Dealing With Low Battery

Question

Hi All,

I'm hoping to use solar for powering micro controller data logging/sensing projects.

Specifically, I have an ESP-8266 (like Adafruit's Huzzah) based project I'd like to run on solar. (the temperature logger I blogged about recently)

The ESP, being WiFi enabled, uses a fair bit of power to communicate, and I would like to avoid any issues with brownout or shutdown at bad times. And also perhaps issues with startup/shutdown flickering.

The challenges include:

1. charging a battery from the solar panel.

2. output of the battery has to provide 3.3v to power the board

3. battery should be sufficiently charged before it provides power to the board

4. battery should shut down the board (cut the power) when voltage goes too low

5. The micro controller should be super energy efficient - ie, sleep most of the time, wake up just long enough to do something, then sleep again.

I'm still having trouble wrapping my mind around points 3 and 4.

Mark mcb1 suggested:

>For the Forget me Not challenge, the EnOcean devices reduced their reporting rate to suit the charge they had.

Which is a great idea - dynamically adjusting sleep and wake times depending on battery voltage. But I think it only addresses part (most) of the concerns.

My more direct concern with 3 and 4 was more outside of the control of the microcontroller: can we keep the battery controller from providing any power at all until a minimum threshold is achieved? ie, if battery pack voltage drops to the minimum 3.2volts, shut off power to the microcontroller, and then don't provide power again until it's back up to, say, 3.4 volts. I suppose a very power-miser type controller could control that part (digispark maybe?). So the battery would need enough power to run the digispark, and the digispark would turn on a relay if it figures there's enough power to run the ESP8266. I'm pretty sure I've seen battery protection circuits out there that will avoid over-discharge that can shut off power to the digispark in those cases.

I guess there's also that detail about not cutting power to the ESP when it's in the middle of communications.

Thanks,

-Nico

ps, This discussion started in another thread, but I thought it could use its own spot, mostly for future reference for all of us

pps, The rough approach is to just set it up and hope it doesn't run out of power much, and else just let it deal with it, but I'm looking for a gentler more refined solution.

mcb1 · Accepted Answer

Seems you need this https://www.adafruit.com/product/3428 A voltage detector is handy for projects where you want to make sure your microcontroller doesn't run at a low voltage. For example, say you have a Feather Huzzah running on a lipoly battery every few minutes. Eventually the battery runs down, and goes below 3.3V, so the voltage to the ESP8266 core is too low, and could cause a lockup or corrupted data. Or maybe your battery is already drained and you're trying to charge it back up, but as you are trying to charge it up, the Huzzah turns on and the spike of current drains the battery immediately so it never gets through the trickle-charging stage. This little chip is a simple solution: It monitors the voltage on one Input pin, and when that voltage goes below 3.3V, it will pull the Output pin low. If you connect that Output pin to the Enable pin on the Feather, it will make sure that the Huzzah does not run if the battery dips too low. Adafruit have a number of Lipo cells that have over and under voltage cutouts. This one cuts out at 3v. https://www.adafruit.com/product/258 They talk mostly about 5v charging, and I haven't really seen a 3v3 output device. This one does have the status available, so you could use it to shutdown the ESP. https://www.adafruit.com/product/390 I would suggest if you have spare I/O, then signal when your sending (or when it's safe to shutdown) and use the controller/voltage monitor to shut it down. Let me know when you get it sorted ... I'll copy it Mark

mcb1 · Answer

ntewinkel I ran across this post on the LowPowerLab site. https://lowpowerlab.com/2017/09/15/weathershield-supercapacitor-tiny-solar-cell/ Using a supercap might be an alternative. Mark

ntewinkel · Answer

Hi Raymond,

Thanks for that link. It looks like a more general 12volt kind of setup. I've done that before with bigger panels and a controller, charging a bank of 12v batteries. Unfortunately the panels failed rather too soon, just weeks after warranty was up

For the remote logger I was thinking of something much smaller, to keep the overall sensor size small.

That's why I thought the AA sized Noma batteries would be a good size (even if doubled in parallel). To charge them I found this "TP5000" battery charger module that can safely charge the 3.2v batteries.

