Month: March 2022

Roger Cheng

Battery Options for Solar Monitor

I finally got my solar power generation monitoring system up and running and I’m having a great time graphing data for visualization. But using weakened alkaline AA batteries I had available was only an interim solution, the next step is to make this into a longer running system.

This project is mildly annoying because I already have a large capacity battery system for capturing power from this solar panel. But I can’t run the panel monitor from the same battery because of different voltage planes: When charging, I measured a little over a volt of difference between input ground and output ground. As expected, there’s also a voltage difference between input positive and output positive. I don’t know the internal implementation of my battery charger, but it appears to be neither “common positive” nor “common ground” so I decided against adding any wires bridging those two sides.

Instead, I aim for an independent system that I can charge from solar panel while the sun is out and run my ESP8266+INA219 for the rest of the day. When the panel is not generating anything to monitor, I can put the ESP8266 into deep sleep. Successfully running this system on extremely weak alkaline AA batteries proved I don’t need a lot of energy storage capacity for this purpose.

On the old school nickel chemistry side of the battery world, I have nickel-cadmium (NiCad) battery cells I pulled from an old Dustbuster that could no longer run a handheld vacuum. I also have nickel-metal-hydride (NiMH) battery cells that could no longer run a Neato robot vacuum. While neither could run a big electric motor, they might still have enough remaining capacity to run a little solar monitor. I don’t have the means to charge them properly but that’s not really required anyway. Nickel chemistry batteries are fairly robust and many devices don’t bother with proper charging circuits as a cost saving measure, which was true for both the aforementioned Dustbuster and the Neato robot vacuum! So these batteries have already lived a hard life.

Alternatively, I could use a lithium chemistry battery. A proper charging circuit is very much mandatory for lithium chemistry batteries. While lightly overcharged nickel chemistry batteries would get warm as they dissipated excess energy as heat, overcharged lithium chemistry batteries aren’t as graceful and could burst into flames. (Which I guess is also technically “get warm to dissipate excess energy.”) Fortunately, since lithium chemistry batteries have become commodity consumer items, so have proper battery management circuits to accompany them. I can repurpose an old USB power bank for this job.

Roger Cheng

Plotting Solar Panel Voltage and Power

Ever since I bought a cheap solar panel array from Harbor Freight, I’ve wanted to quantify its output. Most solar power instruments were designed for house-sized (or larger) arrays, overkill for my little panel. I had instruments from the world of remote-control hobbies to measure voltage and current, but they were only instantaneous values displayed on screen. I wanted more! This motivated my journey evaluating software tools like InfluxDB database, Grafana visualizer, Home Assistant, and ESPHome. Plus building on what I knew about hardware components like ESP8266 microcontroller, MP1584 buck converter, and INA219 sensor. All with the goal of building my own solar production monitoring system.

I had thought I could make the ESP8266+INA219 sensor node run exclusively on solar power, but I gave up on that and have built a temporary setup powered by old tired alkaline AA batteries. Enough to give me a graph!

This was a fairly sunny day, so cloud cover effects were minimal. The solar panel was connected to a MPPT charger. I could see the power levels (green line) climb smoothly from sunrise, reach a maximum at noon, then smoothly fading off to sunset. This was mostly as expected. The power curve is not symmetric because there are a few things blocking sunlight from the east, reducing morning power.

While the power curve was mostly expected, the voltage levels were not. Until this graph I didn’t know solar panels would jump up so high on voltage before producing meaningful amount of power. (I don’t know if the MPPT charger makes a difference here.) If I had known this, I probably wouldn’t have bothered trying to make a solar powered circuit turn on and off based on voltage. Or if I did, I would have tried anyway and knew why it failed. From this graph I learned that if I am to draw upon solar power, I need to base my decision on power and not on voltage. But in order to read power levels, I need power for my INA219! There’s probably a clever way to circumvent this Catch-22 without batteries, but I’m going to take the easy way out and incorporate a rechargeable battery.

Roger Cheng

Running ESP8266 on Tired Alkaline AA Batteries

I retreated from trying to run a voltage & current sensor node exclusively on solar power. I’ll add some rechargeable batteries instead, but before I tackle the complexity of incorporating charging circuit (and charging logic) I took a side trip. I’m running the circuit on non-rechargeable alkaline AA batteries for a few days. And not even fresh batteries, these were already-weakened batteries from which I wanted to extract some more utility before I put them in a Joule thief LED circuit.

