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Tuesday, May 06, 2008

Help Me Learn: How to Design a Solar Charging CIrcuit

I have an general interest in sustainable power systems (my home has a 5-star rating and a 3Kw PV system) but I am not an electrical engineer and have no useful understanding of electrical circuit design beyond very basic stuff (I know what a resister is but I couldn't reliably tell you how resistance relates to current and voltage).

I have two projects in mind that I'd like to pursue, both of which require a bit more knowledge than I have and I have no idea where to go to get the knowledge--all of my resources are focused on small-scale electronics (digital circuits, basic occilators, etc.).

My first project is to create a water feature that is variously powered by wind, solar, humans, etc. where all the different power sources contribute to charging a storage system which then drives an electrically-powered pump of some sort. I'm thinking of something like a 6-volt marine battery, something that can hold a good charge and produce enough current to drive a beefy motor.

What I don't know is how to design a charging circuit that will feed the battery from multiple input sources.

The other project I'm thinking about is modifying an RV to be electrically driven so that it could be, as much as possible, solar powered (e.g., for traveling about the American West during summer). That is, building an electric RV that would run off batteries for cruising and be recharged by a combination of solar, auxiliary generator (presumably a diesel engine that could run on waste vegetable oil or the most ecologically sound fuel available at the moment), or grid connection when parked.

It would need to enable a 200- to 300-mile range on a single charge to account for the (almost) worst case where you have no solar input and must recharge overnight from a campground. The worst case is no solar input and no grid access, so you'd have to run the generator in order to get to the nearest power source (or wait out the clouds without your beer getting too warm).

Some obvious questions are:

- Assuming an Airstream RV (chosen to minimize drag, even though they're frightfully expensive), how much energy would be required to provide a 200-mile range at 55 MPH?

- Assuming a more affordable typical RV, what would the cost from drag be?

- Given current solar panel technology, what output could be expected from the maximum area one could reasonably attach to an Airstream? Would it make sense to include some sort of fold-out panel system for use when parked (e.g., you're stopped for the afternoon at some tourist spot)?

- Assuming worst case of no solar input and no access to the grid, what size of generator would be needed to enable direct operation of the vehicle at say 40 MPH?

All of this would go to answering the first question, which is "is this even practical with today's generally-available and affordable technology?" If the answer to that is "no", then what advances would be required to make it affordable?

We could start with the presumption of a 50,000 USD budget, which is about what it costs to buy a full-sized conventional RV. So if I bought a used one and refit it, could that even be done for that budget?

Another consideration is the value of not buying fuel. With gasoline pushing 4.00USD a gallon and diesel already over that as of May 2008, a 3000-mile trip at say 8 MPG starts to add up pretty fast. That's roughly 1500 USD in fuel costs for that trip. At 4 dollars a gallon, I can recoup 15,000 USD in investment in 10 years of driving vacations. If fuel was at European rates that payback would of course be much higher (and it seems reasonable to expect that U.S. fuel will climb to approach European rates over the next 10 years simply because of both market pressures and increasing social acceptance of the true cost of our life styles in the face of global warming).

So I'm wondering if anyone can provide pointers to resources, online or otherwise, where I could start developing the necessary knowledge to start answering these questions?

I don't think any of this is particularly challenging from either a design or implementation aspect, I just have no idea how to go about learning about it efficiently....


Blogger DaveP said...

Your 3 sources into one needs (old time) zener diods to flatten the variable source voltage to an appropriate charging voltage.

You then need to select which source is 'most usable' (then a way to select 'most usable')

Then a switching method.

Then something to load the other power sources when not being loaded.

Your RV ideas? Ask why none of the electric vehicles do 2-300 miles on a single charge? Then ensure you have a backup smallish petrol engine.

I don't have your email except at isogen Eliot.

12:56 PM  
Blogger FARfetched said...

The first project sounds do-able — you're basically looking for a way to charge a battery using a power source that varies a lot. Wind power is a perfect example. There's some DIY info on that should get you started; it's written on a non-engineering level but does assume you can use tools.

Modifying the RV? Good. Luck. There aren't many, if any, plug-in electric cars that get 200-300 miles on a full charge. The RV has more room for batteries, but you would need a LOT of them to push it that far.

The experimental Mitsubishi MIEV, in low-power mode, goes about 100 miles on a full charge and weighs 2380 pounds. I couldn't find the weight of a self-propelled Airstream, but a Winnebago weighs around 7200 pounds (say 3x the Mitzi). The implication of the article is that, even Li-Ion added about 400lbs to the weight of the car. You're asking to push 3x the weight, 3x the distance… and so you'll need close to 9x the battery capacity.

The important capacity number is Amp-hours (Ah), and the closest they come in the article is "16 kW." A typical Li-Ion battery (18650) is 3.6V and 2.4Ah, so that would be 8.64Ah. If "16kW" is 16kWh (kilowatt-hours), and you need 9x that (144kWh), you're talking 16,667 cells to drive that buggy.

Charging that stack is going to be interesting. Assuming no loss in the charger circuitry, 240V service, and 12 hours to charge it from zero, you'll need a (144000/240)/12 = 50 Amp circuit worst-case. That's a lot of juice.

Solar panels… a 185W Mitsubishi solar panel (coincidence here, I'm just following links) is roughly 5.5'x2.5'. Assuming your RV top is 40'x8', you could line up three rows of panels of seven each, 21 panels total, for a total of 3,885 watts — 3.9kW source vs. 144kW sink… and that's assuming no losses (again) and no clouds.

To make a long long long story short: if an electric RV was feasible, someone would have done it by now. Sorry to be a wet blanket.

I'm working toward adding some solar & wind to FAR Manor, so in that regard you're pretty far ahead of me already.

2:52 PM  
Blogger Eliot Kimber said...

Thanks for the tips.

The RV idea seemed like a long shot with current tech but I didn't even have a solid basis for estimating how outrageous it would be.

I'll check out and see what I can find.

4:19 PM  
Blogger Rob Janson said...

Have you checked out the EEStor Ultracapacitor?

5:31 AM  
Blogger Rob Janson said...

Have you checked out the EEStor Ultracapacitor happening in Cedar Park, Texas? I wrote some stuff here on it:

5:33 AM  
Blogger Eliot Kimber said...

I've been mildly tracking the EEStor stuff--it gets regular press here in Austin. It's definitely an exciting possibility but they're now at least a year behind their original promises of getting product into the hands of manufacturers. One hopes that's because of technical issues and not simply because it's all snake oil. It's definitely an inherently fiddly technology.

A 10x improvement in capacitor capacity would be a major advance so waiting a few years isn't a big deal.

It feels like we're heading for a point a few years from now where low-cost solar cell production will converge with improved ultracapacitors to make it much more affordable to have both affordable home-scale charging and solar-recharging/quick-charging vehicles. Coupled with things like Chevy's Volt platform and work being done in Germany and Japan, it could mean a very different picture for electric vehicles very soon.

Here's hoping.

Also in this thread, I did buy "Practical Electronics for Inventors" ( and found it to be a wonderful book, much clearer and much more concrete any any of the EE textbooks I've (tried to) read. I feel like I'm actually starting to understand how electronics work...

8:31 AM  
Blogger Rob Janson said...

Electric vehicles enemy is weight. I would consider getting a diesel RV, then get a hold of some lithium ion baterriew
s, (smashed prius), then run a 5th wheel for highway cruising. Run ac and rv off the battery pack.

3:02 PM  

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