Asked By: Sher - 10/20/2012

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Hey Sher, the correct way to size a charge controller is to take the maximum amperage that your entire array will deliver, and multiply it by 1.25, this should be your charge controllers minumum amp rating. What you have given us for your question is the specs on the individual cells. I could assume the cells, all 25 of them are wired in series in each of your panels to give you just enough voltage to charge a 12 volt battery. At an average voltage per cell of .56v, each panel would have an open circuit voltage of 14vdc. That is just barely enough to do the job. If this is the case, then the 7.6 amp rating would be the same for each cell, and for the panel in full. Three panels, each delivering 7.6 amps would give you a maximum array current of 23 amps. A 30 amp charge controller would be correct for this, don't get anything smaller. If you think it is possible that you might add panels to your system one day, then consider getting a larger controller now, maybe 60 amps, so you can just plug new panels into the same controller later on. The good news is if you use a 60 amp controller and only end up with the 3 panels years from now, the controller will last longer since it is not operating near its design limits.

I'm curious, did you build these panels at home? Did you have instructions that specified 25 cells? That is an unusual number of cells, most 12 volt nominal panels use 36, giving them an 18 volts open circuit rating. This usually works better for losses that charge controllers and disconnects are notorious for creating. You might also consider skipping the charge controller, but you'll need either a dump load controller instead, which actually waits for battery voltage to get just above design charge voltage before it turns on a dump load to shed the excess charge current. The advantage there is that there are no losses in the charge controller since you don't have one. Or you could upgauge your battery bank to something like 1200 amp hours. It's a large battery bank, but at that point, the maximum charge current would be less than 2?f the battery banks amp hour capacity. In this case, a charge controller is not necessary, because the batteries would never overcharge at that rate. You can read more about this in Richard Perez's book, I'll list it below. Another suggestion I would make is getting a subscription to Home Power Magazine. It's the only periodical that gets into the nuts and bolts of this technology. We started reading it 13 years ago, now our home is completely powered by the wind and sun. They have great articles about people doing exactly what you are trying to do now. We have a system that has no charge controller right now, it's been working fine for 12 years. It has a 53 watt panel, 3 amps maximum, and 4 golf cart batteries, rated at 440 amp hours total, so we are well within the 2?imit.

Try getting more of your information from reputable sources, like the non profit ones listed below, the magazine, and books at the library, and don't put too much weight on information from h***s like me online. One thing I've learned about this over the years is this: There are two things in vast supply in solar technology, sunshine, and missinformation. It is amazing to me how many people are willing to weigh in with answers to your solar questions, who have never laid a hand on a panel or battery. Get informed, and take care, Rudydoo

Answered By: Rudydoo - 10/22/2012