The practicalities of running a laptop on solar power.

July 23, 2014 at 6:41 pm 3 comments

I’m a convert to solar power, having used it at Burning Man to power various things, from a bike light to a sculpture, but mostly GSM base stations. Summer in Nevada has lots of strong sunshine and solar panels have no moving parts for dust to get into, so apart from an overheated charge controller, the experience was pretty positive, provided you do the math and bring enough batteries.

More recently, I’ve been experimenting with solar power here at home in the UK, on sunny days I work in the garden, powering my macbook from a solar panel.

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It turns out that this isn’t a simple thing to do, but I have now got it down to a reasonably reliable configuration.

A 60 w solar panel which folds to a square for storage and has adjustable legs to set the angle.

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It also has a built in charge controller with built in battery charge indication
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The charge controller feeds a small 12v 7Ah sealed lead-acid battery. photophoto 5-1

Which in turn feeds a 12v -> 220v inverter – with a UK mains socket on it.

Plugged into that I have a power/voltage meter and the charger from an original Macbook air 

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Which then powers my 13″ retina macbook (via a magsafe 1->2 adaptor) 

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I had hoped to get away with something significantly simpler (at Burning Man, I just connected the 12v battery straight to the power lead of a fitPc2i and that was that.)

The charge controller on the panel won’t talk directly to the mac, it won’t supply power unless it sees a lead acid cell with at least residual charge, so I need one of them. Unfortunately, the macbook wants ~17v which is more than the lead acid battery supplies, so I need an inverter. The inverter then powers the mac via an Apple power brick.

When I first set this up, it was pretty unstable, the inverter has a low voltage cut out, which kept on going off. It took me a while, but in the end I worked out why.

The retina mac’s brick is rated at 65w – which is more than even the peak output of the panel, the excess was drawn from the (small) lead acid battery, which would then discharge to the point that the inverter shut down. Due to the fact that I’m in the UK, the panels are old, and the inverter isn’t 100% efficient, this happens within 30 mins. 

I have however found a work-around. The power brick from my original macbook air is rated at 45w, which brings me much closer to the actual available power. 

What’s more the power draw is halved (roughly) if the macbook’s battery is fully charged. So if I start the day with a fully charged laptop, I can work for 10 hours and still be on 100%. If the cloud comes over for too long, I can get into trouble, but then I have to top up at the end of the day.

How could this situation be improved? Apple would need to make a ‘solar magsafe’ cable which could take power direct from the panels and program the mac’s charge controller (which manages the internal batteries in the laptop) – then I’d be able to dump most of the components I currently need.

The panel comes with a carry case, which I have used to take it on international flights, on Bart etc. it is quite heavy, but it is manageable. The sealed lead acid battery is (just) below the 100wh limit for hold luggage, so that can fly too. I carefully pack it in the lunchbox seen above to avoid the risk of short circuits, but on the last trip the TSA inspected it (quite reasonably) but didn’t repack it very well, so I found it rattling around, potentially shorting on the the metal frame of the panel.

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3 Comments Add your own

  • 1. Jeff Weaver (@ceoaspirations)  |  August 25, 2015 at 5:42 am

    This is an incredibly inefficient method of transferring DC power to your laptop. All laptops require DC, yet you’re converting DC (from solar) to AC (via inverter), then back to DC again (supplied laptop AC-DC converter). You would be better off running the laptop direct from the battery (place a DC – DC voltage regulator in between the battery and laptop), and plug a DC travel plug into the laptop for an estimated gain in efficiency of 30%.

    Reply
    • 2. babyis60  |  August 25, 2015 at 8:20 am

      Yes, you are right. Unfortunately I couldn’t find a DC->DC regulator which could convert from ~12v -> ~17v at 4+ amps and has the Macbook specific magsafe cable.

      As I said in the post, it would be even better if I could just plug the panels into the mac and let it do the charge control and voltage management, which would save the weight of a lead acid battery and be more efficient.

      I have high hopes of the new macbook – which has an industry standard USB C connector. All I need now is a new 25w solar panel with a USB C power out – and a new macbook of course😉

      Reply
  • […] I’ve decided it was time I found a way to monitor my solar panels (see The practicalities of running a laptop on solar power.) […]

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