c60 for 24V parallel array

dellsworth wrote: »

On the 12 volt you're saying I'd be throwing away a fair amount because of the difference between what the c60 puts out to the bank.compared to what it is taking in. Or to put it another way... Even though it can take in 55V it really only processes 17.5-18.6? I saw it taking in up to 55V (31 according to ERDM specs) and didn't think about where the excess voltage was going. I can understand that.

• 18 volts * 8 amps = 144 watts out of panel into "ideal load"
• 14.5 volts * 8 amps = 116 watts into "ideal battery"

• 30 volts * 8 amps = 240 volt STC rated panel
• 14.5 volts * 8 amps = 116 watts into battery

The 24V is a bit harder to get. The panel is rated at 31 imp. If I pair a couple of 145's (17.86 imp) I get it that they're going to be 35+ compared to 31. Does a hot day derate down to 24V? What's the derating on a pair of 145's?

• 38 volts Vmp * (0.004 vmp per C)(40C+30C-25C) = 6.84 volt drop
• 38 Vmp - 6.84 volt drop = 31.16 volt Vmp-array-40C (hot array)

This brings up a very interesting question for me. I've a customer that's been having problems getting their new batteries to charge. The panels are some cheap 80's they got years ago and it's an undersized array for their consumption. They've had problems for years during the rainy season and were simply too cheap to upgrade the array and ran a generator instead. But it seems to have gotten worse and I even recently put in a smaller bank for them since the previous installer had a bank way too big for the array size. I just thought it was mostly rainy season problems with insufficient consecutive hours of sun. Would the imp have dropped on those panels through aging enough so that they might not have the force to charge up the batteries? It's also a 24V bank. This is a pretty relevant question for all of us. The boom in the industry has been relatively recent and off-grid systems panels are starting to age. Are we going to see more problems with charging banks that previously were sized okay?

If so I suppose we need to start re-configuring the arrays and putting in mppt's to extend their life.

dellsworth wrote: »

Hi Bill,

I get all of the above and I really appreciate your responses. It's like getting a short course in off-grid designing. Did you happen to mean VOC instead of Vmp when referring to 38 volts? You were right that I'd meant Vmp but I'd think you'd use the 31 Vmp instead of 38 Voc in your calculations. That would put it at your original 24V figure which would be safely conservative for design purposes.

That's an excellent point about measuring with the clamp-on between strings of batteries. I've two good (expensive) clamp-ons and do measure output between panel strings frequently (and lots of other things in the old house wiring down here. My Comitán house was built 150 years ago and the wiring is inventive, scary, and fascinating.) But I've not measured between battery strings during load or charging. Just the overall in and out. I'll have to start doing that.

I've replaced plenty of battery banks. I usually size them around the 5% rate of charge figure for the array. What has worked very well is to yank out the poorly performing batteries as they near the end of their lifespan. The rate of charge goes up and (if you designed well originally) you've still plenty for the house consumption - just less leeway. With the brand of cheaper but solid batteries I generally use down here it works out to about 4 years (it's the one component you can get in Mexico that has better value than in the states - Mexican batteries). Isolating the weakest ones at that point and re-configuring the bank usually results in another 6-8 months of life for only about 30 minutes of work. I'd call that a nice percentage of extended life on 4 years. I've not found that setting up a smaller (but still adequate) bank results in any life span differences. Of course putting in an inadequate bank blows them pretty quickly. Fortunately I've only seen those on other guys "designs". As you've pointed out there are differences even in the same components. Banks show they are aging and nearing the end of their lifespan not only because they don't hold the charge as long as they used to but also they charge up very quickly. Under conditions that used to take say 3 hours to fully charge; a used up bank can charge up in an hour. At that point I usually start looking for bad batteries in the bank by simply looking at resting voltage when the system is shut down a few hours (not BAD but more significantly used up). But I like the idea of checking between strings. It'll probably give better indications earlier and also helps isolate problems with bad connections. Also as you pointed out rarely do we swap out a bank because they are used up. But you can extend the life on the bank by refinement.

Interestingly enough down here there are a lot of small mom and pop places that will "rebuild" the batteries. Usually they're just for car batteries but I've checked for doing deep cycle ones. Getting the plates on those are really the issue and they usually DO try to swap them out on the car batteries. I've yet to do it but have considered simply cleaning and swapping out fluid. Usually the plates are more or less acceptable when we swap out the banks. As you pointed out they are not BAD. I'm not convinced it's totally worth it but for some client with no initial capital to swap out an entire bank maybe I'd help.

I consider my work down here almost voluntary considering some of the clients I work with! I've taken up the philosophy of rob from the rich and give to the poor from the point of few that I work less using better, costlier, and prefabricated components for rich folks and use some of their swap outs and do more inventive rebuilding and homemade stuff for the poor ones.

It's good to know that panels stay close to specs. Actually I'd forgotten I'd checked on the previously mentioned array of 80's - probably because I hadn't done it directly. The client had called me. Since I was about 13 hours away I had a friend take over to him one of my clamp-ons. I talked him through measuring the array current when it was about middle of the day but the batteries still hadn't gotten to bulk charge (I'd already changed out the bank). Considering the bit of loss for various things the array output was almost right on spec. So you're absolutely right in that regard. In this case I suspect it's excess consumption the last couple of years with a new renter in the house. He can't be bothered to change his habits like the original owner did to compensate. Also they use a pressure tank for water and the tank diaphragm (or bladder) is shot (and has been for 2 years) - resulting in more frequent pump start-ups. They already contracted with me to change out the tank when I get back up there. With more huge start-up draws on the pump I'm sure it's the main culprit.

I'm a new convert to MPPT's. I've installed just 4 in the last couple of years and the last one was to my own system. I had some left over panels that I couldn't sell (having bought them at higher prices). So I reconfigured and remounted everything I had and it works wonderfully. I was pleasantly shocked at how much additional output I gained simply adding the MPPT. When I did the conversion I first reconfigured my original 12 panels from 6 pairs for my 24V system to 2 6's and put the mppt in place. I got about 15-18% extra current charging my batteries. Then I put in the other 6 that were new and I feel like I could sell electricity to my neighbors (if I had any)! The panels aren't all the same but they are close (120 and 115). I, of course, put like panels in series so the differences were isolated between parallel strings.