I have done the deep cycle battery and charge controller route, with a LCB thrown in too, but have never used a programmable motor controller, which would protect the battery from harm when the pressure switch sticks.
I agree that there is no excuse for these components to be as expensive or as short-lived as they are. Yet I have had each component in my systems fail. The batteries seem to be the most reliable.
The problems may simply be due to the "farmer methods" of installation. My first installations were somewhat open to the elements, but I have since moved any electronics into plastic enclosures (ie. dollar store plastic containers). My experience with failures seems to be consistent with others using the same components, but I'm not sure if their methods of installation are similar.
In any case, there is an opportunity here. In my case, I'd be happy to get a reliable system that gives me some stats, but there could be a larger market that just myself. As an opens source nerd, I see this fitting in to the FarmHack community.
Thanks to all for the guidance. I'm much more psyched than a couple of days ago. Hopefully, I can use our weekend, which happens to be a long one, to get something started.
jremington: The term "linear current booster" was evidently recently invented by the solar power industry and is a misnomer. These are ordinary switching DC-DC converters that increase the current at the expense of voltage.
There is absolutely no excuse for such things to be short-lived or even expensive, as switching power supplies are now in just about every piece of modern electronics. Very few fail after "one season" of use.
A capacitor is unlikely to be useful in this situation, as it would have to be enormous* to power a 9 ampere, 12V motor for even a few milliseconds. Don't forget that DC motors briefly draw the stall current each time they start up.
I would go with a deep cycle battery, a charge controller and a programmable motor controller. All of these components are standard and you can expect many years of service.
*Suppose the capacitor is to power the pump for 0.1 seconds and we allow the voltage across capacitor to drop from 12 to 6 V during that time. The required capacitance can be estimated from the fundamental relationship Q = CV or I = C dV/dt. Plugging in I = 9 amps and dV/dt = 6V/.1s, C= 0.15 Farads!