These batteries have open voltage of only around 1.1V, which is very low by standard of alkalines. (Fresh alkaline AAs measure over 1.5V.) And that open circuit voltage drops very quickly when I put it under strain of powering an ESP8266. So I keep usage brief, with sensor measurement and reporting run of just 30 seconds. (I later reduced it to 15 seconds.) Then I put the ESP8266 to sleep for five minutes. That five minutes give the chemistry inside these very weak batteries a chance to balance out and recover a little power before I hit them with another read+report session.

To measure how those batteries held up under the strain, I was going to wire up another voltage divider to the ESP8266 analog pin A0. But then I saw ESP8266 had an existing provision to report its chip input voltage VCC, no additional wiring necessary, and that capability is exposed in ESPHome. I chose this option out of laziness because it meant I didn’t have to warm up the soldering iron again.

The results: these weakened batteries could still give me a few days of readings before they started fading too much. I could see this in the Home Assistant log of reported VCC values.

For the first day or so, the batteries could deliver enough power that the ESP8266 didn’t see any drop in VCC voltage. But on the second or third day, voltage sags below 2.9V towards the end of my read+report session. This is visible as the lower bound of the VCC graph. During the five-minute sleep period, voltage recovers slightly and this is visible as the upper bound of the VCC graph. As the batteries continue to fade, both the upper bound (how much power the batteries could recover) and the lower bound (how low the voltage sags at the end of the session) decline over time. Eventually, VCC sagged to 1.75V and the ESP8266 could not run far enough to go back to sleep. That’s all I could get out of that particular batch of AAs. I’m happy they could run this circuit for a few days. They taught me some limits of running ESP8266 on low power, and gathered some interesting data along the way.

Roger Cheng

Start Simple with Alkaline AA Batteries

I thought I’d whip up a circuit to run exclusively on solar power. Growing around solar-powered electronics like desktop calculators, I knew it was a solved problem. But after running into obstacles, I have been humbled by the challenges involved and decided to fall back to battery power. A lot of solar power projects have a rechargeable battery somewhere in the mix, and I’m going to follow that precedence in the hopes of simplified energy management.

But as an intermediate stepping stone, I will adapt my circuit to run on batteries without worrying about the charging circuit just yet. I have the components I need on hand: a pile of alkaline AA batteries and a tray for 5*AA batteries in series.

A fresh AA alkaline battery has an open-circuit voltage just over 1.5V, and four of those in series would deliver more than 6V. Plenty for an ESP8266, but I’m not using fresh batteries for this project. My fresh AA batteries go into devices with motors or other high drain use. Once those devices complain the batteries were too weak, I move them into purely electronic devices with lower amperage demands. (TV remote controls, Hackaday badges, Xbox wireless controllers, etc.) When they are deemed too weak again, they go into my pile of AA batteries awaiting Joule thief LED duty. Open-circuit voltage for veteran batteries in this pile hover around 1.1V, thus I needed five of them in series instead of just four.

These 5-ish volts are too low to activate my modified MP1584 buck converter, which would no longer activate until input voltage of at least 13V. But that’s not a problem, because the Wemos D1 Mini clone board I’m using could run on 5V USB power. These batteries are pretty close to that voltage level, so I bypassed the MP1584 and connected the battery tray to existing “5V” pin on this module and used its onboard voltage regulator (which I didn’t trust to handle solar power directly) to deliver 3.3V to the ESP8266 and INA219. This worked pretty well.

Roger Cheng

Solar Startup Still Tricky

I have modified a second MP1584 buck converter module so that it would not activate until input voltage surpasses 13V, comfortably above the output voltage of 3.3V. I want to connect input pins to a solar panel so the associated components (ESP8266 WiFi microcontroller and INA219 voltage/current sensor) would be powered exclusively by the sun.

First is a test run with my bench power supply. Gradually increasing supply voltage starting from zero volts. Thanks to the modification, there were no odd behavior or sounds of a MP1584 trying to work with too low of an input voltage, which is great. As I increased voltage past the ~13V threshold, I saw the blue LED of my ESP8266 blink and it booted up as planned. This time, it was able to find INA219 on I2C bus, which is further than I got before.

Feeling optimistic, I connected this circuit to my solar panel at night and hoped I would wake up to find the system running. Sadly I woke up to disappointment, as there were no logged messages from the ESP8266. Probing the circuit with my volt meter, I confirmed a 3.3V supply voltage was present, but for whatever reason the ESP8266 failed to boot that morning. I manually disconnected and reconnected the circuit board, and this time ESP8266 booted up fine (now it has full daylight power) and started reporting values measured by INA219.

