Solar Powered 12V lighting system

This project is about resilience more than energy saving, though it can also be used to save energy. Total costs should be in the order of £100, but there are ways to reduce that by getting some items from Ebay. The idea for the project came from the Transition Ipswich Energy group in 2011. It had to be targeted at a competent DIYer, and is for small scale lighting. This was originally published on TI’s website, which is no more.

This project lets you run lighting off of solar power, effectively storing sunlight for later use. It can be used at home to keep lighting during power cuts, but the same principle can be used to provide power to sheds on allotments, outbuildings or island sites without mains power. Although I have used lighting as an application, such a system can run an electric fence for much of the year if suitable solar panels and battery are used.

The system as described lets you run one or two 12V 1.8W LED lights through the year – my system was able to run mine through the winter and the shortest day where the light would be on from about 6pm to 11pm. In the summer you can also run a laptop computer power supply independently of the main for a couple of hours (these run typically 40W). That is because in the summer you get far more solar energy and you’ll probably use the lighting less.

If you want to primarily save energy or reduce your carbon footprint using solar power, this is not the solution. For that it is best to get a grid-connected solar power installation which will allow you to save energy and get renewable feed-in tariff payments. That sort of thing is on a different scale from this project, and capital costs are usually in the order of several thousand pounds, but the energy savings are much, much greater. A grid-connected solar PV system does not give you resilience against power cuts, because the anti-islanding systems in the grid tie inverter shut the system down if the main power fails, so that a PV system does not send power back into the grid when it may harm power workers trying to repair faults.

Safety first

12V systems have different safety issues from 240VAC mains circuits. 12V is not considered a shock hazard, but FIRE is your enemy with low voltage power systems because of the high currents involved. The reason these schematics include fuses is to safeguard you from fire risks, do not try and save money by eliminating the fuses, and take note of their locations.

Before working on a 12V system, remove any jewellery such as wedding rings, necklace chains and wristwatches, as if these metallic objects bridge the battery terminals they can heat up causing serious injury.

For indoor use, a charge controller and a sealed lead acid battery are mandatory. Using leisure batteries or car batteries indoors is a serious explosion hazard because of the gases given off under some charge conditions. These can be used outside in well ventilated areas, and leisure batteries usually come with a vent pipe, which should be used to lead any gases to the outside if these batteries are to be used in a shed or outbuilding.

Car batteries should only be used in a car because they lack the vent pipe – when charged in a car there is usually a decent headwind of several miles an hour to clear the gases.

Parts list

• approx 12W 12V solar panel
• charge controller
• 12V battery
• 20A terminal strip (4-way or 8-way if using float charge controller with wire ends)
• 25A fuse and in line fuse holder
• 4-way fuse panel (can substitute with inline fuse holder for single load)
• 2m of 16A red and black zip cord
• spade crimp kit if the fuseholder needs spade connectors

Tools required

This is a picture of the tools I used

You don’t need to use all of these if you are prepared to improvise – a good knife can double up for the wire strippers provided you know what you are doing, but they show what I used.

Parts walkthrough

Some of the items are shown in the pic below

The charge controller is in the middle of the picture and clockwise from the top left is a solar panel, battery and fuse panel. For home use the solar panel shown isn’t big enough. It is about right for a shed system that would mainly be used in the short summer nights.

Solar Panel

The solar panel I used is shown on the right – this is occasionally on sale at Maplin for about £40, and is rated at 12W. That 12W you only get to realise in the sort of sunshine you’d get in California, in the UK you can pretty much assume you will get on average 1/10th of the rated power even in the sun. In practice you do a little bit better – in the sun this can charge at about 0.2A through my intelligent charge controller, giving me about 1/5 of the rated power.

I use the light powered from this system regularly, which is why I need such a large solar panel. If you don’t plan to use it daily, then you can get away with a much smaller panel.

As a rough guide to sizing the panel get one that is rated about 10 times your load. Thus if I have sunlight in winter for about 8 hours and use the system for 4 hours at 2W, I need 8Wh of power. A 12W panel for 8 hours is equivalent to 24W for 4 hours.Allowing for it running at 1/10 capacity in the British weather, it gives me about 8Wh in practice. I get away with that because I use a PWM controller to maximise the use of the solar panel.

Battery

I used a sealed lead-acid (SLA) battery, the sort of thing used in computer uninterruptible power supplies and the like. Mine is a 14Ah 12V gel battery which is about £30 new from Rapid Electronics in Colchester. You could use the 7Ah type commonly found in UPS, either secondhand or these are available from Maplin in town. Maplin isn’t usually the cheapest place for this sort of thing, but you save delivery charges which may swing it.

Two things kill SLA batteries – overcharging and over-discharging. You should aim to never discharge a lead acid battery to less than 50% of its capacity. If your load is a 1.8W LED lamp or a typical electric fence charger, you’re probably running a current draw of 0.2A at 12V, so a 14Ah battery would run my load for 14 ÷ 2 (never run the battery less than 50%) ÷ 0.2 (load current) = 35 hours. This is about three days’ usage in winter, so as long as I get some sort of decent light in the three days I shouldn’t fall short. In summer, of course, I have the opposite problem, lots of sunny days and low usage. The problem of overcharging is solved by the next component.

