💬 Effective Solar Supercap Boost Charger for Small Solar Panel

[ncollins]

@neverdie according to a few datasheets 2v at 200lux. The solar cells delivered were HENGYANG SC-3514.

http://www.vimun.cn/en/ProductInfo.asp?pid=18
http://www.solars-china.com/solars/indoor-solar-cells.pdf

On my windowsill, 2.5v typical in-direct sunlight, 2.7v is the highest I've recorded in direct sunlight.

So far, most of my Aliexpress buys have been fine for my needs. Haven't really gotten close to making a "production ready" module, so for prototyping it's been fine.

One exception: I thought I had a genius idea to repurpose these $1 solar powered keychain flashlights. They had an amorphous solar panel, rechargeable battery, button, leds and a housing...for $1. I bought 20 of them. First one I opened up, I realized it's a lie!

The solar panel is just glued to the circuit. The battery is just a normal non-rechargeable CR2016.

[alexsh1]

@neverdie said in 💬 Effective Solar Supercap Boost Charger for Small Solar Panel:

@ncollins said in 💬 Effective Solar Supercap Boost Charger for Small Solar Panel:

@neverdie they work super well. I've been testing them for a few months now, very reliable.

Great! Nice to have your feedback. :)

I had high expectations for Ceech's LTC3108 design:
https://www.ebay.com/i/331654685113?chn=ps&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&itemid=331654685113&targetid=809743845025&device=c&mktype=pla&googleloc=9028292&poi=&campaignid=6470262913&mkgroupid=80364172271&rlsatarget=pla-809743845025&abcId=1141166&merchantid=115344895&gclid=CjwKCAjw29vsBRAuEiwA9s-0B7exYA8yU9w--t63jcXP7QWO_f05DzJKNxw2XS6pwlLkPkfK1UYqkxoC084QAvD_BwE
but when I tested it, it turns out it needs about 5ma of current to run, which is at least 10x+ more than most small solar panels will deliver under even quite bright indoor lighting. Basically, if a panel can produce 5ma of current, the voltage will also be fairly high, thereby completely defeating the purpose of a chip like the LTC3108, which can be powered from 20mv. In my testing it took a minimum of 50mv and 5ma of current to get it to actually harvest any energy. 50mv would still be impressive, but the 5ma minimum is what kills its usefulness.

One of the best modules @ceech made was this one

https://www.ebay.co.uk/itm/BQ25570-thermal-solar-energy-harvester/332071662285?_trkparms=aid%3D1110001%26algo%3DSPLICE.SIM%26ao%3D1%26asc%3D20131231084308%26meid%3D759b68bcf5834d74bfa46f9df6b8ae4d%26pid%3D100010%26rk%3D1%26rkt%3D6%26sd%3D332393146893%26itm%3D332071662285%26pmt%3D0%26noa%3D1%26pg%3D2047675&_trksid=p2047675.c100010.m2109

It has been running non-stop for many months now

[NeverDie]

@alexsh1 Does it work well even under extremely low light indoor conditions?

[NeverDie]

What the BQ25570 has working against it is a cold-start voltage of >600mv. The LTC3108 doesn't really have a cold start voltage. It can in theory start with as little as 20mv (though in the case of Ceech's module, 50mv).

On the other hand, the BQ25570, once it reaches its cold start voltage, can operate with far less current than the LTC3108. And once the BQ25570 gets going, it can continue operating down to 100mv. That part is very attractive indeed. I think I'll give it a closer look.

[NeverDie]

I found the smoking gun:
"Q. What about solar harvesting applications with the LTC3108?