I don’t know what happened at sunrise. I hypothesize that when the solar panel output voltage rose past 13V, it has still yet to produce enough power to successfully start an ESP8266. So when MP1584 activated, it could supply 3.3V but not enough amperage to supply an ESP8266 through its boot process, putting it in a glitched state that was neither on nor off and stuck there until I power cycled the system. [UPDATE: Further experimentation found this hypothesis was correct, the panel would reach operating voltage well before generating appreciable power.]

I didn’t have my oscilloscope set up to capture the startup waveform to confirm or disprove this hypothesis. It’s clear there are additional subtleties I don’t know about starting up a circuit on solar power. Do I want to invest the time to learn and experiment with this problem domain? After thinking it over for a bit I decided “nah” and abandoned the idea of running everything exclusively on solar power. I’ll retreat to what I know and incorporate batteries into the system instead. Starting simple with household alkaline AA batteries.

Roger Cheng

MP1584 Modification Version 2

I’ve learned some lessons from the first round of modifying a MP1584 module for solar power input, and I thought I would try again. This time I started with a new module that hasn’t suffered the abuse of being enabled at too low of a voltage. To keep it that way, I still need to change the resistors on board so it activates at a higher voltage than the default 3V. Last time, I desoldered a surface-mount resistor, and it wasn’t a very neat job possibly damaging the board. This time I will go with a less invasive process and add more resistors in parallel with an existing resistor. This one between EN and GND was already on the edge of the board so it is easier to access.

I chose to add a 47kΩ and a 22kΩ through-hole resistor in parallel to the existing 100kΩ surface mount resistor between EN and GND. This should lower the effective resistance to 13kΩ between EN and GND. Combined with the existing 100kΩ resistor between EN and Vin, this should result in an activate voltage (EN ~=1.5V) somewhere around 13V.

A quick test with the bench power supply confirmed the new activation point. I then mounted it to a perforated prototype board using two 220uF capacitors as input and output pins. I didn’t put any effort into figuring out the perfect capacitance value, 220uF was just what I had available in a big bag of 100. (*)

Conveniently, using the capacitor’s legs to mount this board solves another problem I had with this type of MP1584 module: the input and output holes on the corners does not align with the 0.1″ spacing on a perforated prototype board or breadboard. But now that I have these capacitor legs, I could bend them to fit available spacing. Their length also leaves plenty of space for me to clamp on allegator clips. Useful for providing power from my bench power supply, and for measuring output with oscilloscope.

With a new candidate power supply in place, the next step is to solder the rest of the board to accommodate my ESP8266 and INA219 boards and test it out.

(*) Disclosure: As an Amazon Associate I earn from qualifying purchases.

Roger Cheng

Potential Explanations for MP1584 Behavior

I thought I could modify a MP1584 buck converter so that it would start up and shut down appropriately depending on whether its input voltage from a solar panel is enough to deliver 3.3V output. It destroyed my circuit instead and I could see what happened under an oscilloscope: when input voltage is close to the on/off transition point, output voltage is a sawtooth pattern with overshoot peak far above 3.3V. Why might this be happening? I can think of a few possible explanations:

  1. When replacing resistors for the voltage divider, I had damaged a part of the circuit critical to proper voltage feedback and regulation. I think this is very likely.
  2. Before I learned MP1584 enable voltage, I was giving it voltage that would trigger the default enable circuit (at 3V) which was lower than the target voltage (3.3V) and this damaged the chip. I think this is also fairly likely. Based on my reading of the MP1584 datasheet, it was not designed for variable-output adjustment with a potentiometer as is done on this board. There is an implicit assumption that the application engineer knows to make sure enable voltage is higher than target voltage and avoid this situation.
  3. On some boost converters, voltage overshoots are expected in low-load conditions. I regard this possibility as unlikely, since MP1584 datasheet was very proud at how it can automatically adjust to handle low loads. I found no mention of a minimum load required for proper operation.
  4. This is normal behavior for MP1584 close to enable on/off threshold. This is wrong for genuine MP1584, but if this chip was a knockoff (always a possibility when buying over internet from lowest bidder) then perhaps the knockoff chip has bad transitory behavior assuming few buyers would notice.

