How best to find the "best" small solar panel of a particular size?

[NeverDie]

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.

[NeverDie]

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.

[NeverDie]

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.

[alexsh1]

@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

[NeverDie]

@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.

[ceech]

@NeverDie Solid choice.

[NeverDie]

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.

[alexsh1]

@NeverDie I suppose you broke it in. Try to do a few more cycles (charge / discharge) and see if self discharge improves

[gohan]

It should get a little better after some cycles

[alexsh1]

@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]

@alexsh1
Well, I'm sure it's easier to solder than the BQ25570. I'm no longer really considering anything that's difficult to solder unless there's just absolutely no alternative.

It looks as though @ceech sells them though already soldered to a breakout board, so maybe he can answer your question.

[NeverDie]

I'm playing around with a coulomb counter, actually a clone of:
https://www.sparkfun.com/products/12052

The 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.

[alexsh1]

I found this interesting fella

https://www.tindie.com/products/closedcube/tps63030-high-efficient-buck-boost-up-to-96/

Anyone tinkered with tps63030?

[gohan]

It's kind of pricey 😧

[ceech]

@alexsh1 I use it. On this board:
http://www.ebay.com/itm/ATmega328p-board-w-NRF24l01-socket-LTC4079-battery-charger-in-Arduino-PRO-form-/331800929918?ssPageName=STRK:MESE:IT

It works.

[NeverDie]

@alexsh1
Is it any better than:
https://www.openhardware.io/view/276/33v-Buck-Boost-DC-DC-Converter
?

[alexsh1]

@NeverDie said in How best to find the "best" small solar panel of a particular size?:

@alexsh1
Is it any better than:
https://www.openhardware.io/view/276/33v-Buck-Boost-DC-DC-Converter
?

Interesting buck-boost converter, but Input voltage is from 1.8v to 5.5v.
I'm looking at the one with a better range of 0.9-5.5V
I need to use two 2.7V super caps in series with a maximum efficiency

[alexsh1]

@gohan said in How best to find the "best" small solar panel of a particular size?:

It's kind of pricey 😧

Not cheap, I agree, but ideally I'm looking to use the whole range from 0 to 5.5V in two 2.7V super caps in series. If you have an idea of a good buck-boost converter outputing 3.3V, I'm all ears

[alexsh1]

@ceech no wonder I like your board. These buck-boost converter works really well with a solar panel

[gohan]

I already have one boost buck converter, I only need the supercap charge controller for 2 supercap or use the 5.5v 4F I have