Thank you for this follow up. After last weeks video I also tested a modified TP4056 charger by adding a 10K potmeter in series with the 1.2K resistor, allowing me to vary the charge current from about 100mA to 1A. Unfortunately the sun was too hazy here and my testing panel delivered insufficient current for a good test. Good to see your tests were succesful and thank you for your thoroughness to publish these extensive tests.

Congrats on the 40k subscribers! Seems only yesterday I was congratulating you on reaching 20k. 50k will be here in no time. Keep up the great work!

Thank you for putting this video together, it's very helpful for me as I am trying to power my microcontroller with a small solar panel. Your videos are packed with info and insights. Keep making great content.

Outstanding systematic study and conclusions ! Thank you Andreas ! 👍👍

Yet another great post on your series. Thank you Andreas😀

You are very methodical and thorough - excellent video series! Thank you.

Really great experiments. Thanks for all of this Andreas, you really put a lot of work in your job. I will use protected one for future projects.

Your tests and results are greatly appreciated

So you finally sorted out your BQ24650 setup, nice! Happy with the result and the good thing about this module is you can push it up to 10A charging current with a little modification and proper cooling but that's a different story or i can say different application.

A great video. I hope you will give us a final circuit design when your series completes. Nice work!

Great video again! I have used the same bq24650 board with a 100w solar panel with the batteries in 3s configuration. This allows you to capture the shortest blinks of the sun with this tiny board, a 100w solar panel will still put out 3 to 8 watts for a few hours on the very worst of winter days which means you do not need that much storage.

Thanks a lot for your very informative videos! I really enjoy them a lot!

Thank you for another great video!

Great pair of videos Andreas, thank you. I have just been trying the TP5100 which I bought last year. In full sun, it's efficiency is close to 100% using my 550mA 6V panel, but at much less than full sun, it really doesn't work and I am seeing pulses of current as though it is continuously restarting. Based on your findings I think I will swap to a TP4056, but we should note that it would need a zener diode to keep the solar panel input Voltage below about 8v (which you might see if the battery was already fully charged).

Have been following your most informative videos for a few weeks now, especially with my interest to build a floating pool water temperature sensing project. I attempted to use the ESP32 but failed dozen of attempts to upload IDE sketch then opted for 8266 with no issues. I have procured a "6V" 4.5W panel with hopes to keep 1 or 2 18650 to power the 8266 and DS18B20 temp sensor, charged overnight. Plenty of sun in the sunshine state, so no issues with that. My main concern is the charger selection, now that you have highlighted many choices, your recommendation for such project. Thanks for all the great content.

A lot of thanks for your systematic approach. Please continue this investigation. This series should be a "must see" for all those people rushing in with their optimistic "i will put my $XYZ on solar, look what i ordered from ebay". My first question for them is always: Do you need to use it in the end or is it just about the learning experience and how to cope with failure and write off? (but off yourse i am a nasty person...)

Good video as always, danke!

Very informative . Thank you!

Thanks you Andrea for your videos. Best regards

Nice video series about solar chargers. Would be interesting to see options for LiFePo4 battery.

Excellent series - now that I've finally tapped into IoT devices with the ESP8266/MQTT, it's very valuable info. My application is the typical thermometer/hygrometer running with an SHT21 compatible sensor, drawing only a few micro-amperes in Deep Sleep on two AA's. The sensor is accurate down to 2.1 volts Vcc; and an update interval of two minutes, plus a few Wifi tricks (static IP, intelligent reconnecting relying on auto-reconnect on power-up) help it to survive on around 0.6ma average, with an on-time of only around a second. A solar cell charging a lipo, or even a couple of AA NiMh cells should in theory make it work forever.

Thank you Andreas.

Thank you. Now i am waiting the next video :)

Thanks a lot, Andreas, very useful!! Maybe it would be interesting to know the consumption when there are no solar radiation, as it would happen most of the time

Thanks for sharing, you are the best👍😀

Cool!! Andreas, you save my time a lots. /Thanks.

This video helps me a lot ! Thnxs !

