💬 The Harvester: ultimate power supply for the Raybeacon DK

[NeverDie]

@Mishka Looks as though an array of BPW34 can actually be packaged together fairly tightly.

It still wastes some real estate, but not as badly as what I had imagined.

I suspect they wouldn't do well under indoor LED lighting though, as they seem to have peak sensitivity at around 900nm, which is infrared.

[NeverDie]

@Mishka The specs on the fx-260 solar calculator says it needs at least 50 lux for adequate power, but Dave Jones tested it on youtube and found that 20 lux was sufficient. Unfortunately, he didn't measured the shorted current, so it's hard to know how it compares to the cell on the solar calculator that you tested.

[Mishka]

@NeverDie The cell probably is not so sophisticated as the one installed in the fx-260. Also, from four visible sections only three are working. Sigh.

It seems the calculator can acceptably work as long as it supplied with 1.3V or more. This roughly estimates to 40 lux for this particular PV cell. However, the display is the resource hog - every additional digit causes voltage drop. Usable number of digits - five or less. After reset the calculator draws 3µA. Every "8" adds up 0.1µA thus reaching PV short circuit values very soon. The IC seems completely unregulated - at higher voltages it draws proportionally more, up to 1mA.

It's also possible to gain little bit more power (about 70mV) if bypass polarity protection diode. However, it might be rather better to employ 10µF capacitor to compensate for keypress actions. With 92µF it's also possible to enter longer numbers, but you have to type them fast, then store the number in memory, then quickly clear display and wait until the capacitor charges. Now you can type another number, press operation, and, finally, MR and = to calculate result :call_me_hand:

[NeverDie]

@Mishka This youtube video ranks the various different types of solar cell technologies:
https://www.youtube.com/watch?v=8t_DFI4O6v4

According to it, the Spectrolab XTE-LILT would be the highest performing, and as a bonus it does its best when under low light conditions, where it has 37% efficiency--head and shoulders above any other brand. Not quite double the performance of the best monocrystals, but almost. However, so far I haven't found it for sale anywhere.

[Mishka]

@NeverDie Well, looks like that is the space mission panel :space_invader: :space_invader: :space_invader: Won't be cheap anyway. On the other hand recently I seen a lot of news about so called nighttime photovoltaic power, who knows how fast it can be delivered to market.

In a meanwhile, I had a chance to play with the Harvester board. Please don't consider everything below as any serious test or a comprehensive review - I just rather tried to understand how the modules are working and what can I get from them.

Following our prior discussion, I've assembled one board in buck-boost mode (the original PCB), and have tweaked another one to the boost configuration. The only solar panels I have are:

• IXYS KX0B25-02X8F. Single panel is annoyingly weak so I connected two of them in series.
• The SCNE SC-1338 from the test above.
• SORBO SB-3077. A big (79x40) panel from solar torch. Didn't tested it like the other two, but noticed about 6V from it.

For the load there is 1500µF 4V tantalum capacitor installed. V_uvp threshold has been set to 2.2V, and V_eoc to 3.2V. Please also note, the MPPT resistor values were set to V_oc=12V (which is too high for all the tested panels) and no adjustments were made to them for any of the tests.

Most of the tests I made in the evening at home where LED lights are the most common source. I used light sensor on my phone to measure light illuminance. Looks like it gives more or less sensible values; such, 50 lux corresponds to a dim light, 250 lux is an office light or indoors illuminance in cloudy day, 1000 lux is outdoors light in cloud day - similar values I've seen in tables over the Internet. After checking illuminance I placed solar panels in the same area and measured most important voltages:

• V_pv - PV panel voltage when it is under load (but see my comments)
• V_store - voltage at the spv1050 store capacitors (2x47µF)
• V_out - voltage on the output capacitor (1500µF)

For low light conditions I waited several minutes for the capacitors to charge and read values only when the system became saturated (or at least look so). Results and my notes are represented in the following table:

The spv1050 datasheet provides quite detailed description on how it works. The only interesting question was - what happens in buck-boost mode if the harvester cannot maintain V_store voltage anymore due to low illuminance. I found that store and the load will be connected as long as V_out>2.2V. After that, spv1050 will shut down the DC/DC and connect PV to the store. I think DC/DC was on because panel voltage raises up from 0.25V to 0.52V when the store disconnects from the load. Why is it 2.2V and not 2.6V as specified in the datasheet, I don't know.