Charge controller

The charge controller’s basic job is to stop the solar panels overcharging the battery. That knackers it, and makes you sore if you have spent £30 on the battery. The simplest form of the charge controller is something that goes in between the solar panel and the battery, which looks at the battery voltage and when it is full, stops the solar panel charging the battery.

This works well enough, and is the cheapest solution. For bigger systems you have to size it right for the solar panel as these have an upper current handling, but this is not an issue for a 12W panel as the lowest spec controller is good for 6A.

I wanted a more geeky one which would optimise the usage of my solar panel, as I knew my usage/sizing was marginal, so I used a PWM controller which tries to match the load specifically to the light conditions to get the most out of the solar cells. It also tells me the battery voltage and load current, and isolates the load if the battery voltage falls too low, which saves me reducing battery life through over-discharge, though this function hasn’t kicked in yet. For the convenience I get to pay £30 for a part that otherwise costs £10-ish.

For a small system you’re generally better off spending the extra on a larger solar panel rather than using a fancy controller to eke out the last of the panel’s performance, but the metering swung this for me.

Fuse panel

LED lights draw a small current of about 0.2A, and using a small fuse allows you to use appropriately thin gauge wire, I used 3A standard twin core lighting mains flex.

The problem arises when you have a fault condition at the lamp end of the cable, say the LED bulb fails short, or the metal of the fitting chafes on the cable and shorts it out. You now have the full battery capacity going into the cable. My battery is rated at a short circuit (CCA) capacity of 84A, and putting that across the 3A cable will melt all the insulation, guaranteeing the short, and eventually catching fire somewhere. I wouldn’t like that very much, so I put a 7.5A fuse between the battery and the cable. By using a 4-way fuse board I can run different things off the battery, so a 15A fuse goes to the 150W mains inverter, and another 7.5A fuse protects another light circuit.

12V lights

The first lamp I used with this was a 12V halogen desk lamp from IKEA which used a MR4 halogen bulb. I simply swapped it for a 12V LED bulb sourced from Ebay, and wired it to my system. The lamp was sub £10. B&Q have a LED 12v desk lamp with a wall wart mains power supply, the LEDs are all in a line and this sells for about £25, which is overpriced but locally available. You need to be able to source a suitable plug and solder it to a cable for the 12V system.

The second was another fine IKEA design which was originally a 240V floor mounting lamp with halogen R50 spots and a SES lamp holder. IKEA don’t seem to have realised how hot these halogen bulbs get. So I gutted the innards and glued a MR4 holder to the SES shell and kept the original cable and switch. Obviously the mains plug wants changing to the 12V system and then a mini-spot MR4 LED lamp was fitted to the socket.

 

Complete installation

All these parts, except the solar panel and the light, are mounted onto a wooden board.

It is worth keeping high-power loads close to the battery and fuse board, typically within 1m to reduce losses in the cables. The solar cells can be mounted on longer cables since this isn’t a particularly high current circuit, and the lighting loads can be several metres away because of their modest power drain. If a mains inverter is used, keep this close to the battery and allow a longer mains cable, since losses are higher in the 12V circuit than they are at 240V for a given selection of cable and a constant load.

Schematic – Cheap Solar Powered 12V system using Float Charge Controller

In this version the Kemo charge controller (or equivalent) is placed between the solar panel and the battery. It prevents the panel overcharging the battery. The loads are simply taken straight from the battery via a fuse panel, in the same way as a car 12V electrical system is set up. Here you must make sure that the load is not left switched on, as otherwise, just as if you leave your lights on in the car, you will find the battery discharged fully the next day. That doesn’t do it any good at all. One of the side advantages of a solar system is that it teaches you to value the power and not leave things switched on needlessly, but you don’t want to get to learn that the hard way by having to replace a couple of batteries.

Schematic – Solar Powered 12V system using PWM Charge Controller

Note that the 25A fuse must be located close to the battery – it protects the battery cable against faults. This particular controller also isolates the load if it is left on and the battery runs down, as well as optimising the load on the solar panel, typically getting up to 50% more from it.

It also shows battery voltage, panel and load current and lets you switch the loads off via the controller.

I used this system for about five years after writing this article in my Ipswich house, powering three reading lamps in two rooms. We didn’t get too many power cuts in Ipswich, but it was nice to be immune to the few we did get in the winter nights; a wood stove and light and a good book makes things more congenial.

I reposted this because the article attracted a fair amount of interest, it seems there are lots of people with outhouses, stables and barns on island sites without mains power. In the intervening years the solar shed light has become a popular product and is often the cheapest way to get a single light, but the batteries are usually small capacity, and rain usually finds a way in so they are short-lived. You’ll usually get more running time and reliability using a bigger solar panel and battery, and keeping everything other than the panel out of the rain, which lets you find your way around inside an outbuilding.