This was highly interesting and informative, Andreas. Thanks. The basic takeaway is that solar power is not only fickle, it is all but a spurious resource for practical use and so unpredictable as to be essentially useless. That said, it is still a lure for people attempting to harness it because, in a direct sense, it has tremendous potential. Converting light energy may not be as good as converting heat energy is another assumption from your demonstration, although, I am not exactly sure how that might be accomplished efficiently. I have myself been keen on designing ways to harness the abundance of solar energy, and always, my ideas come up against the "Complexity Wall". One item that hardened me against solar is not that the sun only shines at best 40% in useful quantity, it is really only available for use when directly overhead (within a narrow range, of just a few hours around high noon) . Just because we can see it a few hours after dawn and before dusk is no use for energy harvesting at all. so, we have to "make our hay while the sun shines" usefully, which means huge collection areas and the control of large current into massive storage units to quickly gather in all we can while the useful energy is available over such short periods. This leads us into the area of impracticality. That is: Cost of solar v 'Something other than solar". Now, I understand we are talking about small applications here. But, when I started watching this series on solar, I was delighted because I know with electrical use, size is not a difference. An amp or a volt is the same regardless of size. The only difference being cost. So, your experiments are completely viable and accurate for any size solar application. And, in the end, you came to the very same conclusions I did some years ago working with whole-house solar systems. It is something that can be accomplished, God knows, we use it here in Las Vegas where we have abundant sun. And still, it is sporadic in use and does not last very long term in a practical engineering sense or economic sense. There should be another strategy for use of it. Thanks again for a most excellent video series. You truly are a treasure.

Sehr gut gemacht!

Hi Andreas Great video I did some independent testing yesterday, as I didn't know you had already done a video about this a few months ago. My tests were just using one small panel ("5V 1W" which is actually 5V 0.75W), into a real lipo cell, under a variety of conditions I only tried 3 main combinations of charging configuration: Direct connection with voltage clam using a Zener (or a string of diodes in series) Using a TP4056 module (mine did not have protection) Using the LM2596 DC to DC converter (the same one as you used) I am pleased to say that I came to very similar conclusions as you did. For me the TP4056 module was the best option because it gave the same charge current as the direct connection, but had the added benefit of providing better battery health management. I did not see in the video what value for charge resistance you used on the TP4056, I found that for my panel (150mA output) that approx 8k seemed to deliver the best current under full or moderate sunlight. If the resistor is too low ( which makes the charge current too high), I could hear the TP4056 start to squeal, which I presume if because its crashing the input voltage from the panel by applying too much load. Strangely the same value 8K, seemed to be the optimum under all moderate light conditions e.g direct sunlight at different angles. As in your test, ambient light does not seem to yield any useful charge current. I have now ordered the CN3791 module, but using the CN3791 attached to a small panel, would not actually be cost effective. Because it better to pay $3 extra on and get 3 more "1W" panels, than to retrieve an extra 20% of the 0.75W panel (i.e 0.15W ) I've done a rather rambling writeup on by blog. www.rogerclark.net/lipo-battery-charging-from-a-1w-5v-solar-panel/

Hi Andreas, thank you for the video. Maybe you can you repeat the tests from the previous video with the protected TP4056. I have made the experience, that the protection circuit has a lot of backdraw.

Good video as always Andreas. I think the TP5000 should be a try. The key factor is the winter radiation levels. In summer I used to charge 3000mAh LiPo Battery with 2 piece of 6V solar panel and a TP5000 charger quite fast. The clouds has large effect on the time which is requires for the full charge. I would like to try in the winter what can I reach.

Andreas, please use the TP4056 with a shunt regulator in parallel to the input. The maximum supply voltage of the TP4056 is 8V. The solar panel voltage will increase when the battery is full and fry the module.

Hi Andreas, please keep up a good work! Being a subscriber for a year, I've watched and enjoyed almost all of your videos. I am little bit confused with a link you've provided for CN3791 module. I wanted to buy it, but description there says that the module is suitable for 12-18V. Is it a wrong link?

