Powering mote 24/7 using only a supercap and solar
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I notice now that leaving the BQ25504 connected to the supercap at night when there's no light to harvest does result is a cumulative drain of about 50 millivolts to the 15F supercap (charged to 3.340 volts) versus disconnecting it during the same period.
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I finished the LTC3501 energy harvester (https://www.openhardware.io/view/281/Solar-Energy-Harvester), but, disappointingly, its MPPC seems to be no match for the MPPT of the BQ25504 for the el cheapo solar cell.
I think the next step will be to modify the BQ25504 breakout board to include the supercap. Going forward that should keep everything neat and tidy and avoid loose wiring.
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I've designed the load switch and the supercap onto the PCB, and I just now sent it off to the fab. The entire board is less than 0.9 square inches in size.
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I'm looking forward to this kind of solution. Keep up the good work
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Another notion I'm toying with is a variation of the proverb "make hay while the sun is shining." In other words, squirrel away as much solar energy as possible when conditions are optimal, and not worry too much about extracting light under truly dim conditions. I think there's a happy medium to be found, and I hope that by playing around with it a bit I'll develop a better sense as to where that sweet spot is.
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Here's the version that I most recently assembled:
As you can see, the 15F supercap is now on the board itself. It works fine.I've since made a few refinements and have sent the new files off to be fabbed. The newest version of the PCB will measure roughly 22mm x 22mm.
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I'm very curious to know what kind of sensor you would be able to run with this project. Of course a lot will depends on the size of solar panel and how much light it will get.
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@gohan
Early tests indicate the 15F charges to 3.6v pretty fast on even ambient light. I realize that's a somewhat vague statement, but I hope to eventually quantify it better (charge time as a function of lux level, panel size, and nominal panel voltage). Nonetheless, based on early preliminary measurements, I'm pretty confident the answer is going to be fairly simple, namely: what can you do with 15F of charge?Before I began this project, I was quite worried that self discharge would strongly govern the answer, but this particular supercap (as contrasted to others I tested) seems to have a relatively low rate of self discharge. Early tests show the same also holds true for its bigger brother, the 90F supercap, in case the 15F isn't enough. The 15F supercap is rated for 100,000 charge/discharge cycles, so it won't be wearing out anytime soon if it's operated indoors under normal ambient temperature conditions. There are probably even more interesting supercaps out there, but for me this one's an existence proof and a fixed point I can gather data around.
If anyone knows of even better supercaps to try, I'm all ears.
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So what can you realistically do with 15F of charge? I mean what kind of battery can you compare it to? Can you use the entire energy stored in the cap and and it keeps the voltage constant until it empty?
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Think of it like a battery whose voltage starts at 3.6v and then over time gradually drops to 1.8v. As presently configured, you would stop when the voltage gets to 1.8v, because both the atmega328p and the RFM69HW require a minimum 1.8v to operate.
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Is my math right if I calculate like this?
15 * 1.8 / 3600 * 1000 = 7.5 mAh
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Not sure the answer to that. However, one thing's worth noting: these supercaps behave more like button cells than capacitors. By that I mean: they can have voltage droops and can even have voltage "recovery" after using them a bit. I really didn't expect that going into this, but it's a very significant effect.
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What do you mean that they behave more like cells?
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@gohan said in Powering mote 24/7 using only a supercap and solar:
What do you mean that they behave more like cells?
By that I mean: they can have voltage droops and can even have voltage "recovery" after using them a bit.
An "ideal" capacitor doesn't behave like that.
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Sorry but I don't understand, I'm not that much into this electronic deep knowledge. By voltage drops are you referring to the small energy drain that the capacitor has?
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@gohan
We're getting off topic. I suggest you read some of the links already provided above.
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Good news! Last night I did some accelerated load testing on the supercap. First I charged it to 3.6v and then I hooked up an RFM69HW mote which woke up once a second to do 3 things: 1. check the voltage level, 2. turn on an LED for 1ms to simulate a sensor load, and 3. transmit a packet containing the voltage data using the RFM69HW.. Bottom line: 14,111 packets transmitted before running out of juice.
Not bad for a first attempt.
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@NeverDie Any updates on your progress? Have you received the new boards? Will you be publishing the gerber files?
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@agdemars said in Powering mote 24/7 using only a supercap and solar:
@NeverDie Any updates on your progress? Have you received the new boards? Will you be publishing the gerber files?
