How best to find the "best" small solar panel of a particular size?
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@NeverDie I just watched this video:
Is this what you have been looking?
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Well, so far the green supercap isn't holding its charge any better. I'll leave it in-circuit for two or three days, though, and see if that improves. It's encouraging that the rate of voltage decrease is decreasing. Some supercaps apparently improve their ability to retain charge if they are forced to hold a charge long enough, though I have no idea why that is. For instance, some capacitor leakage test protocols call for holding a capacitor at its rated voltage for 72 hours before running the leakage test.
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Well, so far the green supercap isn't holding its charge any better. I'll leave it in-circuit for two or three days, though, and see if that improves. It's encouraging that the rate of voltage decrease is decreasing. Some supercaps apparently improve their ability to retain charge if they are forced to hold a charge long enough, though I have no idea why that is. For instance, some capacitor leakage test protocols call for holding a capacitor at its rated voltage for 72 hours before running the leakage test.
@NeverDie you are right - Nick Gammon states exactly the same that overtime leakage "improves". A new cap has to be broken in before leakage is measured.
ESR measurement - I have no idea if it is possible to measure it and how. I think right now we are going down deep into the physics in order to do that
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For more normal capacitors (though apparently not for 10F supercaps), the little gizmo I showed in the pictures above does measure and display the ESR. That's one of the main reasons I purchased it. It's probably not hard to measure either. Measure the unloaded voltage. Then measure the voltage under, say, 1 amp of current. The second voltage measurement will be less, and from that you can calculate the ESR. At least, that's how I'm guessing it's done. Except you'd near to do a near instantaneous measurement, because the longer you drain the current, the more the voltage goes down for that reason too, not just ESR.
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Incredibly, I may have to resort to the Julien Ilett method of measuring capacitance. None of the meters or testers I've seen thus far handle supercapacitor capacitance (e.g. 10F). So, thanks for posting the video! Very timely.
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Incredibly, I may have to resort to the Julien Ilett method of measuring capacitance. None of the meters or testers I've seen thus far handle supercapacitor capacitance (e.g. 10F). So, thanks for posting the video! Very timely.
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@NeverDie the old fashioned way! ;-)
Glad we are doing or reading about the same things at the same time.How long does it take for your device to measure ESR? Did you try a normal capacitor, I.e. 100uF?
@alexsh1 said in How best to find the "best" small solar panel of a particular size?:
How long does it take for your device to measure ESR? Did you try a normal capacitor, I.e. 100uF?
Yes. About 5 seconds.
[Edit: that includes the time it also takes to measure capacitance. It does both measurements (including part type identification) at the push of the button. ]
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Is there a way to do a full disconnect of the solar panel (so as to do an unloaded voltage measurement) without resorting to a relay? I've tried using a PFET, and although it can turn off the current flowing from the solar panel, it still somehow sucks the measured voltage down to about 0.3v above the supercap's voltage. On the other hand, if I physically disconnect the solar panel, then I'm able to then read it's unloaded voltaqe.
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Is there a way to do a full disconnect of the solar panel (so as to do an unloaded voltage measurement) without resorting to a relay? I've tried using a PFET, and although it can turn off the current flowing from the solar panel, it still somehow sucks the measured voltage down to about 0.3v above the supercap's voltage. On the other hand, if I physically disconnect the solar panel, then I'm able to then read it's unloaded voltaqe.
I played around with it a bit more, and I found that if I used some external voltage source that was otherwise not part of the circuit to switch the P-FET "off", then it effectively accomplishes a full disconnect.
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Is there a way to do a full disconnect of the solar panel (so as to do an unloaded voltage measurement) without resorting to a relay? I've tried using a PFET, and although it can turn off the current flowing from the solar panel, it still somehow sucks the measured voltage down to about 0.3v above the supercap's voltage. On the other hand, if I physically disconnect the solar panel, then I'm able to then read it's unloaded voltaqe.
Found a very good on-point thread which addresses the issue I'm having: https://forum.arduino.cc/index.php?topic=115233.0 So, it may be mosfet related, not just circuit related. If worse comes to worst, I'll order the Hexfet it references that supposedly does achieve a full disconnect the way you'd expect.
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@alexsh1 said in How best to find the "best" small solar panel of a particular size?:
@ceech said iThe double cell 15F flat capacitor can only supply 70mA.
It should be enough if not using nrf24l01+ pa lna or rfm69HW. In this case perhaps, it would be better to have 150mA at least to cover those transmitters?
I like the size - It is really flat.If you start to get choosy, that's when it's time to start making your own boards. It's easier than you might think. Otherwise, you just have to accept that the off-the-shelf stuff will never be exactly what you want.
It's really too bad there isn't more of a universal PCB format that's easily edited, so you can simply start with a board that's close to what you want and just customize it a bit (the way one might with, say, software). For instance, even just changing LDO's--unless the new LDO has exactly the same land pattern--can necessitate redoing the board. No problem if you're the original designer, but it's a PITA if you aren't.
@NeverDie said in How best to find the "best" small solar panel of a particular size?:
@alexsh1 said in How best to find the "best" small solar panel of a particular size?:
@ceech said iThe double cell 15F flat capacitor can only supply 70mA.
It should be enough if not using nrf24l01+ pa lna or rfm69HW. In this case perhaps, it would be better to have 150mA at least to cover those transmitters?
