Greetings Jeff,

For the past couple of years I’ve been scouring the local Craigslist for used PV panels in preparation for my ultimate plan to move off-grid. I have been very fortunate in that I have amassed approx. 1.3 (rated)Kw of 12v nominal panels for an average of just about $3/watt. I realize that when dealing with this much rated capability it is normally recommended to up-size to a 24 or 48v system, but to keep things as simple as possible (KISS) I’m planning on sticking with 12v.

The drawback to this is that they are a mish-mash of manufacturers, sizes, and specifications (which also makes it difficult to wire in series for higher voltages). My primary concern lies in the fact that the open circuit voltages range from around 17v up to over 21v. It is my understanding that (in layman’s terms) voltage is roughly equated to ‘line pressure’.

With this in mind I’m wondering if the panels with a higher voltage specification would generate a ‘back pressure’, as it were, preventing the panels with a lower voltage specification from adding to the overall current (kind of like a garden hose trying to augment the flow of a fire hose). I asked an engineer friend of mine who’s been living off grid for almost 20 years now, and even he was stumped.

Chuck Farley

Chuck:

You did not say how many modules you have, but with a total of 1300 watts and all are nominal 12 volt, I assume we are talking about 22 to 26 modules. With your desire to keep everything at a nominal 12 volts, this means you will have all 22 to 26 modules wired in parallel, and that is not good. There are many reasons why this is not practical, but the big one is the UL label on the back that says the size of a series fuse required. For example, a typical module in this size range will probably be listed as requiring a 15 amp “series” fuse. If the modules have an average output of 5 amps, this means if you have more than 3 modules in parallel, it is possible for one module to have an internal “short” and all the current from the other 3 modules will be routed through this failed module and could cause a fire.

To reduce the number of parallel modules, I would set this up for 24 or 48 volts nominal. This means the modules will be wired in “groups” of 2 or 4 in series. Even with identical modules with the same model number you can have as much as a 10% difference from one module to the next, so you want to “match” the strings. Lay them all out where they receive the same amount of sun at the same time, then go down the line on a clear day and mark on the back what voltage each is producing. Next, using a meter that can read DC amps, short the positive and negative leads through the amp meter and also note this reading. You need to be fast as any change in sky conditions will throw off your readings.

Now try and “match” high and low modules in the same string so each separate string has the same voltage and amperage. The problem is not in having slightly different voltage modules in series, the problem is avoiding having multiple strings in parallel that have far different voltages. This is the same problem as hooking up different batteries in series. You will always have one or two individual cells wired in series that are lower or higher than the others, and the battery will settle on a voltage that levels these highs and lows. But if you have batteries with different voltages wired in parallel, the higher voltage battery will try to “charge” the lower voltage battery.

You should use a good quality MPPT type charge controller which will try to sort out any remaining mis-match. If your modules are really mis-matched and it is hard to do the above, you may want to use multiple charge controllers so you will have fewer modules wired in parallel. These can still charge the same battery bank.

Good luck,

Jeff Yago

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