Portable Ebike Solar Charger - build log

John in CR

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I finally obtained cells/panels that make sense to finally build some solar panels for charging my ebikes anywhere. The concept is to end up with series strings that I can fold up and deploy when parked. Each string will be low current with the cells exceeding pack voltage by a sufficient margin so the cells deliver full current. Then each string or parallel group of strings will have a very simple and efficient HVC to protect the pack from over charging. The HVC unit will have a range of adjustment, so I can stagger the cutoff voltages if I use a large array to emulate CC/CV charging.

I typically use 500-1000wh/day on my bikes, so our pretty reliable 5-6 hours of good sun typically through lunchtime should make a solar solution simple. The sun is more powerful here so close to the equator at our 1km of elevation. Ask LFP how badly we got burned before 9am helping me fix a water leak. 8) . When we go to the beach a big array will get double duty as a power source and sun shade, and no need to ration anyone for their beach rides due to fixed battery limitations. Low tide riding on a shallow sloping beach with fine sand that packs hard is a truly unique experience in peaceful fun that I highly recommend.

The panel modules I found are 18V nominal with an open circuit voltage of about 21V, and rated current of about 0.3A . 5 in series should work great for the 82-83V cutoff I use for all my packs. Then I just connect strings in parallel for the charge power I want. The cells are monocrystalline PVs with a claimed efficiency of 16%. With the border around the cells and protective layers between the sun and the cells, they work out to just over 11% efficiency based on total surface area at rated output. Maybe I'll get lucky and my location and elevation net higher than rated output at peak time of day.

In addition to an easy to add up voltage for no losses stepping up to pack voltage, and no expensive charge controller, what makes these modules to me seem perfect is their construction. They're 135mm X 330mm X 2.4mm . The modules are composite sandwich with a layer of PET as the front surface, followed by EVA, the cells themselves, and again EVA. Then to add rigidity and strength the back is a fiberglass board like a PCB. Front and back are flat and smooth with the metal to solder the electrical connections on the back. The edges seem like they need sealing to make them truly waterproof, but I'll just include the edges in whatever I use to seal the electrical connections. They're quite rigid, so the physical structure I need to come up with should be simple.

I'm exited to start kicking the electric company out of the equation for my transportation needs. It won't be saving like the $2k-3k/yr I stopped giving to the oil company, but an extension cord connecting to the sun that easily folds up to put in a backpack will be sweet. :mrgreen:

John

Here's a couple of pics
Solar panels 18V 5W raw front and back.JPG
Solar panels 18V 5W raw front and back with measure.JPG
 
Here's what Fechter came up with for a simple charge cutoff HVC, and said I could share with all. Thanks Fechter, you're a gentleman and a scholar, and I look forward to the day I can payback some. As I understand it, those wanting to run a different voltage will just need to use a couple of different values of resistor so the regulator part doesn't have to handle to large a voltage differential. Note also the higher the current the higher the heat, so I'm going to keep mine to a max of 1A per HVC module, so I will run a 3p string at the most to keep everything very low stress. It only looks like about $10 in parts to me, so multiple HVC's for a big array is no big deal.

Fechters HVC circuit.JPG

Here's the his text he sent me accompanying the circuit.

fechter said:
None of the components are extremely critical. Any flavor 431 will work and the NPN pass transistor can be about anything with enough rating. Most commonly available parts from Radio Shack have 60v limit, but this may be OK as the transistor will really only see the difference between the panel and the pack. The transistor could get pretty hot near end of charge, so should have some kind of heat sink. I'd guess 5W maximum dissipation. Normally less than 1W, so shouldn't need all that much.

The blocking diode shown is optional. It will add about 0.7v drop but will prevent any discharge through the panels at night and protect against some fault conditions. I think the NPN transistor will naturally block most of the current at night, so may be a waste to add a diode. Otherwise a fuse in that position might be a good idea.

The zener diodes D1-2 really can be any zener diode between 50-60v. If you can find a single one that's in that range you could use it. Otherwise you can place any combination of available zeners in series to get in that range. I know RS sells 15v ones and 12v are very common.

The opitimum efficiency circuit becomes considerably more complex depending on maximum panel current. The typical shunt regulator would throw off way more heat at end of charge.

