Re: 24 volt v 12 volt solar advice pls
JD, this is a shortened version of my previous reply but shows there is not much to be gained for using higher voltage panels with such short runs of cable such as in a caravan. Sorry about all the long digit maths but showing it just how it calculated out.
This is the scenario I have gone through with my solar panel investigation and what I have chosen etc. All calculations have been done at a copper wire temperature of 25 degrees C.
My calculations are based on the largest of my choice of panels (18.7 V at 10.7 A = 200.09 watts) at their so called maximum power output voltage at the maximum current and have measured the longest wire run between components. I know they will probably never in real life achieve what the sellers/manufacturers say and it will depend on the panel temperature shade etc but we have to start somewhere.
The wires in the van ceiling that run down from the solar panels are Tycab MWS26503 twin core tinned copper marine wire. Given this wire’s specifications & based on 57 strands of 0.32mm copper gives me a nominal copper area of 4.581888 square mm.
This wire with a length of 2.1 metres will supposedly carry up to 10.7 amps and see 18.7 volts at the solar panel end, but drop 0.0833696 volts (at 10.7 amps) and hence pass on 18.6166304 volts of pressure to the schottky diode. Note that I am not going to use the diodes in the panel as I have chosen the lowest forward voltage drop ones I can find and they are the STPS30L30CT which drop only 0.37 volts and can carry 2 x 15 amps (twin TO220 package) with a maximum Vrrm of 30 volts so ideal for my choice of panels/setup.
Now the chosen wire from the schottky diode connection block circuit board (which I am designing to handle two dual schottky diode packs and combine up to 4 solar panels & have 6MM cable connection bolts) is the Tycab AWC2168032 Twin 6 B&S which has the following specs, 168 stands of 0.33mm copper making it 13.504512 square mm.
The output of the Schottky will pass on 18.2466304 volts after its internal voltage drop of 0.37 volts and the 0.3 metres of cable itself to the regulator will drop 0.0040409 volts so the solar regulator will see 18.2425895 volts of pressure at 10.7 amps on its input.
My chosen solar regulators (thus far) are the Victron energy Blue Solar mppt 100/50 as it can handle more solar panels than I will have and limits the total PV input to 720 watts. There will be two of these solar regulators in my system, a near side and a far side each with two x 200 watt panels and 1 x 140watt panel (540 watts per side or 1.08Kwatt total). And if there is ever a need I can plug in some portable panels when camped in the shade and not have to worry about exceeding the regulators specifications. Maybe I can do some remote on-site welding with them.
Also it can look after the two banks of LiFePo batteries I am planning on using.
http://www.victronenergy.com.au/solar-charge-controllers/mppt10050/JD you will be limited to the physical size of your panels as there are roof hatches, antenna, range hood vents etc that you have to fit them around. Once you know the physical limits then you can start picking panels and see if they are going to fit with the required clearances etc.
Good luck.
Cheers,
ALLAN