I like the size - It is really flat.If you start to get choosy, that's when it's time to start making your own boards. It's easier than you might think. Otherwise, you just have to accept that the off-the-shelf stuff will never be exactly what you want.
It's really too bad there isn't more of a universal PCB format that's easily edited, so you can simply start with a board that's close to what you want and just customize it a bit (the way one might with, say, software). For instance, even just changing LDO's--unless the new LDO has exactly the same land pattern--can necessitate redoing the board. No problem if you're the original designer, but it's a PITA if you aren't.
I do not disagree with you. I have started looking at Eagle software. Obviously, the idea is that I'm going to start with improving somebody's design first and then designing something myself
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@ceech said in How best to find the "best" small solar panel of a particular size?:
MIC5365
I'm going to switch from the MAX8887EZK27 LDO over to: http://datasheet.sii-ic.com/en/voltage_regulator/S1313_E.pdf
which also is the same size SOT-23-5 package.Why? Current consumption. The S1313 datasheet says it should consume a maximum of 1.35ua of current, whereas the MAX8887EZK27 may be consuming as much as 85ua (though I haven't actually tried measuring it). When the overall charging current from the solar panel is only around 50ua indoors far away from the windows, I'm betting it will make a difference. Likewise the MIC5365 also appears to have a rather high quiescent current of around 29ua, at least according to its datasheet that is.
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@ceech said in How best to find the "best" small solar panel of a particular size?:
MIC5365
I'm going to switch from the MAX8887EZK27 LDO over to: http://datasheet.sii-ic.com/en/voltage_regulator/S1313_E.pdf
which also is the same size SOT-23-5 package.Why? Current consumption. The S1313 datasheet says it should consume a maximum of 1.35ua of current, whereas the MAX8887EZK27 may be consuming as much as 85ua (though I haven't actually tried measuring it). When the overall charging current from the solar panel is only around 50ua indoors far away from the windows, I'm betting it will make a difference. Likewise the MIC5365 also appears to have a rather high quiescent current of around 29ua, at least according to its datasheet that is.
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I did the experiment of charging one of the green Chinese cheapo capacitors for 3 days at 2.7v, and then I removed it from the circuit and measured its voltage after 3 days with no charging and no load at all. It came out at 2.54v. So, I guess I would deem that use-able.
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I did the experiment of charging one of the green Chinese cheapo capacitors for 3 days at 2.7v, and then I removed it from the circuit and measured its voltage after 3 days with no charging and no load at all. It came out at 2.54v. So, I guess I would deem that use-able.
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I went ahead and made the switch on the node that I'm experimenting with: I'm now using a LTC3235D 3.3v passthrough boost converter. I switch it on when the node wakes up, do various measurements, then switch it off before transmitting the results if the supercap voltage is above threshold. Everything working smoothly.
I think I may next try putting a buck converter on the solar panel so I can get more juice out of it when its voltage is above 3.3v. I'll be trying this one:
https://www.openhardware.io/view/276/33v-Buck-Boost-DC-DC-Converter
because it's the only one I have on hand. If it seems to help, then I'll look for something more on-point (and less expensive!).@NeverDie said in How best to find the "best" small solar panel of a particular size?:
I went ahead and made the switch on the node that I'm experimenting with: I'm now using a LTC3235D 3.3v passthrough boost converter. I switch it on when the node wakes up, do various measurements, then switch it off before transmitting the results if the supercap voltage is above threshold. Everything working smoothly.
I think I may next try putting a buck converter on the solar panel so I can get more juice out of it when its voltage is above 3.3v. I'll be trying this one:
https://www.openhardware.io/view/276/33v-Buck-Boost-DC-DC-Converter
because it's the only one I have on hand. If it seems to help, then I'll look for something more on-point (and less expensive!).How does LTC3235D compare to BQ25570?
BQ25570 has got buck converter with Vout from 3.3V down to 2V (Vbat is from 5.1 down to 2V), which is just enough to run nrf24l01+ with AVR. -
@NeverDie said in How best to find the "best" small solar panel of a particular size?:
I went ahead and made the switch on the node that I'm experimenting with: I'm now using a LTC3235D 3.3v passthrough boost converter. I switch it on when the node wakes up, do various measurements, then switch it off before transmitting the results if the supercap voltage is above threshold. Everything working smoothly.
I think I may next try putting a buck converter on the solar panel so I can get more juice out of it when its voltage is above 3.3v. I'll be trying this one:
https://www.openhardware.io/view/276/33v-Buck-Boost-DC-DC-Converter
because it's the only one I have on hand. If it seems to help, then I'll look for something more on-point (and less expensive!).How does LTC3235D compare to BQ25570?
BQ25570 has got buck converter with Vout from 3.3V down to 2V (Vbat is from 5.1 down to 2V), which is just enough to run nrf24l01+ with AVR. -
I'm playing around with a coulomb counter, actually a clone of:
https://www.sparkfun.com/products/12052The only downside to it is that it requires a minimum of 2.7v on the input current that it's measuring, which isn't really ideal for a solar panel that might nonetheless be charging at a lower voltage. It also includes its own current consumption in the measurement, and it appears to be about 75ua. In addition to counting the coulombs, you can also measure the average current between interrupts by doing a little math (which is illustrated in the sample sketche).
So, kinda cool. I changed out the sense resistor from 0.05ohm to 10ohm so that I get more frequent interrupts.
