Solar cell support with non-rechargeable batteries



  • Hi!

    So I am playing around with battery powered nodes and two very old solar cells that I have laying around. I am going to try to use supercaps but until they arrive, I would like a setup with non-rechargeable batteries. I am following the threads of @NeverDie and @gohan, from which I learned a lot.

    The solar cells spike up to 7V when in sun, on a cloudy day more like 4.5V, so in direct competition with the batteries. (I don't really have any specs.)

    0_1518796124078_Arduino-Solar-Nonrechargable.png

    In the attached image I added a 1N4148 diode to protect the non-rechargable batteries.
    -) Is this a good thing? Or will they consume to much power and result in a trade-off solar earnings vs. additional power consumption?

    (I know this setup is not as power saving as possible as it uses the Arduino's power regulator. However I destroyed desoldered the Arduino LED.)

    -) Do I need a need some kind of protection e.g. LEDs if the solar cells voltage should be too high?

    Concerning general battery conservation: The guide on how-to measure the battery level uses 1MΩ + 470kΩ attached to the power source.
    -) Wouldn't it save another few µA to to connect to a digital output which would only be set to OUTPUT 0V when measuring the battery level? No additional draining over the resistors.


  • Mod

    The diodes don't consume power, the only concern is the voltage drop and if the output voltage is enough for the arduino regulator. If I were you i'd go with no regulator and 2 AA (or a single LiFePO4 AA) batteries directly and forget solar panel; just strup everything you can to lower sleep current to less than 10uA. Resistor drain is negligible. If you go without regulator you could read voltage directly with VCC library.



  • @gohan Thank you for clearing this up! Couldn't find anything on the consumption of these diodes.
    Using just 2 AA would indeed be the easiest, however I wanted to tinker around 😉


  • Mod

    I am really considering ditching the solar panel and just use a lifepo4 battery, much less hassle and I ca even think to do OTA updates since the process is quite slow and will for sure drain the supercaps.



  • It likely does not matter much in this case, however the diodes do consume power.

    Power consumed in diode = voltage across diode X current through diode

    You could reduce the diode voltage drop (not for power savings but for additional usable battery life) if you replaced the 1n4148 with something like a BAS86 (i'm sure there are others).



  • @johnrob Ok, just like with a resistor. Thank you for clearing this up. I assumed but wasn't sure.

    @gohan So only good weather updates? 😉 From a practical standpoint solar cells are probably more of a hassle but the beauty of the concept...


  • Mod

    The concept is right, the problem is that you would need a 15$ circuit to manage solar charge for supercaps without the problems I am having. In addition you need to keep sensor in a shaded area in order to have more accurate temp readings, so you sacrifice the charging capacity of the solar cell or have solar cell in direct sunlight and have a cable to a shaded area for sensor. So unless you have a big weather station, you could very well do with a single AA LiFePO4 for well over a year or 2


  • Hero Member

    @gohan said in Solar cell support with non-rechargeable batteries:

    The concept is right, the problem is that you would need a 15$ circuit to manage solar charge for supercaps without the problems I am having. In addition you need to keep sensor in a shaded area in order to have more accurate temp readings, so you sacrifice the charging capacity of the solar cell or have solar cell in direct sunlight and have a cable to a shaded area for sensor. So unless you have a big weather station, you could very well do with a single AA LiFePO4 for well over a year or 2

    I'm sorry, but this is hogwash. There's plenty of light outdoors, even if you're in the shade. Also, the solar charging circuit is cheap. The only "expensive" part is a good quality 10F supercap for $2. If you do it right, your node can run practically forever. The only limiter I see might be UV oxidation of the solar cell plastic, which would degrade it over time. The right way to counter that would be to put it behind glass and/or some other UV filter, although I can't say that I've tried that yet outdoors. Probably best would be to avoid cheap solar cells that are encased in in cheap resin and just use glass to protect it from the elements. That should last.



  • @neverdie Hogwash? Not in Texas for sure, don't assume everybody enjoys the same conditions, they don't....


  • Hero Member

    @zboblamont
    OK, fine, the OP can do this simple test to settle the matter for his particular location: hook your solar panel, whatever it is, up to a blue LED. Wherever you can light that blue LED from, you have enough light to confortably power a simple TH node without resorting to exotic technology if you collect the power into a supercap.

    I think you'll be surprised just how many places around your house--both outside and inside--can light that blue LED.


  • Mod

    @neverdie the problem is both charger and booster: if you have a dumb booster when voltage drops below its operating voltage, it starts draining more current than the solar cell is able to provide, so you need extra components to cut the power to booster until supercapacitor reaches at least 1v but at this point you can very well use a circuit specific for supercaps energy harvesting. I have been lucky at the moment that my solar node is now performing well on the north side of the house with no direct sunlight, but if I am going to get several days of bad weather I would still need to jump start the node by manually charging the supercaps


  • Hero Member

    @gohan
    I say avoid all that and just use a dead simple circuit like:
    https://www.openhardware.io/view/382/Tiny-Solar-Charger-for-27v-Mote-Supercap
    and power your node directly from the supercap.

    It's simple. It's cheap. It's tested. It works. Why complicate matters?



  • @neverdie How do you get from 2.7V to 3.3V? With another DC/DC booster?


  • Hero Member

    @gunther I don't. Ditch the LDO on the pro min. The pro mini can run just fine even down to 1.8v. Pick sensors that also do that, and then you're golden. BME280 and si7021 can both do that. Most wireless can also run fine even as low as 1.8v.



  • @neverdie Will try! But that means the NRF24L01 is also out? And am I correct to connect the 2.7V to VCC not RAW?


  • Hero Member

    @gunther said in Solar cell support with non-rechargeable batteries:

    And am I correct to connect the 2.7V to VCC not RAW?

    Yes. Also NRF24L01 voltage range is 3.6-1.8v, so it's not a problem either.


  • Mod

    this way you only get half of the available charge of the supercap


  • Hero Member

    @gohan That's true, but if it's a problem, use a larger supercap. The pricing is non-linear, so, for instance, twice the farads costs less than twice as much. 10x the farads costs much less than 10x as much. I don't think the economics generally favors boosting. You simply have capacity that never gets used.



  • Thank you all for the information!

    @neverdie Ok, so now I have desoldered the Arduino Pro Mini power regulator and I now see a sleep current of 6µA with 3.2V.

    One thing I see is that the node only works down to 3V. With 2.5V it is not stable anymore. This is just the Arduino Pro Mini with attached NRF24L01. Maybe this is expected? I checked visually that I have indeed the 8MHz variant.
    alt text
    I just tried to test it with parts I could get my hands on.

    Could someone please tell me why the Battery Powering page advises to cut Vout? And what is it? It seems to me that this disconnects the Vcc on the shorter end? To what consumption does the connection lead. I find it convenient to have another Vcc pin available.


  • Hero Member

    @gunther said in Solar cell support with non-rechargeable batteries:

    One thing I see is that the node only works down to 3V. With 2.5V it is not stable anymore. This is just the Arduino Pro Mini with attached NRF24L01. Maybe this is expected? I checked visually that I have indeed the 8MHz variant.

    Did you remember to remove the LDO? Because otherwise it gets backfed, which is not what you want.

    Also, yes, in theory you are running it out of Atmel's official spec if running at 8mhz at 1.8v, but I don't know and haven't read of even a single person who has ever had a problem with doing so. Doing this is very common.


 

298
Online

7.5k
Users

8.3k
Topics

89.8k
Posts