Help with power source

Here's my delimma. I'm designing a solar pump controller. Pump is controlled by the atmega monitoring environmental conditions. I want to run my entire circuit to run entirely off solar power - no backup power source... if possible.

Obviously the Arduino needs to have priority over the pump.. or else the atmel will crash and possibly not run correctly.

The pump document says it can start with "as little as 1watt" and (measured) consumes ~500ma at 13.8v.

The solar panel I have is rated 10w short circuit with 14.3v open circuit voltage.

Is it easier to try and make this work by controlling the pump via PWM output from the arduino and measure current/voltage available? or is there a different solution that would give the arduino priority over available power?

You may find that the best way is to charge a battery (during daylight) from the solar panel and then use that to run the Ardunio and/or pump. What was your intention for the system to do during the hours of darkness?

First let me say this is not easy.

measure current/voltage available?

You can't measure the current available you can only measure the voltage / current that is being delivered. If there are to be no batteries then as said above it won't work at night. Therefore you will have to have an arrangement where the power is directed towards the Arduino first. Then have the arduino measure the voltage as the motor is gradually turned on. If this voltage falls below a certain point then the Arduino should turn the motor off, or back off the power (speed) until the voltage recovers. The main problem with this is when there is not enough power for the Arduino. This is known as a "brown out" situation and could be a problem as the Arduino (and all other micro controllers) rely on the rapid turn on of a voltage to trigger the reset pin. You will have to use an external reset IC and set the threshold at about 4V. There is also the possibility that this might cause a condition known as latch up where the Arduino won't start until the power has been removed and re connected completely. It is something you are going to have to experiment with.

Will setting the BOD fuse to 4.7v help on the Arduino end? I did that already.

At night, I don't care if the Arduino shuts off. no solar energy to collect so no need to run or consume energy.

I'm supplying 5.1v from the solar panel via a lt1305 circuit design I had. It takes the panel voltage and bucks it down for direct feed to the micro.

It has a low battery Output pin that grounds when Vin drops below a certain level (Set by a resistor divider across Vin to the LBI pin). Should I tie this pin to the Reset on the atmega or possibly feed it to a reset circuit? Or should I use that in conjunction with a supervisor/reset IC such as max1232?

Should I tie this pin to the Reset on the atmega

Yes that sounds like just the sort of thing you need. There would be no need to have an external reset chip with this. So I think you are good to go. Remember if you are monitoring the +12V you can use two resistors to pot down the voltage before it is fed into the analogue input of the arduino.

That’s exactly what I was going to do.
I believe I tested a voltage divider network of 33k from +12 to the arduino with a 10k to ground and that seemed to scale down ~15v to <5v