Lithium batteries have a commonly-specified discharge ambient temperature range of -30°C to 80°C (-22°F to 176°F), and an optimal discharge temperature range of -10°C to 50°C (14°F to 122°F). However, lithium batteries should not be charged with ambient temperatures below 0°C / 32°F. Charging a lithium battery when it is below freezing can damage the battery, and reduce its lifetime.
There are lithium battery models that are designed to operate at lower temperatures, but unfortunately, the lower operating temperature only applies to discharge, not charging. Even for these low-temperature batteries, the ambient charging temperature still needs to be above freezing. Here is an example low-temperature battery:
I reached out to the manufacturer, and they said “for charging the C18650PCB, you would be looking at a temperature range of 0 ~ 60°C with a max charge current of 1.7A”.
Here is another alternative:
This 4200 mAh battery has a discharge rated ambient temperature range of -40°C to 50°C (-40°F to 122°F), but according to the manufacturer (info@nitecore.com), “the charge temperature is from 0 to 45 degrees”. This is the battery model that I have choosen for my project.
In a different post, I will describe how a zener diode can be used as a battery heater. This can help, but you might not be able to avoid charging the battery at below freezing temperatures in the winter.
Charging the battery repeatedly at sub-freezing (or elevated) temperatures can permanently degrade the battery. You might think that a 4200 mAh battery capacity is an overkill in this application (which for me, uses only the LoRa module, with no sensors or gps), but note that the storage capacity will likely degrade over time, due to charging at damaging temperatures. In summer mid-day conditions, with direct solar heating, the battery is likely to get hotter than 45°C. Charging under such conditions will likely reduce the lifetime of the battery, which is another reason for using a battery with over-sized capacity. And having a high-capacity battery also reduces the likelihood of a total discharge, which is also damaging. Note that not charging below freezing or above 45°C is not an option, since both conditions are likely to be sustained.
The Wisblock box is designed to accomodate a flat lithium-polymer battery, but cylindrical lithium-ion batteries are more appropriate in this application. Lithium-polymer batteries do not last as long, since their gel electrolyte tends to harden over time. Also note that a zener battery heater (which I cover in a different post) will be more effective with a compact, cylindrical battery than with a wide, flat battery. I placed desiccant packs in the enclosure under the RAK19007 (in lieu of a flat lithium battery) to capture moisture. The enclosure will ‘breathe’ through the cable gland as it heats and cools, allowing moist air to enter the enclosure, and the desiccant might help impede condensation.
I lined the bottom of the box with aluminum tape to act as a heat spreader/sink. The battery lifetime can be shortened by exposure to high temeratures (particularly during charging), and conducting heat through the bottom of the sealed box is a good means for cooling, particularly since the bottom of the box is likely to be shaded, and since the mounting bracket will act as a heatsink.
Oddly, the low-temperature 21700 Nitecore battery is much cheaper than their low-temperature 18650 battery, despite the 21700 having higher-rated capacity, so I use the 21700 model. I know that there are much cheaper lithium batteries available than the Nitecore 21700 battery, but I aspire to have a Meshtastic node that will work reliably, autonomously for years.
To make the battery holder fit in the Wisblock box, I had to modify the box by cutting out the two middle lid screw posts using a Dremel, and filling the lid gap with silicone sealant. That leaves the four corner lid screw posts. This modification changes the box rating from IP67 to IP65, which really doesn’t matter, since the solar panel is rated only to IP65. I attached the battery holder to the inside bottom of the box using double-sided tape.
I soldered a pair of JST-PH 2.0 connectors to my 21700 lithium-battery holder in parallel to facilitate the battery connections to the DFR0559 and to the RAK19007. One side benefit of having two connectors is that the RAK19007 connector can be temporarily removed to allow solar charging to charge the battery as quickly as possible. I also added a 2.9V lithium-battery protection circuit (https://www.etsy.com/listing/1743333017/lithium-ion-and-polymer-battery) between the battery and the RAK19007. This prevents brownout battery voltages from being seen by the nRF52840 on the RAK4361, which can cause the RAK4361 to enter a “Super Deep Sleep” state, which will brick the RAK4361 until a hardware reset is performed.
This added protection is redundant with over-discharge protection that is built into the battery, but the voltage threshold in the battery is set too low (2.5V). The higher threshold in the external protection circuit reduces the likelihood of an adverse sleep event, and it better helps protect the health of the battery. The SGM6036 buck regulator on the RAK19007 will generate a 1.1V drop below its input voltage, so it will produce a 1.8V supply voltage to the nRF52840 on the RAK4361 when the battery supply voltage is 2.9V. The minimum-allowed supply voltage for the nRF52840 is 1.7V.
Using the solar array I have installed on the roof of my house as a reference, I estimate that the HX-140x90 should be able to make 8 Wh on a sunny day, and maybe 4 Wh on a cloudy day. The RAK4361 will need about 2 Wh per day. The HX-140x90 really should be sufficient. A fully-charged 4200 mAh battery should be able to supply power to the RAK4361 for more than a week. Over the course of one sunny, summer day, I saw the reported state-of-charge on my prototype rise from 50% to 100%, which corresponds to a 7.8 Wh net gain.
Here is another alternative battery architecture: Meshtastic Devices | Austin Mesh. However, my goal is to create a compact solution based on the Wisblock Unity Enclosure, and the Austin solution is too big. Also note that the allowed temperature ranges in this case will likely impede operation: High temperature (45°C input cutoff, 60°C output cutoff), low temperature (0°C input cutoff, -15°C output cutoff), storage (-20 to 35°C). And finally, note that the Meshtastic firmware does not provide state-of-charge battery monitoring for this solution.
Indeed: If you are more concerned about high-temperature operation, here is an alternative battery you might want to consider:
That battery has a 85°C output cutoff! Note that the DFR0559 is rated to 85°C, the RAK19007 is rated to 75°C, and the RAK4361 is rated to 85°C.
