Powering mote 24/7 using only a supercap and solar
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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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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.
@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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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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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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@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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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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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?
@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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@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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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?