Great series, with useful data. One small thing I wondered about - is there a difference in back-current if there is an unilluminated solar panel still connected to the charger module (eg: solar panel at night time), versus open circuit simulated by (ie: disconnecting your programmable voltage source). Background: (1) solar panels connected directly to a battery without charger or diode will discharge the battery at night so apparently there is a resistive load, and (2) voltage regulators vary considerably in what they do when unpowered, depending on the impedances connected to their other terminals. So a solar panel connected to the inputs of a charger/regulator input MIGHT have higher back currents, than the charger/regulator alone. It would be nice to establish that detail (comparison) before wrapping up this testing and moving to other approaches. I imagine it would be pretty easy to test with your setup.

Great video! Sheds some light on the issue.. by the way: what's the most efficient DC-DC converter to power a microprocessor (e.g. ESP32) from a LiPo?

for us that live where the sun always shines, i would love to see the concept tested with the TP4056 and show it can run through your sunny months putting your maths into practice before you move on to super caps; which also of high interest - since lithiums arent fond of sun and heat (eg, cars and roofs where insurance might get involved). also like to see NiMH and SLA on that note

Again, many thanks Andreas. TP4056 has revealed that my panel is too small. Input voltage drops as sunlight increases. The TP4056 charges with more power but pulls the panel far below an efficient voltage. It charges my 500mA LiPo but very slowly. For topping up a low power, deep sleeping LoRa node with just a few a few daily wake-up and transmits some tuning and a bit more panel area may work. TP4056 does try to charge in ambient (not full sun) light but 10-20mA only.

Hi Andreas I recently checked the MCP module you used and read through the datasheet. What you observed is exactly what the datasheet says, 500mA is the default maximum current for the solar input. This chip is build to get power from solar or usb or both at the same time and has multiple resistors to set the charge current, it seems to be possible to charge with up to 1.3A when you set the resistors correctly and supply the input power to both charge Inputs.

Andreas very nice work!, I have tried the CN3791 on my project, but I could not power my microcontroller in parallel with the battery because the MPPT would not have an output

Awesome Video!!

Really nice series that interests me and a lot of other people it seems. Not sure if you have a long term plan to power something at the end of the series. I suspect that you want to use a pi or so. Would be interesting to see the real world differences between the pi's and maybe undervolting and underclocking the core to get the minimum consumption. Sunday mornings are awesome. Was that your son in the video??

Great video can't wait for the super capacitors version - do you think this work could be adapted to run the esp32 instead of the esp8266? Did you ever work out how to wake up using the ulp co processor ?

Awesome tutorial as always! :) I never expected the TP4056 to be this efficient (or be used as a solar charger)! This is something I can experiment on with some of my future projects.. :D

Hi Andreas, thanks for the great job! Have you investigated the effect of temperature on the charging process of lithium-ion batteries? (if the batteries will be in the winter on the street at -20 C) And have you investigated the charge controllers for solar panels for LiFePO4 and lithium-titanate batteries? (they work better in the cold)

Great video, thank you! I am having trouble finding a battery that is suitable for a device that will be out in the sun, because they are generally not rated for such high temperatures that can accumulate in the enclosure in the sun. Do you have any advice?

I got one of those foldable ones from the supermarket. Can directly plug usb in, might work for stuff like this.

Andreas did you made that video you where talking about, bigger solar panel and super capacitor?

I am building the beginnings of a solar powered outdoor weather station. I am using a 6v 120x60mm solar panel with a short circuit output of 180ma. A CN3065 to charge a 1000ma Li-Ion and using a ATMEGA328 and RFM69. Sensors are a DS18B20 to measure internal temperature of unit, BME280 for temperature, humidity and pressure. TLS2561 for ambiant light and VEML6070 for UV. Putting all things to sleep when not being used it the best way to reduce comsumption, which inturn reduces battery capacity and solar requirements. I believe that based on other devices I have built that I can run this completely of solar year round in the north of England. Maybe I share my work with you one day.