Another interesting test was how much time it will require to charge the 1500µF capacitor to 3.2V. For that I tried different panels with both buck-boost and boost boards. Please note, since the boost board has 4V limit (due to the tantalum capacitor) and pair of the IXYSes are capable to produce up to 8V, I've skipped this couple. The results are in the table below:

You was absolutely right when expressed the concern on the SolarBit panels - they work much better in the sunlight. The SCNE seem also prefers full spectrum to my LEDs.

Evaluate whether this is a good or bad result is possible when considering the load itself. Accordingly to the Nordic, the nRF52840 will consume about 32µC
per heavyweight BLE event, for example, when running at 2.2V (lowest possible value for the spv1050) and sending +8dBm advertisements. For tiny packets it may be as low as 5µC.

The tantalum capacitor stores 1500µF*3.2V = 4800µC - exactly this amount was delivered by the spv1050. This roughly equals to 150 advertisements, or 960 connection packets. If ignore other waste (like sleeping current) and assume 5 minutes as an average result on SCNE-alike panel (indoors, daylight), the system should be able to advertise once every two seconds or send small data packets once every 1/3 second without discharging a battery.

Of course, more light means more power, and there is the night too, but with proper panel and correct location I think it should be possible to maintain positive power balance.

So which configuration to prefer - boost or buck-boost? I still have no simple answer.

The boost is good when you're working in low-light conditions and can keep it low-voltage. I also have impression that the spv1050 is sensitive to the current it can get from a panel. Therefore a couple of single cell panels (i.e low voltage, high current) might be the right choice. On the other hand, it's risky to attach high voltage panels to the boost harvester - neither battery not capacitor will tolerate 6V.

The buck-boost configuration has no such drawback - any panel you may find on attic will very likely to work. Another advantage you might have noticed is that it delivers faster. When fast is fast enough - it depends. But in the low-light when it may took up to 30 minutes to charge a 1500µF capacitor the boost advantage is not so obvious.

[Mishka]

And couple of words about the Harvester LDO - it works. Also, being connected right to the spv1050 store it could be used to bootstrap the harvester, but if connected full time it will soak about 8µA which is too much for a battery powered board. So still kinda hack, and its primary purpose is to supply the board via USB.

[NeverDie]

Your results are consistent with what I've seen with other harvesters.

1. Their #1 goal is to get out of the cold boot mode and to a voltage level where regular electronics can power up, resulting in much more efficient energy harvesting. So, until that minimum voltage is reached, they typically keep all their energy and don't output any. I think that's the right approach, so I have no beef with that.
2. I'm not a fan of built-in LDO's either, and for the same reason: in a marginal situation, you don't want to waste any more operational or quiescent current than you have to.

You might enjoy this eevblog video, where the Dave Jones measurements show that the FX260 can handily power its LCD display in idle mode with just 2v and 2ua, and just barely so at 1v and 1.5ua:

https://youtu.be/_VvEO_m3Owk

It seems to keep its memory alive, for awhile at least, on less than that after the LCD display has gone blank.

I've read that TI made a series of calculators (mostly in the 1980's) called the "anylite" series, which were designed to function without a battery in pretty much even dim indoor light. They used bigger than average solar cells. Asdie from their size, I'm not sure if there was anything special about them. Maybe. I haven't tried one myself, but I suspect so. However, It appears to me that these days though TI has gone the "hybrid" route of stuffing a battery in there, and the size of their solar cell is a lot smaller now. I'm not sure what the value of that is, other than simply extending battery life. The FX260 is one of the few that still offer the solar-only with no battery option. Its LCD display is quite crisp, so it doesn't appear to suffer from doing so.

[Mishka]

@NeverDie What I meant is that in addition to internal LDOs in the SPV1050, there is also the MIC5205 3.2V which is sourced from the USB (can accept up to 16V). This LDO output can be connected to the spv1050 store capacitors via relevant solder jumper on the board bottom, and when connected and not powered it draws about 8µA from the store. So the jumper is kind of requirement.

