Poor nRF52 thread got sidetracked, so let’s start a specific one for this subject of eternal controversy.
The requirement is concurrent charging of one lithium or lifepo4 cell while simultaneously powering the meshtastic device.
About linear chargers:
I think that linear chargers such as the bq24210 or the adafruit board would do good enough in this use case. If you have the right solar panel (MPP around the charging range of a lithium cell, say 3-4V) and and a charger chip that doesnt get confused by the varying output conditions, you don’t need switching topologies and the whole MPPT voodoo. Regular linear lithium chargers tend to get confused by small solar panels and low light conditions. With small panels connected to 18650s current limiting is inherent to the panel itself.The most important thing is actually termination voltage, as discussed earlier.
About Pseudo-MPPT chargers with switching topologies:
These are mostly buck converters that keep the solar panel at a pre-set mpp voltage. I found the CN3791 to work quite well while being cheap and available. One problem is that it always charges until the maximum fixed voltage which should be OK if you use something power-hungry like the T-beam. That way the battery will not be topped up all the time. Plenty of chips with adjustable termination voltage are around.
About energy harvesters.
I own some boards based on the AEM10941 which is a boost mppt solar energy harvester for a single cell. These work very well but are hard to get and somewhat expensive. They also contain 2 LDOs and are therefore overkill. I have a BLE solar sensor node running on this setup reliably for some time now. Works indoors as well. Due to this positive experience with solar energy harvesting I found the SPV1040 appealing. It’s been around for a few years now https://youtu.be/UkFLnVSquu4
The SPV1050 seems to be even more fancy. DFROBOT sell a readymade board for 12 bucks.
What are your thoughts on this topic?