A. In general, you should look at the LTC3105 first for these applications. The LTC3108 requires a minimum input current of a few milliamps (at the converter input) just to startup, and may not be a good load match for a PV cell. Therefore, small solar cells that have a short circuit current of less than a few milliamps will not work with the LTC3108 (or LTC3109)."
https://www.analog.com/en/technical-articles/frequently-asked-questions-thermoelectric-energy-harvesting-with-the-ltc3108-ltc3109.html

[alexsh1]

@neverdie It does work under very extreme conditions (dark or very cloudy days for weeks), but I have not tested it extensively with internal lightning. The solar panel is very close to a window. I may be mistaken by I do not think LTC3108 is designed for low current (< 1 mA) application. BQ25570 is absolutely brilliant.

[NeverDie]

Because of the passage I quoted above, I re-visited my earlier LTC3105 project:
https://www.openhardware.io/view/281/Solar-Energy-Harvester
and gave it a closer look. As a result, I now have it starting the up-conversion at 270 millivolts (a slight derating of the advertised 250mv). It can fairly quickly upconvert from 270mv input to 2.63v output (which is what I picked as a "safe" voltage for charging a supercap). It does consume much less input current than the LTC3108 does, but if you put it in a dark enough room it will stall before starting up, which is a bit disappointing. That said, though, I can fix it using a similar technique that I used for the topic of this thread.

Meanwhile, from amazon.com, I ordered the solar boost charger module that recently won the hackaday prize: https://www.amazon.com/gp/product/B07JFNSPQ3/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1 I'll be interested to see how well it handles a dimly lit room!

[NeverDie]

@ncollins

I'd like to do the same trick with an LTC3108 (namely, pulse it on when it hits a voltage threshhold at, say, 200mv or less), but the trouble is that the voltage detection chips need a supply voltage higher than that to work. So, it's a bootstrap problem that's difficult to get around. Including a small button cell battery as a workaround to that problem feels like cheating, but the alternative might be to build some kind of ultra low voltage trigger, and I'm not sure how to do that. An SCR trigger would maybe work, but it might have a 0.6v threshhold. The EPAD Mosfet's might turn on a 0.2v. Just not sure if it would be a clean switch-on or not.

I suppose using a rechargeable watch battery for the same purpose would feel a bit less like a cheat. Do you happen to know long do they typically last, and what's their rate of self discharge?

[ncollins]

@neverdie this is the data sheet for the authentic version of the watch batteries I'm using:
https://www.sii.co.jp/en/me/datasheets/ms-rechargeable/ms412fe-5/

I tried to order a few of the AEM10941 ICs but they quoted me ~$100. I guess they still don't have a US distributor?

Personally, I'm struggling to justify the cost of the harvesting modules. I think ultra high efficiency indoor solar panels that put out usable voltage at very low light levels + supervisor IC with adjustable hysteresis should provide comparable performance with significantly less complexity/cost.

[NeverDie]

@ncollins Well, you're in good company then. Cypress Semiconductor seems to have come to the same conclusion as you: using higher quality solar cells in series to jack up the voltage. Cypress seems to like the ones made by panasonic.

On the other hand, why not do both? This guy has a design for what looks like an inexpensive boost converter that he says requires as little as 10ua standby and has 90% efficiency:
https://www.davidpilling.com/wiki/index.php/DCDC
Not sure what the start-up voltage is though.

[NeverDie]

@ncollins Maybe TPS61200 would be a good compromise. It's cheaper than the other harvesters, and it has a UVLO centered on 0.3v, so maybe it wouldn't need a separate circuit to keep it from crashing potentially weak output of a solar cell.

My main reservation about it is that it won't be as easy to solder as an LTC3108 or LTC3105.

[ncollins]

@neverdie said in 💬 Effective Solar Supercap Boost Charger for Small Solar Panel:

TPS61200

Looking at the example Single Solar Cell usecase http://www.ti.com/lit/an/slva345b/slva345b.pdf, it seems the expectation is minimum input 3ma at 0.5v? Maybe for startup, but lower current requirements after?

As for the small solar cells in series, I have thought about that too, but occasionally I have sensors that get hit with direct sunlight. I've fried a couple of NRF24 modules that way. I like the idea of using a 5v amorphous solar panel charging a 5.5v supercap + an LDO (with a passthrough?).

