How to power a laptop with a solar panel

Andrew Booker 2008-07-16 15:56:03

A while ago, when I was getting into my time-lapse photography ideas for projection behind Improvizone gigs, I considered one option of parking the car somewhere with a good view, hiding a webcam and a laptop inside, and leaving them to take pictures all day, powered by the car battery. I looked at the huge current being squandered by my laptop even with the lid down and calculated that a day's shooting would drain the battery. So in order to be able to drive off afterwards, I thought about using a solar panel. These are commonly available in a voltage range especially intended for charging 12V batteries.

I looked into to the electricals, sizes and costs, saw that the whole solar panelling thing was going to be eff expensive, and went ahead and bought one anyway for fun. My new photovoltaic module, to give it its correct description, is powering my laptop as I begin typing this article, sitting in the back garden and drinking tea. It can also power the external hard drive, charge my phone, charge my iPod, and probably power most of my electronic drum gear. I'll be trying that later. Right now I know it cannot make the tea.

In principle, the simplest way laptop-by-solar-power could work is by connecting the solar panel directly to the laptop power input. The laptop needs 19V, and my solar panel output can deliver about 3A at that voltage.

I haven't had the guts to try this yet. Unloaded, the solar panel voltage can be as high as 21V, and is around 17V at its rated 5A. I have no idea what sort of internal voltage regulation the laptop will do. If any. In fact I assume all this is supposed to be taken care of in the power adapter. If I pop my laptop by taking it outside on a bright sunny day and wiring it directly to the solar panel, I shall be a jolly unhappy personage. So given that I've already frazzled two or three innocent devices in my solar experiments, I'm using a car laptop switched-mode adapter to make sure my HP gets exactly what it needs. I have a couple of these PSUs and they seem to run happily off 10-19V, and while the solar panel is in full sun, this setup works fine.

When the sun goes behind a cloud, it doesn't. The solar panel power plummets, the switched-mode supply stops working in a dodgy flickering mess that effectively short-circuits the solar panel. No, I don't understand why either. Luckily, that can be stopped easily by wiring a rechargeable 12V battery in parallel. In full sun, the solar panel has enough power to run the laptop and charge the battery. When the sun goes in, the battery takes over, and the solar panel can still contribute a small current.

For the solar panel I'm using a SunPower SPR-90. It's about half a square metre, is completely weatherproof, and sits on top of the pergola in my back garden facing the heavens. For the step step-up to the laptop's 19V I'm using a 70W car laptop converter from Maplin. For the battery, I'm using a 12V 7.2Ahr lead-acid gel battery, also from Maplin. This is junior-school version of the kind of grown-up monster you find in a car. When it comes to my time-lapse application, I will be using the car battery, of course.

In fact I'm thinking of getting a couple of car-strength batteries for the back garden shed. For continuous use on a mainly sunny day the 7.2Ahr one is fine, and it's also small enough to be very easily bring-indoors-able. Over longer periods a small battery is not suitable. Once its internal battery is fully charged, the laptop uses only about 40W. In full sun this leaves at least another 40W spare with nowhere to go but into the battery, overcharging it and sending the voltage up to 16-17V or more. Not only will this boil the battery, it will risk any other 12V devices I might be running too. Equally, if it's cloudy, the solar panel will continuously under-perform, and my 7.2Ahr battery suddenly has about 20 mins to live.

Whereas, a car battery of about 60Ahr can store about eight times as much charge, so not only can it mop up more spare charge from the solar panel without exploding, I can also heave it indoors of an evening and use it to run the the laptop and all sorts of other music stuff, all of which are low voltage devices. It should power all of that for several hours before flaking out. If I were feeling deeply enviromentally friendly, I would get it from a breaker's yard, rather than buy one new from Halfords, although they do deteriorate with use.

As you can see, this is my latest obsession, but in fact there would be a practical benefit to using a battery to power my drum modules, the external soundcard and the laptop. If you power all those with switched-mode supplies, as I've been trying to, you get a lot of nasty interference noise. We put up with it all through the last No-Man rehearsal. Use a battery, and you guarantee noiseless audio. Obviously this is not for taking on an aeroplane, which is a reason for trying switched-mode PSUs in the first place, but it will work for any gig I drive to, which, right now, is all of them.

Apart from some persistent rain, the weather over London recently has been pretty good, with plenty of dappled cloud that makes for good time-lapse. So next I guess I need to try putting the solar panel and laptop inside the car.

The immediate consequence is a big reduction in the direct sunlight onto the solar panel. Only a few hundred milliamps goes into the battery when I lay it on the back floor of my VW Golf with the seats down. Even that small current will still help though, and even if I do drain the battery, if it's sunny when I go back for the car, probably 20 minutes with the solar panel outside on the car roof while I read the newspaper will put sufficient juice back into the battery to start the engine.

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