💬 Effective Solar Supercap Boost Charger for Small Solar Panel

[NeverDie]

@gohan
If you're comfortable having the MCU manage the charge limit, you could simply use a PFET between the solar panel and the supercap. Have the PFET gate connected to ground through a pulldown resistor, so that the PFET initializes to "open" and thus default to charging mode. When you want to shut-off the supercap charging, drive the PFET gate pin high using one of your ATmega328P digital pins. I've done this previously, because I was attempting to automatically measure the open circuit voltage on the solar panel. I don't recall now whether it worked fine just like that, or whether I had to drive the PFET with an NFET, but however I did it, it seemed to work fine as far as disconnecting the solar panel and preventing it from charging anymore.

[gohan]

I'll see when I'll have all the parts

[NeverDie]

@Nca78
Interestingly, in some scenarios it's possible to just do a kickstart and then switch over to non-pulsed mode. So far I've only done it manually, so I haven't yet worked out circuitry to do it or circuitry to decide when it's appropriate and when it isn't. Based on preliminary work, though, it looks as though the same basic technique may be applicable to buck converters also. In a way, kick starting buck converters should be easier to solve, because the starting voltages are (obviously) higher, so finding components which work at those voltages will be easy.

[alexsh1]

Where did you get S-1009N081-I4T1U voltage detector please? Cannot find it on Farnell or Arrow.

[NeverDie]

@alexsh1
Digikey.

[alexsh1]

@NeverDie It is not available on Digikey either currently. Any idea what I could use as a replacement please?

[NeverDie]

@alexsh1
It turns out Digikey does have it, but their own search engine couldn't find it. Very odd. Here's a link:
https://www.digikey.com/product-detail/en/sii-semiconductor-corporation/S-1009N08I-I4T1U/1662-1182-1-ND/6601322

Actually, any of these would work in theory:
https://www.digikey.com/products/en/integrated-circuits-ics/pmic-supervisors/691?k=S-1009N&k=&pkeyword=S-1009N&pv846=33&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=500
except that not all of the would fit the land pattern.

Notably, though, I see they are showing a "new" product entry which wasn't there before:
https://www.digikey.com/product-detail/en/sii-semiconductor-corporation/S-1009N08I-M5T1U/1662-2290-1-ND/7228582
which is a good thing, because it is larger and would be easier to solder.

Are you able to get that one? I could re-do the PCB and customize it for that one instead.

[alexsh1]

@NeverDie Yes, I am able to get both now. Strange thing that Digikey did not find it.

I am confident with SMD soldering. I do not have an oven, but I am using hot fan and do it by hand.

[NeverDie]

This remains the best boost charger for small solar panels that I know of. All the other low voltage boost chargers made by others that I've tried fail, probably because when a small solar panel has low voltage, it typically also has very low current. Solar panels don't produce much current indoors, and if it's outdoors you probably don't need a boost charger anyway. This is the only boost charger for small solar panels that I know of which works indoors under low light conditions.

It's conceivable that a solar charger based around either the ADP5090 chip or the SPV1050 chip might work as well or better, but I haven't tried either one. If anyone else reading this has tried them, please post and let us know how well they perform on small solar panels.

Enocean previously sold a solar charge module, the ECT-310, but I can't find it in stock anywhere:
https://www.enocean.com/en/products/enocean_modules/ect-310-perpetuum/

The AEM10941 claims to work under indoor light on small panels. It won the Hackaday prize, so maybe it's one of the few that actually work under those conditions:
https://www.tindie.com/products/jaspersikken/solar-harvesting-into-li-ion-battery/

[ncollins]

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

One of the first supercap setups I made, "SolarRed". It's been running non-stop for 8ish months sitting indoors on a windowsill.

6 Month History SolarRed

Testing platform for different solar panel / super cap combos

[NeverDie]

@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.

[ncollins]

@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.html

I 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.html

BQ25504
https://www.aliexpress.com/item/32976994195.html

From 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.html

3v Rechargeable Battery
https://www.aliexpress.com/item/32813917590.html

1206 100uf Capacitors
https://www.aliexpress.com/item/32376068793.html

Cheapest amorphous, indoor solar panels I have found: 2.7-3v
https://www.aliexpress.com/item/1854641441.html

Tiny S4 1N5817 Diodes
https://www.aliexpress.com/item/32813213875.html

[NeverDie]

@ncollins Very interesting! What's the max volts that those TRONY solar panels put out? Is it 0.5v or 2.0v?

[NeverDie]

@ncollins For small but high quality solar cells, digikey now has a pretty good selection:
https://www.digikey.com/products/en/sensors-transducers/solar-cells/514?FV=ffe00202&quantity=0&ColumnSort=1000011&page=1&pageSize=500
Those made by IXYS in particular are quite good, especially for being so tiny.

I hadn't had much luck with the aliexpress capacitors. Their self discharge has tended to be relatively high. Maybe you found some good ones though. After trying quite a few I more or less settled on using AVX supercaps instead.

[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.