what solar voltage is best?

[zephar123]

IM working on building some solar panels. The standard is 36 cells form what i have read, but that could produce as high of volts as 19.25!!! is that to much voltage for a c40 charge controler. I want it to be a 12v system. WHat voltage would you recommmend to have as max for a solar panel in that sort of system?

Paul T. Silva

[picmacmillan]

hi paul...i do not claim to be a specialist in this area, but i can tell you what i did...i built 7 panels from the ebay cells everyone bought...i used 28 cells per panel...each panel produces about 15-16 volts , and each one gives out about 3 amps...i put a couple panels in series, therefore my charging voltage is met, and the rest in parallel to bring up my amps...some say this won't work, but again, it is working so far...try going to ifreds website http://www.internetfred.com and he has a very in depth page under solar for what you are looking for..take care..pickster

[sandovalch]

Zephar:

I use 36 cells for my panels, I like to compensate for cloudy days or just to start my charging process earlier in the morning and later in the evening.

From my experience I only get 16.5V before the charge controller for 36 cells, this is because of heat and wire losses. My recommendation would be to be around 17.5V to 19.5V open circuit voltage.

Picmacmillan mentions that the series/parallel does not work for some people, I think it works fine if your panels have less cells in series, but If you wire enough cells in series from the beginning, your always meet your charging voltage.

[zephar123]

ty for your reply sir ill take a look at that site. my biggest question is incase im not clear is really. what volatage should i setup the solar panels on for a 12v system to work with a c40. 13? 14? 15? 16? 19? volts?. Once again ty for your quick reply and im goign to to take a look

Paul

[Hank]

Zephar,

A C40 (Xantrax or Trace) will take up to 120 open volts DC from a PV source. So no worries there. 19 -20 open volts is not a problem charging a 12 volt battery. The battery will drive the voltage down to whatever it is at.

Have fun,

Hank

[zephar123]

thankyou all for your answers I understand now =). I will bge postign up the results with pictures when I get done, now jsut to figure what materials ill be protecting the cells with =)

[old55olds]

Do you think that there is a reason why all the major manufacturers use 36 cells?  At least my kyocera and sharp 120 watt ones do.. Evergreen uses either 36 or 72.  Why not go to one of their web sites and read their instructions. I think that you should use all in parallel. Think of it as a series parallel circuit. You are moving the electrons from one plate of the battery to the other. All current must flow through the panels. If you were to try to force it all through the one panel with another 7 in series with it, the one single panel would have to pass 7 times the rest. I would think that it may get warm.

As I understand it.

Ken

[old55olds]

That should have read with one in series connected to 7 in parallel.

I guess I should proof read it before posting.

Ken

[Vernon]

To get and maintain a full charge on cycling 12V batteries 14.4 volts is needed. A blocking diode is also needed to prevent discharge of the batteries through the panel at night and that drops 0.7 volts under load; add the effects of high temperatures in the Southwest where solar power is most common and it works out that you need the 21.6V or so of open circuit voltage (that is 36 cells at 25C) to maintain rated current and 14.4V considering the blocking diode and high temperature.

I am going to build a solar powered canoe using a 45Watt solar panel, a 55AH 12V deep cycle battery and a 10# thrust 7AMP trolling motor. I will build a power darlington based shunt regulator that will be capable of pulling up to 5 Amps to keep the voltage at 14.4 as charging current decreases on recharge. I am not going to use a blocking diode as it consumes more power than electronics and a relay. The control will drop out the relay at 12.9V decreasing if solar panel output is less than .2 Amps. It will pick up in the AM when the same voltage is attained.

[Svend]

Hi Paul,

A commercial PV module has a drop in voltage output of about 0.4% for ever 1C above 25C (rated temp). In the sun, a module could be 30c above ambient (now 55c). If you live in a warm climate & depending on your mounting system (roof mounts run the hottest & have the poorest air circulation)the output of the module has to be sufficient (after system losses) to hit 15.5v if you want to equalize the batteries. Hope this helps, Cheers Svend

[zephar123]

just wanted to thank everyone again for there help on this question. I put together a 36 cell unit and it seems great i just need to figure out whats up with my c40 charghe control now.