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davestreck
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# Posted: 15 Jan 2013 15:24 - Edited by: davestreck
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I am in the process of putting together a simple off-grid solar system for my cabin in Nova Scotia. It will be a small 12 volt setup using a single 100w panel, a simple charge controller and 3 105AH 12v AGM batteries. I have the panel and the batteries, and just need to settle on a charge controller. The problem is this:
The batteries clearly call for a float charge voltage of 13.2 - 13.4 volts, but all the relatively inexpensive charge controllers I have researched (Morningstar, Sunforce, Trace etc.) maintain a float charge at 13.7 volts or even higher, even when specifically set for AGM batteries. I have read that this type of battery should never be charged at higher than its recommended voltage because it will then vent electrolyte that can never be replaced, but is 0.5 volts too little to worry about? Does anyone know of a modestly priced charge controller that float charges at my recommended voltage?
Any advice from the solar pros out there would be really appreciated. I'm still learning this stuff.
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bobbotron
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# Posted: 15 Jan 2013 16:22 - Edited by: bobbotron
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I suck at electronics, but perhaps this makes sense. After the charge controller perhaps you could stick a zender diode/resistor/something on the line going to the batteries to bring the voltage down those 0.5 volts?
Again, feel free to tell me this is terrible advice! :P
Or, find an electronics geek to hack one of those charge controllers to spit out the correct voltage? I'm learning, given my own inability to absorb electronics know how, it's sure handy to know someone who dabbles in electronics if you're going to get up to tinkering...
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MtnDon
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# Posted: 15 Jan 2013 23:08
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I believe the specs the battery manufacturer gives. After all there is the matter of the warranty. Over time the extra voltage may take a toll.
I think if you have already bit the bullet and bought those expensive batteries (more $ than flooded lead acid) you will also need to bite another bullet and buy a CC more like the Outbacks or upper level Xantrex in order to program exactly the voltage you need.
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MtnDon
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# Posted: 15 Jan 2013 23:14
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Maybe check Blue Sky controllers.
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davestreck
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# Posted: 16 Jan 2013 09:18 - Edited by: davestreck
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Quoting: MtnDon Maybe check Blue Sky controllers.
Thanks for that recommendation. The Blue Sky SC30 charges at exactly the right rate (14.2 bulk, 13.2 float). I'm ordering one today.
Quoting: MtnDon you have already bit the bullet and bought those expensive batteries
Actually, I lucked out with the batteries. They came off of one of our customers' boats (I work at a boat yard). The batteries (originally a set of 4) were bought new in late 2012. The customer had only used the boat once or twice when one of the batteries came unsecured in heavy seas and cracked. Rather than just replace the one, he decided to replace all four. I ended up with the three (basically brand new) leftovers for free. It was definitely a nice score.
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kittysmitty
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# Posted: 16 Jan 2013 17:50
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First, if you listen to the experts, you have no where near the solar required to charge these batterys. That being said, you have more solar for the amount of batterys that I have. Couple of important questions, how often do you go to the camp and how much power do you use when you are there? Best case you panel will give you 400 watts per day, on a sunny day. In a month, best case 12,000 watts per month. We don't get 30 days of sunshine in a row in Nova Scotia. How much do you plan to use per day? How many days per month? Don't want to discourage you, but with a 100 watt panel you will never get to float. There are ways to help. I build a cheap charger using a car alternator and a lawn mover engine. Oh, and I am also from Nova Scotia.
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davestreck
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# Posted: 16 Jan 2013 18:44
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Smitty,
We only get to the cabin for 2-4 weeks per year in late summer. It stands empty the rest of the year. The system only needs to provide about 250-400Wh per day (cellphone, laptop and tablet charging, plus a couple of hours of low-wattage 12v LED light fixtures). I have a 3250W generator that I can use to top off the batteries if necessary, but honestly, we use so little power that I'd be surprised if we'd ever need it. Cooking, heating and refrigeration are propane and wood, and we actually prefer oil or Coleman light most of the time.
When you say a 100W panel will never get to float, what do you mean? If I only take 300 Wh out per day, won't 3 hours of good sun put that back into the bank, at which point the controller would drop to float?
We love Nova Scotia. Where is your place? We are on an island off the Eastern Shore, near Sheet Harbour.
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MtnDon
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# Posted: 16 Jan 2013 19:34
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If the use is only a few weeks a year the biggest duty the PV panels serve is to maintain the batteries with the float charge. There will be no problem with that. However, is that 2 to 4 weeks consecutive or scattered. If consecutive you may have problems getting a full recharge on a daily basis; again though, that is dependent on the actual degree of use.
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davestreck
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# Posted: 16 Jan 2013 20:31 - Edited by: davestreck
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Don,.
We usually do one big trip a year, so the 2-4 weeks is consecutive. Its possible that even given our modest power usage we may well run a bit of a deficit from the battery bank, especially if we have poor weather. But I figured 315Ah of storage was a decent buffer, especially since we have the genny to top off the bank if need be. We have gotten by with just a 10W portable panel for phone charging for the last five years, so we're used to doing without much electricity.
The main reason we went with such a small array was to make the setup less of a tempting target for thieves. I plan on mounting the panel as high up as possible, my thinking being that a small panel at such a PITA height would discourage the run-of-the-mill scumbags. The battery bank and associated hardware will be installed in a hidden "secret room" behind the cabin. We'll see how that works out for us...
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MtnDon
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# Posted: 16 Jan 2013 21:39
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I have a friend who mounts his main panels (about 260 watts IIRC, on the sidewall of the cabin down low. He's in a remote spot but they get a lot of snowmobile traffic in winter. They don't use it often. So, he removes the main set of panels when they leave and slips them inside the cabin. Outside on the roof top is a small panel for floating the panels when they are away. It's an idea you might think about.
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