What about using an adafruit feather with built in charger. Run the solar panel direct to 5v on feather. Let the internal regulator work and charge a 18650 battery. The internal regulator of the feather should handle the 6v from the solar panel.

Andreas, for the MCP based charge controller, try adding a 2K thru hole resistor to PROG in parallel with the 2K SMD if the holes are electrically in parallel or replace the PROG SMD 2K resistor with a 1K. This should get you 1A of charge current.

Awesomeness

I see D2 and D3 appears missing, could you explain what you did to optimize the CN3791 pertaining to resistors, thanks

HI - I have been able to use one 6 volt / 2 watt solar panel to charge 3x18650 in parallel (4.2v), which powers a temperature/humidity/light intensity/relay/door sensor for over a year now. TP4056, 328P-PU, NRLF2401+, DHT22. Also, since January 2017, I have a ESP8266 (ESP-12E) running on 6x18650s in parallel using Deep Sleep with an external interrupt from DS3231 (RTC) to wake up to take a sensor reading then go back into Deep Sleep..... Battery voltage is still at 4.08 volts :)

We're informative!

Hi Andreas, with the TP4056 do you recommend using a diode between the +sig from the solar panel and the module? or it is not necessary? Thanks!

Nice series: thanks for doing this. You should consider re-testing some of these though: I am not sure if they were all used on the same basis. MCP73871: This was not set-up properly. It was run in default mode where output current is limited at 500mA. You need to vary the on-board resistance to allow what you need. The data sheet says this, as do the notes on the AliExpress link you supplied. This will completely revise the low efficiency your test data shows. You saw that input voltage was 6.5 Volts, so at a higher current it would probably be the most efficient of your samples. And in fact it was ordered from China, and the chip might not actually be a real MCP73871: you might consider getting a system from a more reliable source (like SparkFun, Digikey etc). In fact it looks like this was your case: if you read the MCP73871 data-sheet (Section 4.1), it says that when the battery voltage is >100 mV than supply voltage (from the panel) then the device is placed in shut-down mode, and battery reverse current shall be less than 2 uAmp. Since you measured 60 uAmp, something seems wrong. Either in your set-up, testing data or it wasn't a real MCP73871 chip. BQ24650: The system you tested doesn't use this chip (Texas Instruments) but has a clone CN3722. They do admit that. Maybe you should change the labels to reflect that? And perhaps buy a real BQ24650 to test. In fact there is another solar power chip made by Consonance: CN3795 see www.consonance-elec.com/product-E-Solar%20Charge%20Management.html Maybe the CN3722 is not the best choice for your application. In fact there's a good head-to-head comparison of BQ24650 vs CN3722: ukposts.info/have/v-deo/in-AiW6acIqYrmQ.html He says CN3722 is better for < 2A, and BG24650 for > 2A General issue with back-current: This can be controlled according to how you set-up the system. There are several ways, but you can add a Schottkey diode with a capacitor at the outlet to the battery to prevent this, although efficiency drops a little bit. And in some cases you wouldn't do it, because the solar panel itself might have a Schottky diode: no point in adding two. Can you show us a schematic for each device you used, as well as the solar panel, to see what has been used?

Andreas I'm little bit confused because in the last video the clear winner was TP4056 in the contrary to CN3791 which is the best in this video. So which controller should I order for my solar project?

Bitte, die Best Solar Charger Boards von raspberry. super video, besten dank

Nice video as usual. I plan to run an ESP8266 (using DeepSleep) with solar panel and LiFePO4 battery using a MCP73123 chip (Linear Charge Management Controller chip). Which kind of solar panel is most suitable for it ? 5V or 5.5V (around 1W panel) since I want to limit overheat. I live in center of France so sun is not very shiny.

I think your strategy is to plan for a lot of over capacity through the whole year. First an esp32 or 8266 has a sleep mode which can keep the consumption low for instance doing a connected weather station can have a measurement each minute. Second it is possible to have several batteries in parallel. The thing I miss here is if there is overvoltage protection in those charger and if we need to do something if we harvest too much energy. Will there be undervoltage protection in the boost ? Are you planning to use protected batteries ?

