💬 Effective Solar Supercap Boost Charger for Small Solar Panel
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@neverdie Yep, I had the same experience with this LTC3108 breakout.
https://www.aliexpress.com/item/LTC3108-1-Ultra-Low-Voltage-Boost-Converter-Power-Manager-Breakout-Development-Board-Module-Diy-Kit/32867270266.htmlI also intended to try the following modules which have a ton of functionality baked in. Unfortunately, they multiply the cost of a basic node by 2-5x.
BQ25570
https://www.aliexpress.com/item/32903287631.htmlBQ25504
https://www.aliexpress.com/item/32976994195.htmlFrom an energy harvesting perspective, I think rechargeable watch battery + 0.2F super cap + 4 100uf is my favorite setup. Cheap, fault tolerant, very small profile if you use 1206 ceramic capacitors (not shown in this prototype). Based on this design: https://www.allaboutcircuits.com/technical-articles/overview-of-the-ble-solar-beacon-from-cypress-semiconductor/
0.2F 3.3v Capacitors
https://www.aliexpress.com/item/32577753501.html3v Rechargeable Battery
https://www.aliexpress.com/item/32813917590.html1206 100uf Capacitors
https://www.aliexpress.com/item/32376068793.htmlCheapest amorphous, indoor solar panels I have found: 2.7-3v
https://www.aliexpress.com/item/1854641441.htmlTiny S4 1N5817 Diodes
https://www.aliexpress.com/item/32813213875.htmlI'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?
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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?
@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.
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@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.
@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 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.
@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.
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@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.
@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.
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@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.
@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.htmThe 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.
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@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.pdfOn 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.
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@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.
@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.
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@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.
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@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.
@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.
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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.
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@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.
@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
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@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
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@neverdie In direct sunlight, I've seen 5.5v at around 4ma.
These seem to be the cells
SC-3222-9SC-3722-9 (edited).
http://www.vimun.cn/en/ProductInfo.asp?pid=17@ncollins said in 💬 Effective Solar Supercap Boost Charger for Small Solar Panel:
@neverdie In direct sunlight, I've seen 5.5v at around 4ma.
These seem to be the cells SC-3222-9.
http://www.vimun.cn/en/ProductInfo.asp?pid=17Come again? I don't see a cell on that list with the name of SC-3222-9.
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@ncollins said in 💬 Effective Solar Supercap Boost Charger for Small Solar Panel:
@neverdie In direct sunlight, I've seen 5.5v at around 4ma.
These seem to be the cells SC-3222-9.
http://www.vimun.cn/en/ProductInfo.asp?pid=17Come again? I don't see a cell on that list with the name of SC-3222-9.
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Which do you prefer? Those or the SC14351? For powering up the cold start electronics quickly, what I need is high voltage more than I need high current. Your solar cells are inexpensive enough that they might be a good fit for that.
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@ncollins For instance, for the price it might be a good fit for this:
https://www.openhardware.io/view/733/Buck-Energy-Harvester -
Which do you prefer? Those or the SC14351? For powering up the cold start electronics quickly, what I need is high voltage more than I need high current. Your solar cells are inexpensive enough that they might be a good fit for that.
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@neverdie I've primarily been prototyping with the SC14351 because I don't have to worry about voltage regulation when coupled with supercaps.
@ncollins Are you using the Tantalaum's primarily because they're cheap, or for some other reason?
You may be interested in this as a tiny, yet cost effective alternative: https://www.digikey.com/products/en?keywords=728-1067-1-ND
The ESR is rather high on it, but it might be good for running very low current circuits, such as logic. -
@ncollins Are you using the Tantalaum's primarily because they're cheap, or for some other reason?
You may be interested in this as a tiny, yet cost effective alternative: https://www.digikey.com/products/en?keywords=728-1067-1-ND
The ESR is rather high on it, but it might be good for running very low current circuits, such as logic.@neverdie I used tantalum because they were laying around and I was too impatient to wait for the 1206 100uf ceramic caps I had ordered.
Those are definitely interesting. The high ESR on the lower capacitance coin-cell "H-type" 0.47F super caps caused a voltage drop significant enough to trigger brownout on my first few nodes.
I was hoping to try these out, but they seem to have been discontinued https://www.mouser.com/new/Murata/murata-dmh-supercaps/.