With the resistor values shown, the adjustment pot VR1 should give a range of about 74v-82v. Just supply input and measure output with a voltmeter while adjusting.
 
It's alive ! I intentionally didn't recharge after this morning's ride, so my bike it currently being charged directly by the sun :mrgreen: No HVC yet, but it's just one string, so it would take a few days to fill it back up. Interesting though, here it is Dec 16, so almost the shortest day of the year and I'm getting 254mA of a rated max 280mA. That means I may really exceed max ratings when the sun is so brutal in Mar/Apr and Aug/Sep, our solar peaks. I still want to experiment a little, such as see if adding another module in series boosts current at all, and see what the effect is of concentration. With the back of the panel open to the air and shed heat easily I may be able to get away with concentrating the sun. It's so easy to carry a roll of reflective mylar film that increasing output by 1/3 or 1/2 may be realistic.

Now it's time to figure out how to make them portable, either folding (say using plastic hinges) or easily assembled from modules to panel. Anyone have any slick but cheap and easy ideas?

Here's what my well under 1kg 20W panel looks like. just 10 easy solder connections, a handful of screws, and a pair of 0.75" square balsa rods looks like....super simple for now just to get up and running. I wish all my ideas came together this easily working exactly as expected. 8)

First Solar panel charging.JPG
First Solar panel back.JPG
 
Ooops, almost forget the diode. The first afternoon cloud came and I had to go running out to disconnect. :lol:

That was close :oops:
 
Looking good there, Einstein. :lol: :lol: :lol:

Are you certain the V is high enough to input into the battery ?

I count 5 panels at 12V.
 
Thank you for sharing John and everyone else involved, i subscribed. After the first time i plugged in to charge, i wondered if something like this could be done, but i dont have much electronic experience, i'll be watching and waiting thanks again steve
 
Thanks guys. Harold as I noted in the first post they call them 18V nominal. The OCV (Open Circuit Voltage) was 106V for the 5. They are designed for a 12V car battery charger, so it may turn out that I do need to add a 6th module. I'll know better early am tomorrow when I check voltage and current with and without a 6th. I had enough of the balsa for 2 more, so those are ready to go for tomorrow, which might just be able to top off the battery during the course of the day tomorrow.

I'm starting to have those thoughts again about a road rocket ebike that I almost never have to plug in to charge, plastering it with solar to make it aero and make the aero tail able to fold out for much more panel area when parked. The 250W of modules I have on hand would make it pretty easy. Time for me to order a nice chunk of the 10W modules too, some for permanent on a couple of ebikes, some for a home based charging station, and some for a modular portable charger.

John
 
I saw that first post part, and did some thinking. :roll: :roll: :lol: With the big cells we have, they run no load at 18V, .5V X 36 cells. I want to see what you actually come up with, and, might be able to get a few with you on the next order.

If you haven't hit it exactly, you are very close. Nice job. :mrgreen:
 
John, Those are interesting panels. I like your idea. How much did they cost?
 
drewjet said:
John, Those are interesting panels. I like your idea. How much did they cost?

This was just a sample, and I'm still negotiating pricing. They weren't a product for sale, just modules that were at an intermediate step of becoming a car trickle charger like the ones that retail for over $10/watt. I did find some of those really cheap that I was going to buy to cannibalize, but they weighed 1kg for a 4W unit due to a glass face and a bulky charge controller. Shipping was going to make the price ridiculous, since I wasn't willing to invest in an ocean freight shipment of a bulk order of an unknown, especially since I didn't know if the cell module was attached to the heavy glass.

John
 
At 7am I got 75% of rated output and over 80% at 9am, so I definitely have the right module count for my 20s lipo / 23s lifepo4 packs for which I'm able to use the same chargers. I highly recommend that voltage for flexibility, especially since paralleling the 2 is better than either alone. The lipo replaces the useless surface charge of lifepo4, by turning it into a useful voltage range. Then as the lipo gets to about 50% of DOD the A123's take over. Later if you go past about 2/3rds of total DOD the lipo takes back over and stops the A123's from falling off the shelf.