I don’t know which explanation is correct, and right now I’m not terribly interested in risking additional components to test these hypotheses. But I plan to do the following which should mitigate all of the above:

  1. Never use this specific modified module again.
  2. In the future, avoid enabling MP1584 when input voltage is lower than output voltage.
  3. The MP1584 module has a small surface-mount capacitor across both its input and output. For additional buffering, I will add larger capacitors to both sides for additional buffering.
Roger Cheng

Putting Modified MP1584 Under Oscilloscope

Something didn’t go according to plan with my latest project and destroyed a few components. The prime suspect is my recent modification to a MP1584 buck converter modules I bought on Amazon. By default, it would activate if input voltage is over 3V. This is far too low to deliver 3.3V output, so I modified it to defer activation until input voltage is nearly 12 Volts. A simple test with volt meter and simple LED was successful, but when I connected the ESP8266 microcontroller and INA219 sensor I released the magic smoke.

Before I connected those components I had checked the voltage output and it looked fine on a volt meter so I suspect there’s something with a transition behavior. This calls for an oscilloscope and I have a cheap DSO 138 that is better than nothing. Since the time I bought the kit and put it together, something has gone wrong with its voltage reference and now absolute voltage readings are unreliable. However, I think the relative shape of the waveform still resembled reality.

I first checked typical startup behavior: what does the 3.3V output look like when I connect this modified buck converter to 13V input? I set the oscilloscope to hold data upon rising edge trigger, and captured this:

The shape looks reasonable. It corresponds to graph in the MP1584 datasheet and shows the advertised soft-start capability to avoid a voltage overshoot on startup, which was my first suspect. I then turned the oscilloscope to continuous scan mode and adjusted my power supply voltage up and down. When I stay well above the input enable voltage of ~11.8V everything looks good, the buck converter maintained a steady 3.3V output. (It reads 5.27V on the display due to the voltage reference weirdness I spoke of.)

But things started looking dicey as I dropped close to 12V. I started seeing a sawtooth pattern to the output voltage. Instead of holding at 3.3V, I started seeing the voltage drop then jump upwards, with the magnitude growing wider and wider as I dropped input voltage. The worst was when I dropped into range below the 11.8V activation voltage, but still staying above the shutdown voltage…

“Well, there’s your problem.“

Without accurate voltage reference I don’t know exactly how many volts it whipped through, but at a guess it looks like it dips just below 2V before overshooting above 4V. This would definitely destroy components designed for 3.3V. It’s great that I see what is happening, now I need to think about why it happens and decide what I will do about it.

Roger Cheng

MP1584 Modification Did Not Go as Planned

I wanted a DC voltage/current monitor module to be powered by the same solar panel it is monitoring, which meant it shouldn’t start running until the panel started delivering enough power to run the circuit. I modified a MP1584 buck converter module so it would not activate until the voltage rises close to 12V. I wired this MP1584 module back into a monitoring circuit with an INA219 voltage/current sensor and ESP8266 WiFi microcontroller.

Before I powered it up, I used a meter to verify I hadn’t done anything silly like shorting Vcc to GND. This circuit also had a Vin+ and Vin- plus I2C communication lines SDA and SCL. I verified none of these were shorted to any of the others.

I then connected Vin+ and GND input pins to my bench power supply. I started with zero volts and slowly raised it until close to 12 volts, when I saw the blue led on my ESP8266 board blink signifying startup. I opened ESPHome dashboard so I could see the logged output from this ESP8266, and saw that it powered up and connected to WiFi successfully. However, it failed to find the INA219 as an I2C peripheral.

My first hypothesis was that I made a mistake soldering I2C signal wires SDA and SCL. I probed those connections and verified I hadn’t crossed those wires and there is electrical continuity. But ESP8266 still reported no response from an I2C scan.

The next hypothesis was that I used too thick gauge of wire for I2C signal lines and its higher capacitance had degraded I2C signal. I replaced the 22AWG wire with 30AWG wire, verified they had continuity and I hadn’t accidentally swapped SDA and SCL. But still no response on I2C scan.

While looking at the ESPHome YAML file looking for a configuration error, I smelled the distinctive scent of dying electronics. My first act upon turning back to the workbench was to turn off the power supply. Then I saw the INA219 was very obviously quite dead, a hole burned into the top and charred remains surrounding that hole.