An arduino pro mini 3.3v with voltage regulator removed when put to sleep draws just a few uA also when connected to nrf24l01 chip since it has a very low quiescent current too.

As always, very informative. Maybe the goal, being completely independent and running on solar energy forever is a bit too ambitious, specially if you live in Switzerland. I would consider a small solar panel as a way to be independent during summer and maybe spring and autumn, but during winter I might still need to change the battery once or twice if it is a very bad winter. I think that would be pretty good still. Yes, you will have to replace the battery during the year but without a solar panel you will for sure replace the battery multiple times. Would that make sense?

What is the specification of the solar panel and the battery and what max charge current? I have successfully ran esp8266 for a month off a 1000 mah battery with 10 min interval, using the valuable information from your previous videos.

I think the lm2596 could perform better with higher input voltage by stepping up output current. I think now it's just working more like linear regulator with just opened internal bjt switch. Atm I think the output current is the maximum current of solar panel in short circuit. Am I right? Best! I appreciate Your work and videos!

Great work Andreas. I appreciate all the time you have invested in this project. I am puzzled by the results for the BQ24650. It is supposed to be an efficient energy harvesting IC that can produce useful power even in low light conditions. I was about to purchase a module based on the LTC3588 which is another well regarded energy harvesting IC. After seeing your results it looks like I should instead just rely upon a cheap TP4056. As regards the choice of device, the ATmega328 and ESP8266 / ESP32's all run at 3.3 volts (and lower) and that seems to be the sweet spot for reduced current. I am looking forward to your tests with supercapacitors. I suspect that running them at 2.5 v (with protection) and feeding the voltage directly into low voltage CPU's will be the most efficient technique as it will avoid the need to convert voltages. Perhaps using solar cells producing 3 v or so will be the best option for them?

Where did you find that BQ24650 board? It's not on Amazon or Banggood. And I can only find the unmounted chip from Digikey.

thanks

By the way, my Voltage and current measurements were all made using the INA219 devices - they seem really well suited to this application and my whole board (Arduino pro mini, Nokia 5110 LCD, INA219 and BME280) only draws about 8mA. The Arduino could easily go on a diet and lose its power LED and regulator, but in UK summer it is happily maintaining its power supply vertically behind a window facing South. The Arduino could also easily go to sleep, but my aim is to build a solar powered weather station using the Pro Mini and use an ESP8266 to push the data to a server and remote displays. Obviously angling the panel has quite a lot of impact on the current output. I wonder if (for small panels) a simple pair of model control servos could steer the panel about and make good use of evening sun which would normally not be seen by the panel. For this, the Arduino would need to know the time and use the library that calculates the bearing to the sun every half hour or so. My panel was producing about 250mA in the evening against 550mA at noon when angled towards the sun, but nothing usable otherwise.

Spannend wirds dann auch noch beim Anlaufverhalten nach einer Kompletten entladung der Batterrie. Alle Regler die ich bis jetzt angeschaut habe schützen zwar die Batterie vor Tiefentladung, laufen aber viel zu früh wieder an, und bleiben dan in einem power on - power off loop hängen, da das Solarmodul nicht genügend Strom liefert um den Akku und den Bootvorgang zu versorgen.

I've been playing with some of the small panels you show and came to the same conclusion, in the UK they are too small to generate useful power. Now if we could somehow generate power from clouds and rain... :)

I believe the MC73871 did not perform well in your tests because the chip is programmed via an external resistor to charge at max 500 mA. This can be changed by varying the resistor, so possibly the efficiency of the charger could be improved in your results if the resistor had been changed. Anyways, great videos as always, thanks for what you do!

Do you know of a good 6S or higher voltage solar charger that can be used as a power backup? So it charges or supports pass through power or draw from batteries?

