I've heard that RAK boards will get stuck in a deep sleep mode or something if the voltage goes below a certain threshold, and need to be power cycled or reset. Haven't tested myself, just something my local mesh group has said they experienced.

So, I design battery packs for a living. 115F ambient operating conditions are doable assuming you dont add solar loading. Can you paint your enclosures white? Or cover them? A high quality 18650 will operate fine up to 50C especially with a very light load and a low charging current. Do you have more details on your setup, chipset and average load?

I developed a simple circuit using 1 Farad supercapacitors and a power management IC, no battery. It buffers a 5v solar panel and provides smoother power when the aun is up. It even trickle charges the supercapacitors from just a bright security light outside the window at night.

The effect is that the capacitors store the trickle until they are full, then deliver a burst of steady full power, enough to power a node for 30 minites when busy. So with this circuit, a node cleverly placed near an outdoor light would operate continuously in daylight and intermittently at night with some artificial light nearby.

A bit of a buffer battery is good.

I've got the opposite problem of lithium batteries freezing. But three NiMH batteries in series is close enoigh to drop-in. They're ok with cold, maybe also heat?

Or, capacitor to even out the load. Or a super-capacitor if you want to last all night long. They'll consume everything to charge up from empty, though.

I ran a PirateBox on a Pi 0w by connecting it to a pass-through USB battery pack, then the battery pack into one of those tri-fold Anker (or was it Aukey? Been a few years) solar panels. I hung this up in a window that got good sun most of the day (being in San Diego was of benefit here) and that thing would run off battery after sundown and the battery would charge once the sun was back. I wasn’t measuring any voltage or other metrics, but it worked fine.

Have often thought about taking one of the Heltecs in my drawer and firing up a node that way. I still have that same battery in another drawer. I have many drawers.

Edit: fkn typos.

Yeah, it's completely possible to just buy one of those solar panels with a USB connector and plug it right in. I've done it before. But you're going to experience multiple daily outages if there's so much as a cloud going over it (unless you install a giant panel), not to mention entire days where it's cloudy.

Put a daylight sensor in line with one of the leads with a cutoff some percentage higher than the cutoff of the device.

Out of curiosity, what about a supercapacitor? Since it doesn't store energy chemically, it might work better and not become a spicy pillow.

Was just watching a video about it yesterday. I don't think this is the link I watched but there are dozens of builds if you search YouTube.

https://youtu.be/d2vQ87Th8DI

LiFePo4 are way more stable than Lithium Polymers, even LiPos, which are used in every EV, sitting in the hot sun for days on end...

LiPos are obviously a stupid choice. They should only really be used when you need extreme discharge rates, or really is sizing.

“daytime only” node

There's lots of ways to store energy besides batteries...

Depending on how much you'd want it at night, or just how bored you are, you could do something with a simple motor/generator combo, have it pump water up to a mini tower, and then when solar kicks off it flows back down turning the pump into a generator and powering the solar till the tanks empty.

Almost definitely not worth the hassle, but would be a pretty fun project to fuck around with.

I’m just nervous that it will under voltage the device over the long run if that makes sense.

There definitely should be some kind of capacitor. Likely built into your panel, but you want to make sure there's one somewhere