Sunday, June 12, 2011

Solar Powered Camping - Lessons Learned

Lessons learned from the Solar Powered Camping project

  • Constraints help!:
    • It needs to power a tent fan for a few hours a night (all night would be GREAT, but a few hours minimum)
    • We had to make it fit in the locking Rubbermaid tote bin with all his other camping gear
    • It needs to be lightweight, and durable enough to with stand the packing / unpacking / setup process
    • An 11 year old needs to be capable of setup / teardown with a minimum of adult help
    • We had a deadline (we knew the day he headed to camp!)

  • Be flexible, a better design often emerges: 
    • We designed the solar panel stand "on-the-fly" and a great design (SIMPLE manual tilt) emerged while we were playing with fittings. 
    • A better, less expensive, and smaller battery option proved to be best. (see below for more...)
  • Prototyping close to the store (or in their parking lot!) can save lots of driving, gasoline, and frustration
    • We setup in the Lowe's parking lot, and made 3 return trips inside for additional fittings. Even if I'd purchased extra (good practice), I still would not have thought to grab the fittings we needed.

  • Don't forget Ohm's law
    • I'd done my calculations on runtime, etc. using the current draw from the Coleman camping fan / light, and the capacity of batteries. When we decided to do an "experiment" to see how long the fan would run, it ran for twice the time. I'd forgotten that the Coleman fan was a 6v fan (everything else was 12v). It was using half the POWER that I thought it was using...
  • Electronic solar chargers exist for a reason!
    • Overvolt protection, Undervolt protection, battery voltage sensing, battery type charging logic, temperature compensation - these things all maximize the little current available from the solar panel. Any other solar project will include an electronic solar charger.

  • Bigger isn't always better
    • I'd selected this big 7Ah sealed lead acid battery to maximize runtime. Problem is, this battery takes a VERY long time to charge if run down too far (undervolt). The fans do not have an auto-shutoff timer, so taking the battery all the way down overnight is very likely. 
    • We decided to go with the smaller Coleman rechargeable batteries that fit right in the back of the fan. They are 6v batteries, and even when run down, the solar panel could full charge them in a day. 
    • The smaller size also allowed him to take 2 of them, which automatically means two nights of charge as he heads to camp - and a backup if he has a problem charging one day.
  • Verify Specs, manufacturers take too many liberties
    • The "12v" solar panel output 24v. It is current limited (by the panel's efficiency), but still. We added an LM317 voltage regulator. An electronic solar charger would be better in the future...
    • The panel said that it had power-drain protection (assumed a diode...) - but I found nothing. We added a diode.

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