Hey Andreas, i know this video has been posted a long time ago. But i would like to add an advice. I use the MCP73871 that in in your test it's current output was fixed on 0.51A. That's because that board contain a resistor to do that current control with the objetive of do a slow charging on batteries. If you change that resistor you can reach higher output levels.

very good thank plz keep it experiment

Andreas During peak times our controllers are discarding energy because we can't overload the battery and then 10 minutes later clouds reduce the panel output. Would it be possible to regain some of this excess energy if we placed some super caps in between the panel and controller.

Hi Andreas, Thanks a lot for this interesting review! I am planning to build a solar powered beehive scale controlled by an Arduino Uno and ordered a 12V/5W solar panel and a CN3791 regulator with 12V input voltage. Question: what kind of regulator, if any, would you use between battery and Arduino? I want to prevent the batteries from damage by excessive deload (2-3 Li hydride 3.7V, 5k mAh batteries). Would TP4056 be an option? What is the highest voltage that can be applied to the Arduino barrel power connector (would 3x3.7V in series be ok?)? I am a total newby to the world of electronics and would highly appreciate your feedback. Best wishes from Basel, Christian

How about the new TP5000 ?.It can be used for 2A.

Hi Andreas, thanks again for the great video! I was disappointed when you mentioned that small solar panels were not feasible for controllers like the ESP8266 :( Are you sure that is correct, because according to your first video about solar panels (#142) I made the following calculations: In Belgium we get 1020Wh/m2/day in December, so calculating 15% solar panel efficiency and 66% charger efficiency, I should be able to get about 100Wh/m2/day, or 10.10mWh/cm2. My ESP8266 project (my heating fuel tank level gauge) only uses very little because it measures only once per hour. During measurement it consumes about 70mA and this lasts only 15 seconds the rest of the hour is spent in DeepSleep at about 0.20mA, I calculated this to a total of 40mWh per day. So for this 40mWh I would only need a solar panel of 4cm2, which I ordered, today looks to be a good day, hopefully I can do some tests outside :)

Hi, quick question if I may. Would a 6v panel connected to a TP4056 be able to charge a mobile phone? Thanks in advance.

If you don't need Wifi/Bluetooth,and just need a simple uC to perform whatever task,you might consider the MSP430 chips. They can run on very little power.

Mr.Andres i think you are wrongly mentioning the buck ic its LM2596 thats what you shown in the video . but you are mentioning it as LM9526 . excuse me if am wrong .

hope the microAmp microcontroller will soon be available on aliexpress, banggood, ..

The ams1117 also come in 1.5v boards too.

your speaking pace (words/minute) really increased for new videos. If that's your brain changing and not some video post processing - that's impressive

I wonder where i could possibly send you some modules ???

Wie viel Energie vebrauchen diese Schaltungen im Dunkeln aus dem Akku ? Ich habe eine Schaltung mt einem TLV431 gebaut, der die Solarzelle über einen BD136 kurzschliesst, wenn die maximale Ladespannung des Akkus erreicht ist. Bis dahin fliesst die gesamte von der Solarzelle abgegebene Energie über eine Diode in den Akku, der die Spannung der Solarzelle während des Ladens auf seine aktuelle Spannung herunterzieht. Bei sehr ungünstigen Bedingungen (bewölkter Himmel, leichter Regen) habe ich mit zwei seriell geschalteteten kleinen 1W/6V Solarmodulen noch einen Ladestrom von 5,4 mA gemessen, bei intensivem Sonnenschein waren es 108 mA (genau Richtung Sonne ausgerichtet, aber in einem vertikalen Winkel von ca. 30 Grad zur senkrechten Achse). Die Schaltung mit dem TLV431 verbraucht im Dunkeln nur 0,02 mA (20 Mikroampere) an einem Blei-Akku mit 6,7 V Maximalspannung

What about a simple 4.2v zener diode circuit? For small panels

I have that semilunar one with the many wires coming out of it, still in a box somewhere. Anybody any experience with that one?

Based on the specification MCP73871 can deliver up to 1A Fast Charging current that is programmable by the resistor. I think that this might put this module on the top and not the bottom of your list. The maximum available charge current is also limited by the value set at PROG1 input that by default might be set to 500mA. Please check that module that was used for your test has 1K resistor for PROG1 and also that SEL value is set to High. I assume that you are using DC input and not USB mode.