The charger module can handle input voltages between 4.5 and 9 volts. So I'm hoping to find some little solar-light sized panels that will stay within that range when in the sun. I have a fair number of little salvaged panels I'd like to start with.

That's a custom solution, but a much easier way to go is to purchase one of those solar battery packs designed to charge cellphones, and use the 5v USB output to power it - the Witty can take 5v directly, and for my ESP I built that little 3.3v adapter. I later also found little 3.3v modules that do the same.

Cheers,

-Nico

mcb1 · Answer

So it's ok to hold a board in extended reset? I beleive there may have been a similar discussion when jw0752 was doing his thing with the battery timer. The trick would be if the particular micro goes into a low current mode while it's held low. Presumably the clock is dead, and all the outputs are also low or tri-state. Mark

ntewinkel · Answer

Great finds, Mark! Thanks I didn't know about voltage detectors, but that's a neat little gadget. I have a Huzzah here (part of a shopping cart contest prize ), and I see it has the enable pin. The other ESP boards don't have it, so it looks like Adafruit added some smarts to their boards. It looks like the easiest solution is to just purchase the Adafruit solar kit = battery + solar panel + charging board. And +voltage detector and Huzzah. Not the cheapest, but with Adafruit you know it's done well. But when have you ever known me to take the easy route? ok well... Mac, Arduino, RPi... but besides those times (and ignore the electric bicycle assist too)... And ok I admit I have lots of little easy to use modules for all kinds of specific purposes... But I digress, ignore that last line. I'm wondering what the enable pin on the Huzzah does, and if that can be generalized to other boards. Thanks, -Nico

ntewinkel · Answer

>I pondered: >I'm wondering what the enable pin on the Huzzah does, and if that can be generalized to other boards. Thought I'd read up on the Huzzah some more, and found this: EN (CH_PD) - This is the enable pin for the ESP8266, pulled high by default. When pulled down to ground momentarily it will reset the ESP8266 system. Which corresponds to a pin on the ESP chip itself, and it looks like other boards simply call it RST. And even the very limited ESP-01 has the ch_pd pin. So it's ok to hold a board in extended reset? Which is what Adafruit seems to suggest: " If you connect that Output pin to the Enable pin on the Feather, it will make sure that the Huzzah does not run if the battery dips too low. "

ntewinkel · Answer

Further to the idea of using a Digispark as a power controller: http://digistump.com/wiki/digispark/quickref About halfway down the page is a sketch: " The following will display on the LCD the current voltage coming into the Digispark without having to use a pin to measure it! " Which opens up the possibility of using the Digispark (or any similar board like Adafruit Trinket, or even bare ATTiny85 chip) as a gate keeper: 1. Modify the above mentioned sketch to monitor high and low power levels 2. Listen for "active" signal from ESP board 3. Super low level is monitored by hardware - ie, the battery protection circuit shuts off all power at a certain point. 4. When Digispark starts up, relay (or transistor) is off so no power goes to the ESP board 5. When Digispark detects voltage is above a safe level, it powers up the ESP board 6. When ESP power is on, and Digispark detects voltage going below that safe level, it waits for "active" signal from ESP board to go low, and then shuts off power to the ESP board. 7. Repeat back to step 5 The assumption is of course that the ESP board is programmed to be a good citizen to not keep "active" high for extended periods of time. -Nico

urkraft · Answer

Hello Nico,

Interesting project! I too have been contemplating a similar one. Is this link of interest to you?:

http://www.solarpoweristhefuture.com/how-to-charge-a-battery-with-a-solar-panel.shtml

Good luck!

Regards,

raymond

ntewinkel · Answer

Another note for safekeeping and sharing:

You can get AA sized LiFePO4 batteries that have a nominal rating of 3.2 volts. You can also get charger modules designed for them.

I have some by Noma that my inlaws accidentally bought (and used to blow up their solar lights). Each is 600 mAh.

From what I have read, fully charged is at about the maximum level the ESP handles, but it spends most of the battery life around 3.2 volts, then drops sharply when it runs out.

-Nico

Dealing With Low Battery

Top Replies