First off, my apologies for a long time away from the blog: between family, life, work, travel, and other commitments things have been busy and the blog had fallen by the wayside.
Background
That said, recently I’ve started getting involved with Meshtastic, a license-free, open-source, off-grid mesh network that can run on cheap, low-power hardware and allows people to exchange text messages with others, either through direct messages and private or public chat “channels” (akin to a “chat room” of old). I’m interested in this as both a fun way to socialize with nerdy people in the area, but it also looks very handy for communication in emergency situations like after an earthquake.
I recently installed a solar-powered Meshtastic node on my roof’s antenna mount (currently also host to my house’s GPS antenna) using RAK Wireless WisBlock hardware which is known for being quite low-power. These modules can run from USB, a battery (e.g., a lithium-ion cell), and even have an onboard TP4054 lithium-ion battery charger module connected to a port labeled “solar”, and their product descriptions refer to the device being intended to charge a lithium-ion cell from a small, 5V nominal solar panel.
RAK’s choice of the TP4054 as an onboard charger chip was a bit puzzling to me: it is a fine charger chip if used with a fixed supply voltage (like from USB), requires minimal external components (one resistor and two capacitors), and does a great job charging Li-ion cells, but it lacks any ability to reduce its current consumption on the fly to match a variable-output supply voltage like a solar panel that might be able to supply plenty of current during the peak sunny hours during the day, but only a limited amount during the mornings, evenings, or during cloudy days.
Various charger chips exist to charge batteries from solar panels, including the CN3065 linear and CN3791 “MPPT” switch-mode charger. Both adjust their charging current automatically to not overload a solar panel, but the CN3065 has a more limited input range than the CN3791.
CN3791-based charger ICs are available from various vendors for reasonable prices, I don’t like the most commonly available model for a variety of reasons. In particular, they try to keep the solar panel voltage above a value set by fixed resistors, but I wanted to be able to easily adjust the target voltage.
My Chargers
To rectify these shortcomings, I designed a charger board based on the CN3791 chip that has, among other improvements, an adjustable MPP voltage, reverse-polarity protection, an (optional) battery protection chip, and a separate load path so the charger can accurately measure the charging current and battery voltage.
I have more information about the chargers here. Current pricing (no pun intended) for small quantities of the Mod 3’s are $20/each and are hand-made in the US by meafrom US-made PCBs and globally-sourced components. Larger orders and orders for different Mods (e.g. with different battery protection chips, or none at all) are available with a little more lead time. Contact me if you’re interested in buying some.
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