No, and for a host of reasons. The main reason is this: for most applications, it's overkill. Because this thread had become such a monolog, I continued to evolve the design offline, and along the way I've found that you can get perfectly good results with just an inexpensive mini solar panel, a capacitor, and a simple blocking diode. For the audience on this forum, that's all you really need.
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You are kind of pioneering in the use of supercap and sensor nodes. I'd like to be part of the discussion, but unfortunately I'm not that good at electronics so I can't contribute much. Probably you are right at using a standard cap you can get comparable results, but I believe you get smaller size with our project
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@NeverDie Indeed, keeping it simple is usually the best approach. I am nonetheless interested in learning about the circuit you developed if you're willing to share your observations.
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Would it be feasible to implement mppt in software? From what I understand, the atmega328 pwm works in power-save mode (but not in power-down). Power-save mode consumes 1uA. Could the charging speed be controlled by pwm and adjusted periodically by the mcu, to keep the load on the solar cell at maximum power?
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If I understood right NeverDie said that the supercap can be charged directly from the solar panel with just a blocking diode, then add a boost-buck dc-dc converter to have constant 3.3v
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@mfalkvidd said in Powering mote 24/7 using only a supercap and solar:
Would it be feasible to implement mppt in software? From what I understand, the atmega328 pwm works in power-save mode (but not in power-down). Power-save mode consumes 1uA. Could the charging speed be controlled by pwm and adjusted periodically by the mcu, to keep the load on the solar cell at maximum power?
Like you, I've also wondered what kind of MPPT might be built by leveraging an mcu and just simple components. There is a guy on youtube ( https://youtu.be/JXSRXUiUA6M ) who built his own arduino MPPT charge controller from scratch, though for a much bigger solar panel. For me, the real quesation is: why aren't there more chips that offer a proper MPPT? It seems like a rather bizarre gap in the market.
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Is that mppt really needed even if you are charging a capacitor?
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@gohan
Let's put it this way: there's probably nothing I could achieve with MPPT that I couldn't achieve with a sufficiently bigger solar panel. So, if space isn't a concern, the economics tend to favor buying bigger mini solar panels, because mini solar panels are fairly cheap from China (cheaper than buying the BQ25504 chip). On the other hand, for large 100watt or above solar panels, it's more economic to add MPPT, because an MPPT charge controller is cheap compared to, say, doubling an already expensive solar panel.That said, there may be other features of the chip that you may want anyway, like charge termination or a "battery_OK" pin to power-on your atmega328p when the voltage gets to an acceptible level. So, you could create those by other means, or you could buy a package with those features and which also happens to have MPPT at some incremental cost.
So, it's six of one, half a dozen of the other.
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What I finally arrived at was this:
https://www.openhardware.io/view/396/Simple-Solar-Supercap-Charger
which is both less expensive and easier to assemble.
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here is another circuit I found http://www.discovercircuits.com/DJ-Circuits/supcapvoltlim.htm
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@NeverDie actually I have done that, feeding an attiny85 and also an Atmega328 ("barebones" arduino) 2x1.5F capacitors. all on a 6 volt solar cel and a low drop regulator
The gardenlamp is quite handy as well, but better put some sealant on the edges of tha small solarcell as they do tend to leak. You will find everything has rusted inside after using it a season. Nevertheless, have been able to run an attiny on it
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The gardenlamp is quite handy as well, but better put some sealant on the edges of tha small solarcell as they do tend to leak. You will find everything has rusted inside after using it a season.
Yeah, corrosion is a huge problem with the chinese ones because it seems many of their components are incredibly prone to rusting (even including their wire!). Go figure on that one. Potting epoxy will cost you much more than the garden light. I think polyurethane spray foam may be the cheapest way to weatherproof them, and it's readily available.
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@Ed1500 said in Powering mote 24/7 using only a supercap and solar:
@NeverDie actually I have done that, feeding an attiny85 and also an Atmega328 ("barebones" arduino) 2x1.5F capacitors. all on a 6 volt solar cel and a low drop regulator
Yes, for outdoors it's bright enough you don't really need a boost converter. So, your solution may be the least expensive DIY for outdoors. Interestingly, the chinese garden lights do use a boost converter. They use a special purpose IC specifically made for garden lights.