I repeated the same basic probe I performed on this circuit before I powered it up, and this time my meter said Vcc is shorted to GND. Not good! I separated the components and measured them separately. The MP1584 power module did not short Vout (a.k.a. Vcc for the other two boards) to GND. The INA219 sensor module did, which is very likely related to why it died. And mysteriously, the ESP8266 microcontroller module reported the same… how the heck did it continue running with its Vcc shorted to GND?

Dumping both the INA219 sensor board and ESP8266 microcontroller board in my box of fried electronics, I took a closer look at my modified MP1584, suspected killer of electronics.

Roger Cheng

Raising MP1584 Enable Voltage by Replacing Resistor

I examined my MP1584 module and learned it was activating at far too low of a voltage. I want this buck converter to deliver 3.3V, and generally buck converters need input voltage a few volts (~2V) above the specified output. By that rule of thumb, my project shouldn’t activate until somewhere north of 5V, but it was activating at 3V and making a sound I could hear as it tried to perform an impossible task. This can’t be good.

I thought I would try modifying the board with different resistor values to raise its input enable voltage level. I will leave the low side resistor as-is at 100kΩ between EN and GND, and replace the high-side resistor. Looking through my commodity resistor pack, I thought a 220kΩ in series with 470kΩ should do the trick. Having 690kΩ between Vin and EN, and 100kΩ between EN and GND, should result in a voltage divider that activates EN (1.5V) when input voltage rises to approximately 11.85V.

At first, I thought I would have to switch to my small soldering iron tip to remove the existing high side resistor. But before I switched, I noticed my normal soldering tip is almost the same width as the resistor, allowing me to heat up both sides at the same time for removal. It left a big glop of solder doing so, but that wasn’t too hard to clean up. I then soldered the two resistors, in series, between the EN pad and Vin pad. Since these are through-hole resistors and not surface mount, it was not elegant. But it seems to work.

Slowly increasing input voltage with my bench power supply, I didn’t hear unhappy sounds at 3-5V. Nothing happened until I was close to 12V, at which point the MP1584 came alive and started delivering 3.3V. Success! Or so I had thought… and proven wrong by a puff of smoke.

Roger Cheng

Probing MP1584 Enable Pin

I want to use a MP1584 buck converter module for a solar-powered project, and to explore its behavior I used a bench power supply to vary volage input from zero volts up to expected operating voltage. I heard an audible screech from the circuit within the 3-5 volt range and decided not to go further until I better understood what’s going on.

The first step to problem solving is always to Read The Fine Manual, in this case MP1584 data sheet published by Monolithic Power Systems. The first surprise was the big read “NOT RECOMMENDED FOR NEW DESIGNS REFER TO MP2338” stamped across every page. I guess this chip is getting phased out by Monolithic, and at some point I will have to learn about a different chip. But that doesn’t matter today so I proceeded to read about its startup behavior. Specifically the EN (Enable) pin on this chip. According to this document, MP1584 will start up when EN is above 1.5V.

Great, what does that mean for my little board, purchased from Amazon from the lowest bidder that day? I laid the board flat and started probing. From what I can tell, the EN pin is connected to a straightforward voltage divider ladder, using two resistors of equal value. These little surface mount resistors have “104” printed on them, indicating they are 100kΩ plus or minus some percentage of tolerance.

Here is the same image in light grayscale, and I filled some color into the relevant areas.

EN trace is in blue, and its circuit board trace immediately goes under the chip to emerge to the left moving up. Running between the two poles of a capacitor(?) it reaches the two voltage dividing resistors. The top resistor bridges between EN and ground, which I filled in black. The lower resistor bridges between EN and voltage input, which I filled in red. Vin can be seen connecting to the Vin solder pads to the upper right.

A voltage divider with two equal value resistors means voltage of EN will be half of input voltage. To test this hypothesis, I soldered a wire to this pin so I could measure its voltage as I vary the input voltage. A small 5mm LED module was connected to MP1584 output. This is a self-cycling unit that quickly flashes through different colors from RGB mixes.(*) I used it here because it tolerates a slightly wider range of input voltage and current than just a single diode, plus it is inexpensive and disposable in case something went wrong with this experiment.

Clipping a voltage meter to the blue wire, I quickly confirmed the hypothesis is correct. EN voltage is approximately half of input. Therefore, if a MP1584 activates at 1.5V, this circuit will activate with 3V input. This is a problem when the potentiometer has been adjusted for 3.3V output. This is a buck converter. It lowers voltage and could not raise it! No wonder it was unhappy and screeched its displeasure.

But now that I have a basic understanding of how this module decides to come alive, I could modify it to suit the project at hand.