number of cells in a solar panel for 12 volt battery bank

[wiboater]

 I have a piece of tempered glass that is 71.5 inches by 31 inches. I can make two 36 cell panels under the glass or since I have enough room to add 6 more of the 5 x5 cells I could have 78 cells total. two 39 cell panels or one 78 cell panel. Can I do that or is haveing 78 cells all wired together in series to much voltage?  Does it have to be an even number of cells like 36 or is 39 ok? I have enough glass to do one more panel this size also. I tried this on another topic about why 36 cells but it timed out.

[wiboater]

I've done a little more reading since I asked this question and it looks like the answer is no on the 78 cells in series on a 12 volt system. I'm still not sure about having two 40 cell sections for the 12 volt though. In other words each one at 20 volts. Also I'm not sure how that would match up with the one 18 volt 36 cell panel I already built. If there is any doubt I'll probably just stick with the two 36 cell sections made like the first single one.

[Tritium]

An easy way to answer your question is look at commercial panels and see how they are built and wired. Go to some manufacturer sites and study their panel specs. Also look at sites selling many different panels such as www.sunelec.com or Northern Arizona Wind and Sun or Alt-E sites (google their names for the URLS).

Short answer: most commercial panels built for 12V battery charging use 36 cells in a series and then parallel series strings of 36 cells for more amperage output.

Thurmond

 

[wooferhound]

As I understand it ,  a panel made to charge a 12 volt battery to 14.4 volts will read 23 to 25 volts open circuit, I don't think a little more would be a bad thing.

[Rover]

You state that you already have a glass panel, without stating  specifics on the transmission efficiency which will effect the total efficiency.
Knowing that may also help in determining how many cells/per panel.

If you have good transmission efficiency, depending on the type of controller (pwm vs mppt), too many cells could just be wasteful, with a pwm just throwing away the excess power.

[wiboater]

The glass is from a storm door, it is clear glass but does have a greenish tint just like the tempered clear glass I bought for the first panel I built. Because of the glass size I didn't want to have that 5 inch blank space at the top and bottom. That's why I was asking about adding the extra cells. I have been looking at panels on the Web. It's hard to see details though.  I've decided to play it safe and just go with the 36 cell two panel configuration. They gave me 20 extra cells when I ordered the 200 and I broke  a few and  still have to test the rest. So I better play it safe in case I have some bad ones. Thanks for the replies.

[tecker]

 19 to 21 volts for a 130 watt panel is the norm . These panels use EVA so the water clear glass is a must .I think leaded glass is alright in that reguard .