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@NeverDie indeed, I just put a line of silicone sealant around the edge of the PV. That stopped a lot already
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@NeverDie It was indeed more of a concept test 'can I do this with one of those lamps'. I first tried to just use the power to the LED with the nicad in place but the NiCad just won't last long coz of the continous recharge, so I took it out and put supercaps in there. Store enough charge to get through the night. Supercaps also do better in the cold than Nicads , though I admit I brought them inside when it started freezing.
But those lamps are just great little boxes for a contained attiny 13/25/45/85 project
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@Ed1500 said in Powering mote 24/7 using only a supercap and solar:
But those lamps are just great little boxes for a contained attiny 13/25/45/85 project
Exactly so. They're cheaper than most project boxes, and then you get the solar cell "for free".
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@Ed1500 said in Powering mote 24/7 using only a supercap and solar:
@NeverDie actually I have done that, feeding an attiny85 and also an Atmega328 ("barebones" arduino) 2x1.5F capacitors. all on a 6 volt solar cel and a low drop regulator
Exactly which supercaps are you using, and where are you sourcing them from? It would be good to compare notes.
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@NeverDie I have 5.5V 1.5F Supercaps from Aliexpress. Forgot which supplier but they do well.
I put them in an RC circuit to measure and though that might not be totally reliable, they were above 1 Farad
Ah, found them. Although. It was a year ago, the ones I have say 1.5 F but seems they now sell 1Farad
These: https://www.aliexpress.com/item/2Pcs-Ultracapacitor-1-5F-5-5V-super-capacitor-1-5Farad-farad-capacitor-5-5V1-0F/32632431698.html
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@Ed1500 Yep, rechecked mine. 1.5 Farad, but the 'my orders' link now points to 1 Farad caps
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Thanks for looking that up. I keep coming back to this one:
https://www.digikey.com/product-detail/en/illinois-capacitor/106DCN2R7M/1572-1287-ND/5410638
if only because it seems like a good value for money, and it seems able to run any mote for quite a long time.
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@NeverDie 10F for 1.95 is definitely not bad... but not sure what the shipping would be
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Good point. I use first class mail from digikey, which arrives in 2 days and which costs around $3.65 I think. If you order before 8pm, it ships the same day. So for me, since I usually order late in the day, it's almost like it arrives in a day and a half.
I don't think I would order just a single supercap from Digikey, unless I was getting a bunch of them, but as I'm ordering a lot of parts from Digikey anyway, the incremental shipping cost is typically free.
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@NeverDie realise though it is 2.7 Volts
sure, 3.65 is peanuts if you order a bunch.
I see they have a branch in Netherlands as well. I would pay 1.81 euro for that capacitor.. but I would pay an unbelievable 18 euro shipping? Apparently because they still ship it from the USA. Also, need to answer a bunch of question what I am going to use it for, will i resell it etc. ... So I guess DigiKey is not a viable option for me
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Don't complain about your shipping costs from Digi-Key guys. These are the rates for me, insane !
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I should point out that even without a boost converter, I'm getting good charging indoors in most locations. I've been investigating boost converters just in case I want a node that's virtually bulletproof with respect to charging from low light conditions, or which needs to power 3.3v or 5v sensors. For instance, the banggood PIR sensor on the other thread requires between 2.7v and 3.3v to power it. It uses so little current though that it should be possible to power it continuously from even a solar node, so I'll be attempting that fairly soon. Obviously, doing it outdoors is nothing new and would be a piece of cake by comparison.
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Isn't there some place in Vietnam that stocks parts for re-sale so you don't have to order internationally and pay usurious shipping rates? Or is there just not enough demand to support that? Surely, China must have good options for you beyond just Aliexpress.
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@Nca78 Wow. I guess you are a loyal Aliexpress customer
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@NeverDie Actually I had expected that for DigiKey Europe as well... but it says 'all shipping from USA'
Well, China also has Banggood
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I ordered the 5.5v 4F ones, I hope they arrive sooner or later
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@gohan said in Powering mote 24/7 using only a supercap and solar:
I ordered the 5.5v 4F ones, I hope they arrive sooner or later
That could be interesting. Do you have a link?
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@Ed1500 said in Powering mote 24/7 using only a supercap and solar:
@NeverDie Actually I had expected that for DigiKey Europe as well... but it says 'all shipping from USA'
Well, Europe has a Mouser.com presence, I think. Plus, surely there's other stuff there, like in Germany or the UK or something.
