Arduino on battery getting unstable if temperature falls

Question

i am building an outdoor sensor with a combination of a custom Arduino board and a battery (+solar charging). I experience strange behavior when the temperature falls below a certain value (i can reproduce it by putting it in the freezer), regardless which type of battery i am using. The custom Arduino runs without power-regulation, straight from the 3.7volts the battery can provide. How can i prevent this to happen? I guess a little heating would be not really a solution as i am trying to consume as less energy as possible. Would a (step up) regulator possibly help? Any other ideas?

---Update--- Thanks to your comments here i figured out, that the AVR 328p should not be operated below a voltage of 3.7v if run at 16Mhz. As i am running with a 3.7v LiPo Battery that could easily explain my problem i guess. My first try to decrease the processor speed to 8Mhz with a Prescaler worked, but the GSM Fona does not seem to like it. So i would like to try something in between like 10Mhz or 12Mhz. As the prescaler does not support this, my question is: can i just use another crystal or would i have to modify something in the code/bootloader as well?

Answer 1

To answer your modified question:

Running the ATmega328p at 16Mhz with 3.7v is out of spec - see this EESE answer for reference or the ATmega datasheet - so the obvious solution would be to lower its speed to 8Mhz.

But since you also mentioned that there is an additional GSM breakout board involved (Adafruit Fona), which expects UART (in auto-baud mode) Rx/Tx connection, it might not be that trivial. Further exploration of the Fona library's source code reveals that it has several hard-coded delays (see Adafruit_FONA.cpp::begin, for example), but its core seems to be implemented inside the Adafruit_FONA::readline method, which uses an underlying Stream, which in turns comes from a Software/Hardware serial object, as shown in the FONAtest.ino example that comes with the library.

From my experience, Software/Hardware serial should work fine on 8Mhz, except maybe when using high baud rates, which is solvable. The hard-coded delays doesn't look like they should cause actual damage except for delay, but you could cut them in half just to see if you can make the library work.

Additional aspect that you should consider is the comment from Adafruit's Fona product page, regarding pin Vio:

This is the pin that you MUST drive with an external voltage from 3V-5V to set the logic level converter

So, despite that from the datasheet, the Fona seems to have a voltage regulator, you might still want to consider adding a decoupling capacitor between Vcc and Gnd of your battery, to mitigate transient current draws from any component in your circuit.

Answer 2

It's most likely that the cold weather is messing with your batteries. I don't know what battery types you've tried, but this site says that Lithium batteries (Not Li Ion, but standard single use, expensive, lithiums) hold up well in very cold and very hot environments. For rechargeable batteries the site recommends NiMH batteries. Of course, we all know that lead acid batteries hold up well in cold (Or else our cars wouldn't start!), so those are also a viable option. Lead acids can be purchased in somewhat small forms, but they may still be too big for your purposes.

For the other part of your question, yes a regulator will help. It's rarely acceptable to run directly off a battery without a regulator. Since your application is low power you'll want to use an efficient switching regulator (Search for modules, not individual chips). Remember that a battery's output changes based on its charge level, temperature, current draw, and other things I'm likely forgetting. Using a regulator mitigates these effects as regulators are less affected by temperature and current draw than the battery that supplies it (Hopefully).

So to summarize: Use Lithium batteries, or NiMH if you need to recharge, or Lead Acid if you need a ton of capacity and size isn't a limit. Then get a switching regulator module. Best of luck with your project!

Answer 3

When using battery, always use a step-up converter, something like LTC3525 or NCP1402. A direct connection of unregulated battery voltage to an analog input can give you a good information on the remaining capacity while the step-up converter still provides stable 3.3V for operation.