Wiring solar panels in parallel?

[ZuidWester]

I have some solar panels, which produce 23 volts each. Is it possible to wire two or more in parallel rather than in series (two of which produces 46 volts - more than I need).
I would like to use them to charge a 24 volt battery set up (two 12v 120 ampere batteries in series). Is 23 volts enough? Questions, questions...

 

[steve_cronin]

This 23V is I assume the reading across the output of a panel with no battery connected? This is a little high but 19-21 volts is quite normal as an open circuit voltage.

You will need at least 16.5 volts to charge a 12 volt battery fully and around 33volts to charge a 24volt battery. I would suggest, in your case having a 24volt system supplied by two 12 volt batteries in series, that you use one panel to charge each 12v battery. The two panels in parallel will have little or no effect on a 24volt battery.

There is no problem using the two panels in parallel to charge a 12volt system. In fact my boat has two 55watt panels in parallel and these now adequately supply the fridge over a 24hr period in the Med. "Now" that is, since I started force venting the compressor/heat exchanger with cool air from the bilges instead of from the hot saloon as per the standard set-up. Not as efficient as water cooling but the next best thing.

Steve Cronin

 

[steve_cronin]

More...

Pardon me for mentioning it but...

I was told by a county council Traffic Technician that the panels used in battery powered roadside displays are of a high voltage because they are used to charge an 18volt battery. This battery voltage was chosen to deter boat and RV users "re-cycling" valuable roadside, er... " finds"

Steve Cronin

 

[Joe_Cole]

Steve has answered the question but, just to clarify things.......

If you connect the solar panels in parallel you will still get the same voltage as a single panel (but you will double the current)

You will have to connect them in series to double the voltage.

 

[ZuidWester]

Thanks for the reply

I got the panels from a local factory which produces PV facades (Saint Gobain). They are seconds due to visual impairments. They were great value at about 60 quid each (I estimate they must put out 60 watts peak), so I bought a number of them to sell on.

Not sure why, but the man at the factory warned against wiring them in parallel. I guess I should ask him why not!

 

[Anonymous]

Re: Thanks for the reply

[ QUOTE ]
Not sure why, but the man at the factory warned against wiring them in parallel. I guess I should ask him why not!

[/ QUOTE ]The probable reason is that if one panel is in sun and the other is in shade, then the shady panel becomes reverse-biased. I am not sure whether this can damage the panel but it will probably start to conduct, 'stealing' the power from the sunny panel. The solution is to put a diode in series with each panel to prevent a reverse current. Ideally you would use a Schottky diode which has a very low volts drop when conducting, though if the open circuit voltage of your panels is high (19V or more) and the wiring run is short and in good low resistance cable, then an ordinary silicon diode should suffice. Schottky diodes are quite expensive - get them from Maplin or RS Components. By the way, RS do supply non-trade these days and have on line shopping with full data available online.