I've enjoyed the calculators video. BTW my calculator from which I took the solar panel has similar characteristics. Regarding reuse of calculator panels, first of all they must match the application voltage. If speaking about the nRF52, it has to be 1.8V to 3.6V (or up to 5.5V), but a small panel can't supply it under load. Also, the radio can draw up to 16 mA which is possible only with larger panels. Of course, a capacitor can fix online use of a solar panel, but in order to work with lights off a secondary battery required, hence the solar charger.

And now it runs out into a game of coulombs. Today I've tested the boards in direct sun light. They charge the 1500µF capacitor in few seconds and then just idling. It looks like this power excess is more suitable for a battery rather a small capacitor. But a 2032 size supercapacitor could also make it. It looks slightly more interesting strategy to fast charge battery when you can and then work from the battery, than struggle to charge the battery in the lowest possible conditions and hence limit charging speed. Please also note, it is usually easier to build high-voltage panels rather high-current ones.

Anyhow, there are too many influential factors so a practical test is required. I think I have ot try two competitive configurations placed in exactly the same environment:

• 2 x KXOB25-02X8F panels (5.53V, 6.3 mA) and the buck-boost board
• 2 x KXOB25-05X3F panels (2.07V, 19.5 mA) and boost only board

[NeverDie]

@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:

But a 2032 size supercapacitor could also make it.

Maybe, though I'mskeptical of capacitors that don't list ESR in their datasheet.

This one is kinda interesting: https://www.digikey.com/product-detail/en/UMAL201421A012TA01/490-13963-ND/6152301/?itemSeq=317110112 I found it in the supercap category of digikey as a 50F capacitor, but it's actually a small surface mount rechargeable battery with what is described as a long cycle life (still has 90% of its capacity after 5K charge cycles). Looks as though it has been discontinued, but there are still a few left in inventory. If used as a backup of last resort in combination with a supercapacitor, maybe it would last a very long time. Looks as though its self discharge rate is pretty good at normal ambient temperatures, so that might be possible. If you averaged only one charge cycle per week, then it would effectively last 100 years and still have 90% of its capacity remaining. Sounds like it could be a really good way to avoid and/or manage the cold-boot phase of whatever energy harvester you might end up using.

[NeverDie]

@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:

So far the Raybeacon was finally assembled and sent to Nordic for tuning - you may have noticed updated pics on its page.

What sort of tuning is it that Nordic does? This is the first that I've heard of Nordic doing it.

[Mishka]

@NeverDie They do review schematic and layout of their SoC and then can do antennas tuning. For the tuning two fully assembled devices (including housing) are required.

[NeverDie]

@Mishka Without modifications and under dreadful low light conditions, all of the fancy pants commercially available ultra low voltage energy harvesters I've tried for solar seem to be much more current limited than voltage limited. And by limited, I mean the difference between working and not working. For that reason I wonder (?) if maybe for the boost-only configuration you'd be better off with a 0.5v cell of the same dimensions. I haven't made up my mind on the matter yet, but that's where my thinking is starting to trend.

[Mishka]

@NeverDie Yeah, perhaps at low light a weak PV battery simply can't sustain forward current for all the semiconductors to make them work properly. I think the 0.5V cells may be interesting to try too. My only concern is that the SPV1050 requires at least 0.15V to work which is roughly at 15% threshold of the log-alike illuminance-voltage chart - the 2V panels will have it at about 4% and look most promising. Please note, 2.6V buck-boost requirement is at 29% for two 4.5V panels.

I want to give a try to all three kinds of panels, but little bit later using newer PCBs where I hope it should be easier to switch harvester into the boost mode. For now going to fix found issues and release v1.0 as buck-boost only.

[NeverDie]

@Mishka I ordered a 0.5v sunpower solar cell to play around with. About 22% efficiency. It's different from typical solar panels in that all the wiring is on the back of it. No metal tape on the front of it, which is at least partly how it gets the higher efficiency (by using 100% of the surface that's facing the sun). It's also slightly flexible. I'm unsure, but it may possibly (?) be cut to size. All I know at present is that if you cut it in half with scissors, both halves continue to work (this was demonstrated on a youtube video).

