For completeness sake there are also many LifePo4 battery options that have heating pads that will activate below a certain temperature. Even Amazon have about a dozen brands with heating pads.
To answer the question look for "lifepo4" and "heated" [1] but it sounds like that's too big for your use case. In your case just use a regular and smaller deep cycle battery in a very waterproof enclosure. Bury the enclosure at least 6 feet under ground to keep it from falling below 50F / 9.9C without heating pads. At least 4 feet deep with 2 feet of gravel under the enclosure to keep water from pooling around the enclosure. Fill the bottom of the enclosure with desiccant pouches to absorb condensation. Use rodent-resistant cabling in conduits and a lot of sealant where the conduit is fastened to the enclosure. The enclosure should then be inside a rodent-proof vault. This could be pressure treated redwood or synthetic material rated for burial. Lay a foil marker in the hole above the vault and keep it centered as you bury the vault so that you can find it again.
Being under ground will prevent both over-heating and freezing assuming your circuit is per spec designed to run cool. This can also reduce tampering. If you are not concerned about serviceability of the circuit board then you can put it in a small enclosure and fill it with epoxy so that the entire thing is water proof. There are epoxies designed to conduct heat but not electricity.
This assumes of course that your setup is meant to be stationary.
I'm searching for "self heating power bank" on amazon.com and get various random crap (self heating lunch boxes, regular noname power banks, heating pad for pets...). Could you point me to some specific examples?
Thanks. These are a little big (100Ah, 200Ah, 50Ah...), heavy and expensive compared to what I'm looking for. For reference, I'm currently using a 5W panel, and a single 18650 element, costing a few dollars from AliExpress. Works fine for my usecase (Meshtastic nodes) except for the charging below 0°C thing.
At that scale you are probably better off building your own solution: a low-power microcontroller with a temperature sensor, a small heating element (search "heating foil"), some voltage sensing with the ADC on the battery and solar panels, and a bit of simple logic.
If you are mostly concerned with low-temperature charging you could even power all that from the charging side only.
At 5w, a simple temperature sensitive switch, 5w resistor, and some wraps of insulation might be enough. If below x temp, just shunt all current to the resistor, otherwise power the charger.
