Solar Power in the Field (part 3)

In the last post in this series we figured out how much power we’d need. Now the really important part – we choose our panel and battery and stick it all together.

Lets start with the battery. This is pretty straight forward, because for most people, this will simply be whatever car battery they can get their hands on in the country. If you need a lot of juice, you might need two and run them in parallel (more on that in my final instalment on alternatives to solar). Or if you only need a little juice, then you might consider a smaller battery (if available) or attempt to charge some AA/AAA rechargeables directly.
For large amounts of power storage, it seems that the best solution is still Lead-Acid batteries. They’re heavy and hazardous (*nod to david n*). And therefore, not a good thing to pack on a plane, especially in the current climate. But you can store a fair bit of juice in one. They come in sealed, maintenance free form, and the type you’ll often see in a car, which require top ups from time to time. They come in different voltages: 6, 12, and 24, and different storage capacities: 1 amp hour up to 120 amp hours. Your typical car battery is a 12 volt, 35 amp hour, unsealed lead acid battery. So that’s 420 watt hours using the formula from my last post (watts = volts x amps). For our system we’ll be using a 12 volt battery.
Lead acid batteries are quite temperamental. Their ability to take a charge varies based on the ambient temperature, and they’ll discharge faster if not done at room temperature. So be sure to keep your battery somewhere not to cold or hot. Also, I hope I don’t need to say, keep them away from children. While a battery may be rated at 35 amp hours, you should only be using around the top 30 or 40% of the capacity, otherwise you’ll damage the battery, and it’ll be harder and harder to charge it. That means that 35 amp hours really means about 12 amp hours of usable power (or 144 watt hours). Once in a while its probably OK to use 50%, but don’t make a habit of it! So be sure to take that into account when buying one.
Next is your solar panel. Depending on the time of year, the peak time for collecting solar power is roughly from about 10am to 3pm. You’ll do better if you angle the panel towards the sunlight throughout the day (rather than just lying it flat on the ground, or some custom built structure). So each day you’d hope for at least 5 hours of good charging time. So a 30 watt solar panel will give you roughly 150 watt hours of power a day (this is rough, charging the battery results in some loss of power). So if you use the top 35% of your standard car battery (~144 watt hours), a 30 watt solar panel is going to fill up your battery again in roughly a day.
So, using your estimated daily usage, make sure your battery can supply that amount and be topped up within a day (preferably half a day). Your daily usage is going to be fairly high if you plan on using a laptop and video camera every single day, but probably wont be all that high if you’re just charging AA batteries for your torch and audio recorder.
There’s a few bits of equipment you’ll need to plug it all together. First of all get a charge regulator. These devices will stop you from over-charging your battery which will probably damage it. A good one will also tell you when to stop using your battery too so you don’t damage it from over-draining, sometimes even cutting out the power so you don’t damage the battery.
One thing to watch out for is the maximum amps that the charge regulator can take. If your solar panel outputs higher amps than the charge regulator can take in amps it will damage it (my example charge regulators can take a maximum of 6 Amps and 20 Amps as input). The same goes for maximum voltage. Many charge regulators will also stop the electricity flowing back into the panel at night, assuming your solar panel doesn’t have a blocking diode to make sure this doesn’t happen. (just an aside: if you do damage your charge regulator, you can “fly blind” and connect your panel directly to the battery… but it can be dangerous if you over-charge your battery. If your panel doesn’t have a blocking diode then you must unplug it in the afternoon before the sun goes down!).
Some people recommend taking a digital multi-meter into the field. These are cheap and light weight and amongst other things you can use them to measure the voltage on your battery, which is a way of seeing how full your battery is. As a battery is charged, its voltage increases, and as it is discharged the voltage decreases. The operating range of voltage varies from battery to battery, and depends on the ambient temperature. Typically, you’d be operating in the range of 11 – 13.8 volts. See the battery documentation for more specific information. Although a multi-meter saved me several times in the field (my solar panel broke within the first month and I had to get creative…), I don’t generally recommend using one unless you know what you’re doing. If you use a multi-meter incorrectly you could short the battery and cause yourself a lot of harm (it may even kill you). So instead I recommend using a charge regulator with a battery capacity indicator.
You’ll probably want to get a couple of Cigarette Lighter Socket to Battery Clips adapters to plug your stuff into. Maybe some double adapters if you’ve got a lot of equipment (check to see if it has a fuse too!). Consider a housing for your battery too, one that will stop kids from touching the positive and negative terminals on your battery (which must be avoided of course!). Russell Emerson here at Sydney pointed out an excellent battery housing which is both very safe and makes it easier to lug your battery around. Russell’s model incorporated LEDs to indicate battery charge levels too! I just wish I could find a supplier… The casing is acid proof in case the battery is damaged or spills. And finally, it has a convenient car plug built into the casing, which means no fiddling with wiring once your system is up and running.
Wiring up your system should be covered in the manual for the solar panel and/or charge regulator. If anyone needs it though, contact me and I’ll post a generic description of how to connect the wires. Pay particular attention to the polarity (+/-) when making connections. Make sure when you’re wiring anything to the battery terminals that there’s no chance that the positive and negative wires will ever touch. A car battery is able to discharge huge amounts of power very quickly (that’s why they’re used to start cars). This also means that you should never hold on to both ends on the wire! Auto mechanics often tell the story of the stray spanner that falls onto the battery touching both terminals. The sudden surge of power fuses the spanner onto the terminals and causes the battery to explode… so please, be careful!
In my final instalment in this series, we’ll look at some modifications to this set up. Specifically, we’ll look at alternate power sources such as a petrol generator or wind power, and how to hook up batteries in more complicated configurations.

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