I faced the same quandary in January 2018, when I setup my first device to run in the winter. Up until then, my particular application (agricultural-related) had no use in the winter, so I typically removed the devices from the field in the early fall. I did some reading on LiPO’s in cold weather, and concluded that it might be okay to discharge them at freezing temps, it is apparently not good at all to charge.
I went with the following setup:
- 5 Ah 12V sealed-lead-acid battery, feeding the Vin pin on the electron, with no LiPO connected to the electron’s battery connector. (About $25)
- 5 W 17 V solar panel ($20 on Amazon). The mounting bracket that I use puts the panel right at 45 degrees, which is almost ideal for my 40 degree latitude. I think I paid another $15 for the mounting bracket, which seems like too much. I’m sure there’s a better/cheaper solution for this.
- “Battery Tender” solar battery charger as the charge controller ($25)
The electron is always cloud-connected (always breathing cyan). No lower-power or sleep modes in use.
Normal operating voltage is about 12.5-12.8 V. In the middle of the day when the charge controller is “topping off” the battery, I see as high as 13.8 volts. This means that I’m somewhat violating the Vin spec on the electron, though not violating the TI power management IC (PMIC) voltage spec. My understanding (based on other threads here) is that the electron’s limit of 12V is related to thermal concerns with the power dissipation of the PMIC.
My assumption is that there is not likely a thermal problem in my particular application, since my current demands are always low on average. I.e., I’m not charging a LiPO from Vin, so the only “high current” (and therefore higher power disspation) ever seen by the power management IC are the short spikes that occur when the cellular modem is actually transmitting. Since these are short spikes, the PMIC should not overheat. Though, I haven’t done measurements or calculations to prove this, so take it with a grain of salt…
The battery voltage is connected (through a voltage divider) to one of the electron’s ADC inputs, so I have a log of battery voltages over the 10 months that the unit has been out in the field. I’ve seen the battery voltage drop as low as about 11.8 when it’s really cold and cloudy/snowy for multiple days, but this occurs rarely where I am in Colorado. Even in the winter, most days I see the charge controller top off the battery by late afternoon.
So far so good on this setup. I think I plan to switch to this in the long run for all of my deployments because, though it’s a bit more pricey than using the supplied LiPO, it is really a much more robust power solution than a small solar cell plus the 2 Ah LiPO that comes with the electron. And, it seems simpler to me than trying to use heaters, insulation, fans, etc, to keep a LiPO in a safe operating temp range.
Jeff
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