Complete Beginner - DC lighting for off grid island

Re: Complete Beginner - DC lighting for off grid island

Welcome to the forum.

What you're missing is the distance factor. The further those low Voltage wires have to run and the greater the current on them the lower the Voltage will be by the time it gets to the other end. As such the lights could be very dim, or even not work at all.

Two 5 Watt lights is 10 Watts, about 1 Amp on 12 VDC.
This page has a basic V-drop calculator: http://www.powerstream.com/Wire_Size.htm
Now if you know the distance involved you can determine a wire size that will minimize the Voltage drop.

You would also need charge controller(s): one per panel & battery install.
The batteries will need to be large enough to supply the Watt hours expected to be used, and the panels & controllers large enough to recharge those batteries in the available sunlight.

It's quite possible for this to work. And if not at 12 VDC then a little extra power allotment and a small inverter can send 120 VAC (or 230 VAC probably in that location) quite a long ways.

Re: Complete Beginner - DC lighting for off grid island

What you need are some hard numbers to work with. Principally how much power is going to be consumed per household.

If you run 5 Watts of light for an hour that's 5 Watt hours. On 12 VDC it's about half an Amp hour, so two of those lights per hour would be using one Amp hour. From there you multiply out. For example ten hours of run time makes 10 Amp hours, which means a minimum 20 Amp hour battery for that location and 40 would be better (the lower discharge level means longer battery life).

So if you had four small batteries like this for example http://www.solar-electric.com/unbamo45agms.html And then each household brought theirs to the central recharging station that station would need to be able to handle all four batteries.

Which is where it gets interesting. For in fact they would not be used as one battery bank, so each would have a different level of charge and need. That means you either have to charge them individually, or have four controllers/panels, or take your chances with how they recharge all at once.

If charged individually it's pretty easy to do: you're looking at ~5 Amps of current at most so a panel like this http://www.solar-electric.com/so85wa12voso.html and a small controller like this http://www.solar-electric.com/sg-4.html

But if you look at the costs you see that such a panel is pretty expensive per Watt, so it may be cheaper to go larger. In which case you can get a dual battery controller like this http://www.solar-electric.com/modubachco25.html and charge two batteries at once from a single panel (ideally 175 Watts @ 12 Volts but you probably won't find such a thing) or a pair of 130 Watt panels http://www.solar-electric.com/solartech-spm130p-s-n-130-watt-multicrystalline-solar-module.html

Sorry I'm throwing a lot of options and possibilities out here, but there are a lot.

Another thing that should be considered right from the start is how much more power is likely to be used/needed in the future? Buying a bunch of small equipment now (which tends to cost more for the capacity) and then needing to expand to larger stuff later is something you should want to avoid.

Re: Complete Beginner - DC lighting for off grid island

Atnot wrote: »

Does 'how they all recharge at once', mean that we could just buy a single panel and a single battery controller and hook the batteries in parallel and hope for the best?

Yup. You treat them all as one larger battery and get ready to throw out the ones that are used more when they don't get charged enough. So 4 * 35 Amp hours would be a 'virtual' 140 Amp hour battery and you need a panel and controller capable of delivering up to 14 Amps (about 245 Watts).

This is the part that I am really confused about. I don't know how you calculate the current/size of the panel required. I found this calculator at The DIY World. It determined we could use a fairly small panel (10 watts based on 10 watts of lights used 6 hours, with 6 hours of charge time). I got the charge time from a climate history websites calculation of sunshine hours per day based on the average time the sun is not obscured by clouds.

Well the peak charge current rule-of-thumb works well in most cases. 10% of the battery's "20 hour" Amp hour capacity multiplied by the Vmp for the nominal system Voltage. In this case the above 245 Watts is calculated as 10% of 140 Amp hours being 14 Amps * 17.5 Vmp for a 12 Volt system = 245 Watts of panel.

According to those calculations I could use a 20 watt panel for 2 x 35 amp hour batteries. Which of course is much different in cost from an 85 watt panel each battery. I am guessing this smaller panel and battery setup would be harder on the batteries and limit their life a lot? Or is this calculator really not very realistic to real world application?


Atnot

On a 12 Volt system a 20 Watt panel will produce a tiny amount of current: 20 Watts / 17.5 Vmp = slightly more than 1 Amp. That is not going to do the job for two 35 Amp hour batteries. It wouldn't even keep them up if they were not being used. Trust me on this: I've fixed more inadequate systems than I can keep track of, and insufficient PV is the #1 problem.

Re: Complete Beginner - DC lighting for off grid island

zoneblue wrote: »

Im confused. If you have one battery in each house, theres no practical way to charge them from a single source. When the batterys are in parallel its no longer sperate batterys, and the wire lengths will affect the charge rates unevenly. You generally want large short wire between controller and batterys.

Actually I've designed systems like this before: everyone carries their battery to the central charging station in the morning. It may sound like a pain-in-the-anatomy to most of us, but in some locales it is far more feasible than long wire runs or lots of individual systems. The 'charging them as one battery' method is most apt to cause trouble.

How many houses are we talking and how far apart are they? That makes a difference.

Four houses, but as yet that all-important distance between has not been revealed.