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