John

edit- sorry I miscalculated the 9am figure the first time. It's 11am and it's at 90% of max rating now. 3 strings going now and .75A output. I think I'll fill the pack today for my first ever solar charge....an even bigger EV grin tomorrow morning. :mrgreen:
 
Man, that's going to feel awesome when you do that first ride on sun power. Good job working on this. I still have yet to take the plunge but I am working on it.

Today I found some big coroplast signs out for bulk pickup on the side of the road (will be used for making a tailbox on my recumbent. Easily bungie-ed them to my rear rack and sped home like nothing was there.

So for all you solar bike doubters, this is what a square meter of surface area looks like. That's 1kw of solar power, so depending on your panel efficiency, 100-180watts in high sun, WHILE RIDING.

That's enough power to do the bulk of the work for a low profile recumbent at 20mph. I think a 'perpetual motion machine' is in the works here....
 

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That's 3 days now without connecting to the utility co for transportation, and my main bike is almost full. I should be able to top it off in the morning. I have a long ride tomorrow. My buddy at the destination has a couple of cases of panels, so I'll have to get him to crack a box open to fill back up while at his place since mine aren't all built out and portable...yet. I'll need about 10ah, so even with what I have it wouldn't be enough to refill from 11am on. I hope he's willing to crack his open, because I'm going to try to go solar only for the rest of the year. 8)

Time to order up some more panels and get the solar charged central pack idea up and running, which will also open up the potential for ebike charging as fast as I want on solar power, something I don't currently have even with a wall plug.

I've formulated a plan for the portable panels, which is using cloth as my hinges, and concentric sizes of fiberglass tubing for structural supports and legs. Overall panel thickness should come in under 5mm, so a lot of power will fold and stack neatly in a backpack.

John
 
AussieJester said:
Well John, you definitely live in the right part of the world to make this viable, I wish the best in making this setup work for you...fact is I would like to copy it lol
KiM

Thanks AJ. Now's the time to get cracking, since the sun is nice and high for you guys now. Does the weather cooperate this time of year?
 
Still no juice from The Man since Saturday. I did over 40 miles yesterday, and thanks to a higher power array at my friend's place my bike is at about 85% SOC with some more charging time today. That's even after some hard riding on this morning's errands, embarrassing 2 motorcycles and a car actually had the nerve to think it could pass me. We ended up at the same destination, but I was parked and locked up before he even go into the parking lot. LOL !

John
 
yep we are into our summer months now John most days will be cloud free 30C+ right through to March or so... all good charging with Sun one problem is, it's too bloody hot to ride anywhere lol

KiM
 
AussieJester said:
yep we are into our summer months now John most days will be cloud free 30C+ right through to March or so... all good charging with Sun one problem is, it's too bloody hot to ride anywhere lol

KiM

Pretty much the same here, but through Apr/May, and the 30"s is down at sea level. Up here in the central valley it's mid 20's and perfect as long as there's a breeze, which is almost always. The heat at lower elevations means I'm either on my bike creating my own cooling breeze or in the water. :mrgreen:
 
How is $3/watt for these type of small pre-built panels? I'm about to pull the trigger to do the same type of slow charger. Small enough to fold up and easily carry on the bike.

I also like the idea of reflecting more sun onto the panel to increase output.
 
John I was always thinking about this. But I was thinking about a partial mirror setup to help feed the sun and focus it on fewer panels.

I have a funny vision of on of those strainers that you can open up its like a bowl that closes like a camera iris... And you use this to reflect on the solar panel.

But very cool non the less.
 
Arlo1 said:
I have a funny vision of on of those strainers that you can open up its like a bowl that closes like a camera iris... And you use this to reflect on the solar panel.

Those are for steaming vegetables I'm told. I use it for straining pasta though. The design would work on a larger scale I'm sure.
 
http://solarcooking.wikia.com/wiki/Collapsible_Parabolic_Cooker
http://zool33.uni-graz.at/petz/parabolic-mic/
http://solarcooking.org/plans/

http://www.google.com/patents/US7423609
 
How's the panel holding up?

I just finished putting mine together tonight. 85v 0.3a. The panels I got were really heavy - they say 1.2kg (must be less) each for 5w. Then I bolted them to a plywood sheet which I cut out sections of. Total thing is 11lbs :shock: Yeah I may not be carrying it around too often.

I'm on the lookout for some lighter weight panels that would be more portable.

Thanks for the links AW.
 
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