# Engineering Roundtable - Burning Man Edition

Engineer Chris went to Burning Man, and needed some solar power for his project. Thus, a Roundtable was born.

On this edition of Engineering Roundtable, Chris Taylor shows us the steps he took to set up a solar power supply to run his LED weather balloon project at this year’s Burning Man festival. He covers the five major parts of any solar power system and explains how to choose a battery and solar panel, setting up a charge controller, and using pulse width modulation to adjust a load. Plus, BURNING MAN.

As always, if you have any questions/comments/ideas/amusing gifs/sandwich recipes/tasteful jokes, we invite you to leave them in the comments below for the community to enjoy. See you for the next Roundtable!

• Great build, and great Roundtable talk. Thanks! Also…

Why do engineers constantly confuse Halloween and Christmas?

–Because Oct 31 = Dec 25!

• Oh that took me a minute!

• I feel so ashamed, I had to go look this up! But once I did, I laughed. Not sure my wife will appreciate it but I think this might just make dinner table conversation tonight! Thanks!

• Well done. Can’t believe I’ve never heard that one.

• That was pretty good though…

• I just barfed.

• How would you calculate the current draw of the LED strand while being dimmed via PWM? Do you just multiply the current draw with no dimming by the duty cycle of the PWM signal?

• Theoretically, that’s the way to calculate it. The way I actually did it was the empirical (read: lazy) way. Cheap, off-the-shelf products like the LEDs I had don’t often exhibit proper theoretical behavior, so I just plugged the system into a bench power supply and turned the LEDs down until the current was lower, but the difference in brightness was negligible. That current happened to be 6.5A, so that’s what I used for the rest of the calculations.

• You’re ignoring the fact that solar panels don’t deliver constant power all day, they peak at noon (For obvious reasons.) as such, you need to use the local Solar Insolation value… that’s the number of hours at max wattage you get for your area in the season you intend to use it when in the sun all day. Meaning: A Solar Insolation value of 5 means that if you leave the panel in the sun all day…. you get 5 hours worth of the panel’s rating. Despite the fact that the sun was up for 11 hours.

Let’s see. Burning Man is in Nevada, in the desert. You will probably get 5.0 to 5.9 Solar Insolation hours per day at the height of the summer solstice in June. That was 3 months ago… Let’s call it 5 during Burning Man just to push it. You might get less.

Anyway… that means you need twice as much solar panel as you have. At least.

The battery types used in solar systems are ALL lead acid. The major types used are flooded lead acid (Golf cart batteries…. 6 volt per battery, you have to carefully monitor the charging and add water.) and AGM, which means “Absorption Glass Mat.” AGM batteries are 12 volt batteries, and are superior but more expensive than flooded lead acid batteries. I’m hoping the battery you have is of that type.

• Although one should take into account that in poor weather the system is less likely to be used, so you could go with a less conservative value (I looked at the NREL Red Book and got about 7-8 as reasonable).

You are absolutely right though that especially for systems looking for year round use more power will be needed. Thats how I have a 800w system driving a 3 amp load (extreme case).

• Actually, no. The NREL Red Book lists Nevada at 5.1 with south pointing panels (No tracking.) for the month of September. You get as high as what you note with 2 axis tracking… but nothing like that was mentioned by Chris.

If you intend to use the system year round, it gets even worse. Then you should use a 20% discharge per day rate and further factor in a weather rate of at least 2:1. Those batteries start getting REALLY expensive!

Chris doesn’t have that problem, and can probably assume he can pretty much wreck the battery during Burning Man. By this I mean the expected life of the battery… Deep cycle AGM batteries intended for solar applications are typically specced for 500 cycles at 50% discharge. If you discharge the battery more per cycle, you get less… less discharge per cycle… more cycles. Chris can burn that battery down to zero charge pretty much every day for the Burning Man, and it will still work.

The problem will be when he tries to recharge it during the day. There won’t be enough power to recover the battery to 100% before nightfall with a wimpy 80 watt panel.

• Outstanding explanation, Chris. You’ve answered a number of questions I had about multi-amp solar powered projects. This one goes in the “useful information” bin for sure!

• Nice explanation, mostly, but you do need to correct a few things …

First, car batteries are not sealed lead-acid (typically). They are usually flooded lead-acid batteries known as “starting batteries”. The difference is the lead plates in a starting battery are thinner than in deep-cycle batteries allowing them to deliver a lot of current for a short amount of time.

Sealed lead-acid (SLA) batteries are what you find in uninterruptible power supplies and such. The difference is they are sealed so you can’t get to the electrolyte (acid) and they won’t spill unlike a flooded battery which will if you tip it on its side. It appears that all new SLA use “absorbed gas-mat” (AGM) which is some kind of fiberglass (fiberglass-like?) material that suspends the acid as opposed to the older gel-style SLA batteries which used a gel to do the same thing. AGM batteries work much more like deep-cycle batteries than gel cells which tended to die if deeply discharged. (I’m skeptical that all the batteries labeled AGM really are — it seems that in just the past few years, every single SLA says it’s AGM, but I suspect new labels for the same old gel cells made in China … so far I haven’t found a teardown test to confirm or refute that suspicion, but I digress.)

For the same capacity in amp-hours, a flooded deep-cycle battery will weigh more and cost more than a starting battery. And for the same capacity, a SLA battery will cost more than a flooded battery. SLA batteries also come in smaller sizes (

• This is true. Car batteries are never “Sealed Lead Acid”. (Well, there may be one or two super expensive ones.. but in general? No.) Even “Maintenance free” car batteries are flooded lead acid types, the difference is the caps are not removable, and there is a greater acid capacity.

Sealed Lead Acid batteries are of 2 types, “Gelcell” and “AGM”, both of which use a valve system to control the electrolyte.

-> From the replied to post: “(I’m skeptical that all the batteries labeled AGM really are — it seems that in just the past few years, every single SLA says it’s AGM, but I suspect new labels for the same old gel cells made in China … so far I haven’t found a teardown test to confirm or refute that suspicion, but I digress.)”

I agree! I bought 2 new batteries for my electric hand cart 2 years back… and the new batteries claim to be AGM… but I don’t buy it. They sure look and act like Gel cells to me. But I’m not about to disassemble them to be sure!

• How did the system actually fare on the playa; any suggestions for improvement? Do you have a schematic available for your PWM controller?

• Oh wow! Wish I knew you were at that camp! I was camped a couple blocks away (3:30&I with the 4' giant discoball). Waited in line for the pendulum for about an hour and saw that getting cranked up. At first people thought it was a giant tetherball. Pretty sweet tho!

• Less talking, More Burning Man. :D. Glad you folks were out there. I had some DYI super bright wearable blinkies, thanks to you guys and another company that resembles fruit. Was really surprised how long they lasted on a 1300mah lipo battery.

• Nice project, and good presentation. However, I think using an Arduino to generate the PWM is overkill – you could have done it with a simple 555 timer chip, and used the difference in cost to invest in more beer. :-)

Or were you thinking about a more sophisticated usage than just the knob-variable brightness?

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