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Comment by cuu508 | Hacker News Reader

> Things that have a small solar panel, a battery, and no grid connection could benefit from this.

As a side topic, what are the currently available options for this?

I've heard about sodium ion (safer but still not sub-zero charging friendly, also no easily available solar charging controller boards), lithium titanate (same), and plain old deep-cycle lead acid.

For small solar-powered projects, it would be nice if there was a power bank on market which supported solar charging, and could either be safely charged in freezing temperatures, or had built-in temperature sensor and reject charging when it's too cold.

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Most consumer LiFePO4 power stations have this sensor and cut charging below 0C, still allowing discharge up to around -10C.

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.

Can you point to any specific examples? Do they support solar charging? And what wattage panel would be needed to support both charging and heating?

Just look for ‘self heating’

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?

Self heating lithium iron phosphate battery [https://battlebornbatteries.com/shop/products/batteries/heat...]

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.

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.

[1] - https://www.amazon.com/s?field-keywords=lifepo4+heated

According to this sodium ion battery product, it still charge quite happily at -20 degrees C:

https://midsummerwholesale.co.uk/buy/eleven-energy/4-5-kwh-s...

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