UKposts does not make it easy to go from this video, to the promised successors. Do you have somewhere a list of all your videos? Even better would be some kind of mind map showing linkages...

But can I supply the 6Volts to an esp8266?

I built the circuit by using TP4056 as battery i chose 1500mAh 3.7v phone battery. And the circuit charged my battery. Only problem is that, to charge up the battery, the load ( esp8266) must be disconnected. The problem is, i cannot disconnect the load and wait battery to charge up. Is there any chargerboards which feeds the load and also charges the battery at the same time?

In my opinion the main issue with the ESP32 or ESP8266 is the WiFi. This is a great solution if you have mains power available. But if not, you should better use a different transmission method. Maybe BT would be better? As long as the sensor is accessible from time to time but has no access to mains I am using my "Homematic" components with a battery - alkaline or lithium primary cells. The Homematic components are transmitting @868 MHz using an optimized protocol to keep power consumption down. So a battery is good for severals years of service. This is much cheaper and much more reliable than a solar cell, LiIon battery, charger etc. And most probably also more ecofriendly. But of course not so interesting.

Do you think there's merit in using a low power micro-controller, directly powered by solar, varying the R(prog) of the TP4056 to constantly try to find the MPP? I built such a setup, and can report that it appears to work. I used the panel voltage as the basis to find MPP. The logic just said: if the panel voltage collapses, we're drawing too much current, so reduce the charging current of the TP4056. However, I don't have the test gear to fine-tune the setup and check if my results are better than a plain unmodified TP4056. I suspect that in full sun there won't be much difference, but in low-light conditions we might see a slight improvement. But, I'm not sure if these modifications are worth the effort.

Hello Andreas, thanks for sharing these videos on solar charging. I am now studying the design of solar chargers. As your videos show the topic is indeed very much complicated with lots of hypothesis along the way… I think your opinion and figures on linear vs switching solar charger are wrong. You postulate that chargers charge at maximum voltage (4.2V). This is not the case. Li batteries charge at constant current for 85% of the charging cycle with a voltage that is increasing. This means linear chargers will be far less efficient during 85% of the charge. Plus one could prefer to charge only to 85% since it would preserve the battery and it is difficult to optimise the charging of the remaining 15%. Also in real condition you will have to oversize the battery and most of the time will not charge at 4.2V. So the all concept of comparing the charge at 4.2V is not correct.

Excellent video! I'm a little confused... it sounds like you are giving up on the smaller solar panel. I don't see why it can't work as long as you have a good battery, and you aren't running the ESP-8266 all the time. Your videos have already shown that the ESP can run on next to nothing as long as it is put to sleep for some period of time. Surly, you can make something useful with a small solar panel, and a good battery, and an ESP that only wakes once every 5 or 10 minutes. If the battery charge is low, it can wake less often. Having temperature, or moisture, or other data once every few minutes is still very valuable and often more than what is needed. For example, the rumen loggers I've coded run for years on a 9-volt battery, waking once every few minutes. The limiting factor is the shelf life of the battery.

Which of these chargers has load sharing capability?

I see that the BQ24650 boards are $21.00 no including shipping + tax AND they don't do 1s for Lithium only 2s, 3s, and 4s YET we only need a 1s high quality solution because 1 Li-Ion 18650, etc... would do the job perfectly.

Maybe, you can use the "ESP-NOW" protocol for the remote node... This should be the "low-power" communication for ESPs. There is already a library for this... I found a comment here: blog.hekkers.net/2015/04/06/esp8266-good-enough-for-a-battery-powered-sensor/#comment-68028 "Just for information. My sensor application uses ESPNOW (that is a low-power communication protocol of ESP8266, not WiFi), and I got 30 days for 1 sample/30 sec with eleloop 2500mAh. So It is expected to work 1 years for 1 sample/6minites." Did you ever measured the power consumption with the ESP-Now?