Hi all, I have a solar hot water differential controller I built from an EtherMega (See simplified drawing) that has been running successfully for the last 3 years (Much to my surprise) but now wish to add freeze protection by adding another SSR (SSR2)to switch on the pump at night should the temperature get to near freezing, this source would take power from the same solar charged battery that runs the Arduino board as opposed to running off Solar panel 1 (SolarP1) as it usually does during the day . Theoretically both SSR1 and SSR2 would never be on at the same time but to make sure there is no chance of current flow from Solar panel 1 (solarP1) back to the battery bank would adding blocking diodes D4 and D5 do this, if so have I got them in the right orientation and place and what sort of diode would be appropriate, thanks in advance for any input. 
I have been looking at it for a while, but I think it is not right.
In your circuit, SSR2 switches the low side (the ground) of the pump, but SSR1 also switches the motor. So you have two relays that do the same. SSR1 does not use a ground at the output, I don't know if a SSR can handle that.
What SSR's are you using ?
Would it be possible to simplify it ?
Why would you want to use the solar panels in a different way ?
Are they the same ? Can you use them both at the input of the charge controller ?
Using directly solar power or battery power (charged by the solar panel) makes no difference during the day.
So I would use the battery power and a single relay to switch the pump on and off.
Hi thanks for the reply, SSR1 is a SY-4093 http://www.jaycar.com.au/productView.asp?ID=SY4093 and has been working fine for 3 odd years in the circuit switching power from SolarP1 (40Watt) (Yep overkill I know) to the 15 W pump http://www.ebay.com.au/itm/Brushless-Water-Circulation-Pump-5W-suit-Solar-Hot-Water-/250899162946?pt=LH_DefaultDomain_15&hash=item3a6ac16742 I am not sure what you mean by "SSR1 does not use a ground at the output" SSR1 is switched on from arduino via ground and digitalpin and closes circuit from SolarP1 to pump (Open to correction though) . SolarP2 is (120 Watt) charging a 30aH battery bank supplying the arduino and other systems. Yes both SSR's would turn on the pump but SSR1 turns on power from SolarP1 whereas the proposed SSR2 would supply power from the battery bank. I just want to avoid feeding current from SolarP1 directly to the battery bank via SSR2 if they are by chance on at the same time. The reason I would like to keep the pump running on its own panel is that it self regulates the flow rate depending on the amount of sun radiance which balance's the heat transfer nicely without too much on/off cycling, granted using only the battery bank and one SSR would simplify the system, but hey where's the fun then lol system also has 8 x onewire temperature sensors, 3 solar water panels, multiple heat exchanges in a preheater, RTC, logs to SD card and serves web page to local network also has data dump and bypass to pump and still have to add a heat dump to avoid summer overheating problem (The worlds most complicated solar hot water system lol)
Thanks again for you input
I was wondering how the SSRs could be used. I think they have a opto-mosfet inside. So one of the outputs does not have to be connected to ground, but your SSRs do have a '+' side and a '-' side.
I didn't know you want to balance the pump power with the solar panel. That is a very simple way to control it, but now your circuit is not so simple anymore. Making a simple circuit requires that the Arduino controls the pump. I don't know if you are willing to do that, a wrong sketch could make the pump go nuts.
The circuit with the diodes 'might' work, but it's a little freaky.
It is possible to make it less freaky and still use SolarP1 to balance the pump power.
You use the SSRs for low-side switching (a switch or SSR in the ground wire), I would like to change that to high-side switching.
That way a common ground can be used, which makes the circuit easier to understand.