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@gohan seen those for 15 USD/10 and for 6USD/5
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You are right @Ed1500, I receive so many shipments from AliExpress that now the postman is fed up with puting non-delivery notices in my mailbox, now he leaves the packages at the management office of my building, where they advance the tax for me when needed
@NeverDie yes I have some local shops, with very cheap delivery options, less than 1$ a;nd delivered the next day when they have stock. But unfortunately the inventory is pretty limited, I use them when the price difference is not worth the wait from AliExpress.
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@gohan yes, saw those. please keep us informed abt yr experience with those
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@NeverDie Oh dont worry, plenty of local online shops here for electronic components plus distributors for adafruit and similar.
However, I can order in china and have it delivered to my door often for less money than just the gas would cost me to drive to a local store, or to have it delivered from a local webstore.
The only thing is that sometimes I have to wait 2-3-4 weeks
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These look quite inexpensive:
https://www.aliexpress.com/item/30-x-ultra-capacitor-2-7v3f/32570929215.html?spm=2114.10010108.1000023.15.qq5cZ7
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@NeverDie indeed. but 2.7 Volt is a just a bit of a difficult value to work with and putting them in series makes the capacity drop, but I think one could run the attiny 13-25-45-85 from 1.8V so that might be a good combo
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Or, maybe this then:
https://www.aliexpress.com/item/Universal-Capacitors-Ultracapacitors-2-7V10F-super-capacitor-10F-2-7V-low-ESR-fast-charging-small-power/32688216790.html?spm=2114.10010108.1000013.4.G3WV1v&traffic_analysisId=recommend_2088_2_81019_new2&scm=1007.13339.81019.0&pvid=1ecf1af6-9158-4f9f-99ff-acaa16690ce2&tpp=1
It's just a wild card as to whether they actually perform according to the advertised specs, though. That's why I think it's helpful if we all share what we're actually using, and whether it meets expectations or not.
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@Ed1500 said in Powering mote 24/7 using only a supercap and solar:
@NeverDie indeed. but 2.7 Volt is a just a bit of a difficult value to work with and putting them in series makes the capacity drop, but I think one could run the attiny 13-25-45-85 from 1.8V so that might be a good combo
Yeah, and the RFM69--which is what I use--runs as low as 1.8v as well, so it's possibly a good match.
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@NeverDie 10F now we are talking. But yes I very well understood yr request to share experience. I acn only say thet the 1.5 F's i had performed well and with an RC network thrown together, they seemed in the right ballparc... but apparently seller now sells 1F and I cant vouch for those
The supercap is still a very viable alternative for smaller devices, so I will probably buy more in future and thus interested in which ones are good or not
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@NeverDie ah yes it does, but the RFM is usually connected to a microcontroller
So that is limited choice
i wonder if 1.8 Volt would seriously affect the range of the RFM69 (running mine from 3.3)
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Maybe a small booster to 3.3v could be used, since power efficiency shouldn't be a huge deal since the you will have the panel charging most of the time
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@Ed1500 said in Powering mote 24/7 using only a supercap and solar:
@NeverDie ah yes it does, but the RFM is usually connected to a microcontroller
So that is limited choice
i wonder if 1.8 Volt would seriously affect the range of the RFM69 (running mine from 3.3)Interestingly enough, the lower voltage doesn't seem to affect the range.
ATmega328p's can run at 1.8v also. In theory it should be set to run at 4mhz, but neither I nor anyone I know of has had problems running at 8Mhz down to 1.8v. If you run with BOD off and internal 8Mhz resonator, it only consumes about 150na while sleeping. That's why I haven't bothered going to ATtiny's. Maybe there are other reasons to do so though?
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So, I just ordered this:
https://www.aliexpress.com/item/3PCS-fala-capacitor-2-7v10f-super-capacitor-high-current-Low-ESR-fast-delivery-2-7V10F-ultra/32715563451.html?spm=2114.13010608.0.0.qFbHqn
and this:
https://www.aliexpress.com/item/CNIKESIN-2PCS-Fala-Capacitor-2-7v10F-Super-Capacitor-High-Current-Low-ESR-Fast-Delivery-2-7V10F/32804681189.html?spm=2114.13010608.0.0.qFbHqnfrom the same seller that gohan used. It's a risk, but if either of them check out, I may buy more of that brand.
I'll make a posting after I receive them to say whether they seem to be good or not.