I want a harvesting module that boosts and dumps voltage into a very small supercap 2.7v with enough capacity for one boot/wakeup + sensor read + transmission. Once the small supercap is charged, the input voltage boosts up to 5v and is redirected into a larger storage supercap. , the the small super cap should be charged by the storage supercap when charge input is too low to be boosted. If necessary, the storage supercap could be boosted to charge the small supercap.

Basically, do whatever it takes to get ready for the next transmission, then store in a reserve. If under voltage, boost it, if over voltage route through LDO, else passthrough.

[NeverDie]

@ncollins I think the booster designs are good fun for seeing just how far one can push the limits. So, with that in mind, I decided to do an exploratory breakout board for the most extreme booster out there , namely the LTC3108:
https://www.openhardware.io/view/732/Extreme-Energy-Harvester
For solar cell applications in dim light it may need to be managed though, because otherwise it may (?) collapse the input voltage and simply stall. i.e. it needs to be pulsed. If I'm lucky, maybe with a large enough input capacitor it will pulse itself. If not, it will need some kind of external control to manage it. Unfortunately, datasheets rarely provide that kind of insight, so in most cases one simply has to build and test in order to know for sure.

Maybe a trigger circuit simiar to this would work, but it woud need to work at a very low voltage level, and therein lay the rub:

Maybe one of these jfet oscillator circuits would work at an appropriately low voltage:
http://www.dicks-website.eu/fetosc/enindex.htm

The trick would be getting it to work not only at a very low supply voltage, but a very low supply current as well.

If anyone has ideas or suggestions, please post.

[NeverDie]

@ncollins I've noticed that a number of the dollar store "solar calculators" are fake as well. They give the appearance of being solar powered, but in reality they run on a small battery instead.

[ncollins]

@neverdie I was surprised to find the deceptive $1 solar keychain actually used a decent indoor solar cell. I can't find similar spec'd solar cell for less than $3 (at 10-20pcs). Despite the blatant false advertising, they're actually worth salvaging, especially with a solderable CR2032.

[NeverDie]

@ncollins Is there a better forum than this one for discussing solar energy electronics? I've posted what I can, but it's difficult to find help/collaboration here.

[ncollins]

@neverdie Not that I've found. A thread here, a thread there, but nothing focused. Might make sense to create a new mysensors forum category, Energy Harvesting?

Forum is a good place for conversation, but I think a wiki would really help document the results of those conversations. Also a place to provide some intro material: What is energy harvesting? Why pursue it? The easy route, the advanced route...

I'd gladly help with the content.

[NeverDie]

I recently went looking on some of the Joule Thief forums in the hope of perhaps finding some useful circuits there. However, the problem that I repeatedly ran into was that because the inductor used by Joule Thief devotees is typically hand wound from whatever junk is laying around, it looks as though it may be difficult t to repeat their successes. It might have been easier to leverage their work if they had used standardized parts.

[NeverDie]

@ncollins said in 💬 Effective Solar Supercap Boost Charger for Small Solar Panel:

@neverdie I was surprised to find the deceptive $1 solar keychain actually used a decent indoor solar cell. I can't find similar spec'd solar cell for less than $3 (at 10-20pcs). Despite the blatant false advertising, they're actually worth salvaging, especially with a solderable CR2032.

How much voltage/current/power can they produce?

I've noticed that some solar cells, especially cheap ones, can leak away their current almost as fast as they can produce it.

The "fake" solar cells in your photos loosely resemble these amorphous solar cells from Panasonic: https://www.digikey.com/product-detail/en/panasonic-bsg/AM-5907CAR/869-1013-ND/2165198

[ncollins]

@neverdie In direct sunlight, I've seen 5.5v at around 4ma.

These seem to be the cells SC-3222-9 SC-3722-9 (edited).
http://www.vimun.cn/en/ProductInfo.asp?pid=17