Also, on a different topic, I found this solar bluetooth beacon that cypress semiconductor had been selling for a while, but which is now discontinued:

Similar form factor to what you are attempting with the Nordic, except that in their case they crammed everything onto a single PCB, because all they needed was a Bluetooth beacon and nothing more.

For its energy harvesting chips, Cypress went the route of requiring a minimum 2v for their energy harvesting to work, but their harvester can operate with just around 1uWatt of power, which if I'm not mistaken, means it can operate on about 500na, which is far less than the Linear Technology energy harvesting chips that I have so far tried.

Lastly, but largely off topic, I found it interesting that Cypress's new generation of ARM mcu's (called PSOC 64), and due to be released sometime soon, will have dual processors (akin to the ESP32), but one operating on as little as 0.9v and the other on as little as 1.1v. And those will have integrated bluetooth and wi-fi as part of them to make for a single chip solution. That will surely help dim lighting scenarios for solar harvesting. Exciting times we live in.

[Mishka]

@NeverDie Cutting panels should just work. I'm unsure how do you attach wires to it though. I'll be grateful if you will share your findings.

This Cypress BLE sensor looks very sexy. It's definitely not so flexible as the Raybeacon, but it no doubt sets the tone and is ahead in both design and technology. I'm now thinking should I switch my nRF52 board from simple two-layers to high density interconnect and WLCSP - the aQFN-73 still not too easy to solder anyway, but it imposes serious space constraints.

I like the size and shape (still considering rounded square though) - it's about what I expect from a battery operated embedded development board. Also, I found the board area is enough to build a custom DIY module using 0603 components. But routing and component placement on the main board is somewhat tight. Maybe issue a "pro" version - with all features of a development board, but integration level of a BLE module? At least it will allow place the buttons symmetrically :)

[Mishka]

@NeverDie said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:

I found it interesting that Cypress's new generation of ARM mcu's (called PSOC 64)

Yeah I remember how I played Doom on $1500 386-based personal computer, and now I can play it on $5 SoC powered by a coin cell. Awesome!

[NeverDie]

@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:

I'll be grateful if you will share your findings.

Yes, of course. I ordered it just today, so I'll update you after it arrives if I learn anything from it. I suspect the wiring issue you flagged may allow the width to be reduced but prevent the length from being shortened. Anyhow, if not this one, then probably some other solar cell exists that can be trimmed to fit after-the-fact--hopefully one with good efficiency!

As for the cypress solar beacon, it actually does a little more than just that: it broadcasts the temperature and humidity from a couple of onboard sensors. Nonetheless, Cypress describes it this way: "The Solar BLE Sensor is ultra-low power and works with just solar energy." ... and yet, isn't that a small battery I see on it? Maybe it's rechargeable, and cypress opted for that so as to save space as compared to a supercap? I haven't looked into it, but that's my guess.

I'd say they did a good job of squeezing it down to the size of quarter. It makes me wonder just how much smaller it could be made. Interestingly, even though the solar panel takes up a lot of board real estate, it's not much worse than a coin cell battery. So, if it could still run on a solar panel that's even smaller, then the whole thing could be shrunk even more, and then solar would be an even more tangible bonus than merely not needing to change batteries.

[NeverDie]

@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:

still considering rounded square though

I think one can argue that for a solar node a square/rectangular PCB (with or without rounded corners) is perfectly valid. I think the circular designs were primarily just trying to match the minimum footprint of a CR2032 or similar battery, since without solar that's what limits the smallest PCB size possible.

[Mishka]

@NeverDie Well, round shape has property of manhole cover - it can't fall through :-)

Another thing I'm thinking about is to get rid of CR2032 and build it on a 3rd party module and a supercapacitor. This way It can be much smaller, but number of other drawbacks may come. Smaller size usually means worse radio performance. Also, the smaller the board, the higher integration required thus severely impacting production cost. This especially may affect expansion modules which are usually a DIY thing.

[NeverDie]

@Mishka If you do decide to try for a smaller design, there exists a tiny (3.2mm x 2.5mm) 11mF supercap that's rated for 10,000+ charge cycles and that might perhaps be just barely enough capacity to do some minimum amount of work:

https://www.sii.co.jp/en/me/datasheets/micro-battery-2/cpm3225a/

or possibly one of the variants that the same company makes.