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I recently purchased one of these:
https://www.aliexpress.com/item/Mega328-Transistor-Tester-Diode-Triode-Capacitance-ESR-Meter-MOS-PNP-NPN-M328/32685741297.html?spm=2114.01010208.3.92.KtC8xSand, for the price, it seems to do a decent job of measuring capacitance and ESR.
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@NeverDie I have used Attiny's mainly for their size. Pricewise, compared to a pro mini clone it is a bit foolish, unless you use the smd versions.
I had an attiny+ 433Mhz transmitter built in one of those garden lamps with a moisture sensor at the base where it sticks in the soil. Worked well, Not ideal, but it was nice to play around with
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@NeverDie interesting. May get one too
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@Ed1500 I got the bigger brother of that one. Look at the workshop discussion. I can't wait to start testing this stuff with all the voltage regulators I got and see how much energy I can actually store in the same 2 caps both in series and parallel
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@gohan keep us updated
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@gohan said in Powering mote 24/7 using only a supercap and solar:
Look at the workshop discussion.
Uh, what workshop discussion would that be?
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@gohan said in Powering mote 24/7 using only a supercap and solar:
Well, after much digging, I think I finally found your post. Is this what you're referring to?
Is it better than the one I posted?
I'm thinking of possibly getting one of these:
http://www.ebay.com/itm/261114892135
if only because the company that makes it also makes a popular low budget signal generator.I don't know that I actually need it though, except to verify that stuff purchased from Aliexpress actually is what it purports to be.
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Actually I went for the 2017 version on aliexpress that also has a pulse generator
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If anyone interested for some ideas of supercaps protection https://youtu.be/NsTAyD2i3rc
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@gohan said in Powering mote 24/7 using only a supercap and solar:
If anyone interested for some ideas of supercaps protection https://youtu.be/NsTAyD2i3rc
Useful video if connecting capacitors in series. If using just one capacitor, though, I wonder whether there's any advantage to using the described shunt regulator circuit compared to simply using a 2.7v LDO voltage regulator?
By the way, if your supercap's will be in a higher than room temperature environment, you may have to de-rate their max voltage. In general, supercaps won't last as long at high temperatures. Worth checking the datasheet for the details if that's a possible concern.
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I'll experiment when I'll get the small adjustable dc-dc converters and see how they go.
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@NeverDie The unforgettable "guy with the swiss accent" videos
Though very useful information, not so applicable to me as I am using 5,5V capacitors on a regulated 3V3 line
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I think the Swiss guy presents an interesting notion, and it's worth watching for that. However, I think his part choice of the TL431 is a poor fit for my solar application. According to the datasheet (cf page 6 of http://www.ti.com/lit/ds/symlink/tl431a.pdf), the minimum cathod current required for regulation is between 0.4ma and 1ma. From indoors, that's generally more current than I'm receiving from the solar panel! Now, double that, because you'll need two of them.
Also, it can't handle very high currents either, which is, I presume, why the Swiss guy resorts to the transistorized circuit and/or the chinese supercap protector PCB circuit.
HOWEVER, I notice that TI has a more recent successor to the TL431, called the ATL431, which appears to address both of those shortcomings: http://www.ti.com/lit/ds/symlink/atl431.pdf
Also, the ATL431 price would be much less than the Chinese transistor supercap protector board. The ATL431 price is about 58 cents, quantity 1 (http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=229180358&uq=636316574771858018)
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@NeverDie Oh that is good to know, thanks
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Has anyone seen this solution for balancing the supercaps?
Active Balancing Supercapacitors With SAB Mosfets – 06:12
— Mad Electron Engineering
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@gohan
Sounds better than the Andreas Speiss method. Thanks for posting it!
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Has anyone figured out how to do use this chip?
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@gohan said in Powering mote 24/7 using only a supercap and solar:
Has anyone figured out how to do use this chip?
I haven't pursued it, because I haven't heard a strong argument as to why two or more supercaps are better than one for powering a mote. The economics of supercaps seems to favor a one supercap solution.
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Well it is all related to what voltage you need to store I'd say: if you need to use more than the standard 2.7v you need more caps in series (for example if you want to make a 12v pack for your car)
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@gohan
Well, if you need more than 400F... maybe then. I'm sure there's a crossover point somwhere if the storage requirements get high enough. At that point, though, it would seem you're doing much more than powering a sensor mote.Anyhow, I look forward to seeing what you come up with. Maybe you've found an interesting use-case that I haven't even considered.
