihara on April 29, 2013 at 2:02pm

Oops. I meant to post this in the Off-Grid group. Feel free to answer if you have one. This is part of an IBC based system, but I would have been more informative in the title.

Permalink Reply by Dan Ponton on April 30, 2013 at 8:54am

Really good question. Hear are my non-expert thoughts. Watts are made of volts x amps x phase. Volts are like water pressure while amps are like water volume. You need a minimum pressure at all times, that s why the cutoff is 11V. Also with reduced watts to the motor your output will reduce, and it is not linear so the output will reduce more the the % of watts reduced.

Permalink Reply by Jeffrey Ihara on April 30, 2013 at 11:34am

Thanks, Dan. I hadn't considered nonlinearity in the relationship between watts and work, presumably imposed by the mechanism of the pump. The discussion has been pretty active over on the off-grid group (I re-posted there after realizing my mistake). You might want to have a look at what's been said and offer your input.