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not for near future... I am still preparing a bunch of other things
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I notice Julian Ilett seems to enjoy playing around with these 700F supercapacitors:
Julian's Postbag: #99 - SuperCapacitors and Computer Chips – 17:13
— Julian IlettEbay seems to have the lowest price: https://www.ebay.com/itm/1pcs-2-5V-700F-Automobile-Super-Farad-Capacitance-NEW-L85/262714791208?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
Anyone here tried them? I'm curious as to their self-discharge rate when they're not being charged.
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Also, although I'm doubtful about the brand, these 500F supercaps might be interesting because they're relatively inexpensive:
https://www.aliexpress.com/item/1PCS-AMS1117-5-0-DC-DC-Step-Down-Voltage-Regulator-Adapter-Convertor/32328707306.html?spm=a2g0s.13010208.99999999.269.QVr0W1
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so far what I saw is that good branded supercaps have much better self discharge compared to the cheap ones
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Am noticing that on Aliexpress you can now get an entire LTC3558 board for about the same price as just the ltc3558 chip on digikey.
https://www.aliexpress.com/item/GY-LTC3588-LTC3588-Energy-Harvester-Breakout-LTC-3588-Energy-Harvest-Collector/32755828984.html?spm=a2g0s.13010208.99999999.277.HJ7HHPSo, for low light energy harvesting, it might be interesting.
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@neverdie I think the size is a killer for me. I watched Julian's video and also remember this one:
Using capacitors instead of a car battery for 3 years and counting! – 06:57
— CatrinisinGreat caps and the price is really good too.
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@alexsh1 I purchased one of the 700F supercaps after my prior post just to see, and sure enough it has a pretty terrible self-discharge rate: about 0.4-5v per day. With a boost converter, maybe it would be good for powering an LED garden light at night.... Not sure what else it might be good for.
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@neverdie That's really bad - 0.5V a day. Questionable quality I guess?
I can see that similar capacitor AVX branded is much more expensive:http://uk.farnell.com/avx/sccy1kb707plble/super-cap-700f-2-7v-weldable-pin/dp/2856928?st=700F
Prices are coming down gradually. I suppose soon enough we may see AVX priced more affordably.
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Cypress Semiconductor has an interesting energy harvesting chip intended for wireless nodes:
http://www.cypress.com/file/219311/download
The chip looks a little easier to solder than some of them.
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@neverdie said in Powering mote 24/7 using only a supercap and solar:
I notice Julian Ilett seems to enjoy playing around with these 700F supercapacitors:
Julian's Postbag: #99 - SuperCapacitors and Computer Chips – 17:13
— Julian IlettEbay seems to have the lowest price: https://www.ebay.com/itm/1pcs-2-5V-700F-Automobile-Super-Farad-Capacitance-NEW-L85/262714791208?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
Anyone here tried them? I'm curious as to their self-discharge rate when they're not being charged.I charged up this supercap 5 months ago and then set it aside. Checking it now, it still has 0.6v left on it. I thought it would be at zero! So, maybe it really did hold a useful charge for much longer than I thought....(?)
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On the cheap supercaps I have they hold the 0.6v, they don't go to zero
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@gohan said in Powering mote 24/7 using only a supercap and solar:
On the cheap supercaps I have they hold the 0.6v, they don't go to zero
Interesting. So, if we were to put a few high Farad garbage Chinese capacitors in series, using a balance circuit, such that each just holds 0.6v, then I guess we could actually hold a meaningful charge for a very long time? Then, using my new efficient boost converter, we could drain most of the energy out of them:
https://www.openhardware.io/view/628/Efficient-Low-Current-Boost-Converter
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Apparently the answer is yes:
My 6 series 2.7V 500F supercapacitors w protection board for 16.2V 83.333F unit – 03:16
— electronzapdotcomIt also nicely illustrates that what's really needed is charge balancing across supercaps, not merely overcharging protection. Otherwise you are much more limited by the weakest supercap in the series chain of supercaps.
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what I meant is that they don't actually go to 0v even if you short them as you will always be able to measure some small voltage. I am no expert but my bet is that that 0.5/0.6v is kind of their bottom line where they don't hold any more useful energy below that.
For balancing there are SAB Mosfets that help to keep all the supercaps on the same voltage, but the reverse charge issue is the same for all series circuits, also LiPo worn out batteries can get to 0V and then getting reverse charged with all the consequent risk of fire.
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