Hi All, I've been thinking a bit about solar charging and have come up with a few options. First, this is a pretty good application note laying out the basics of Maximum Power Point Tracking for solar panels: http://www.microchip.com/wwwAppNotes/AppNotes.aspx?appnote=en561417 Also, this is a good overview of charging Li-Ion batteries in general: http://sound.whsites.net/articles/lithium-charging.htm Here are some ideas with pros/cons and approximate costs at 1000 and 10 quantities. 1. Use reputable MPPT ICs BQ24650 ($2 @1k, $4 @10) http://www.ti.com/product/BQ24650 LT3652 ($3 @1k, $4 @10) http://www.linear.com/product/LT3652 LTC4121 ($3 @1k, $4 @10) http://www.linear.com/product/LTC4121 Pros: High Efficiency Excellent Documentation and Support Available at all major parts distributors Wide input voltage range Reliability/long part lifetime Cons: Cost 2. Use Chinese brand MPPT ICs CN3791 (maybe $0.35 @ 1k, $0.68 @10) http://www.consonance-elec.com/seriesCN3791-E.html Pros: Wide Input Voltage Range Decent Datasheet tons of working modules available from Ebay/Aliexpress Cons: Less Efficient (I believe they use fractional open voltage MPPT) Not widely available from US parts distributors Unknown Quality/Reliability (probably good though) 3. Use Name Brand Li-Ion Charging IC MCP73831 ($0.40 @1k, $0.50 @10) Pros: Simple/Compact Widely Available Reliable Cons: Low Input Voltage Range Bad Efficiency (No MPPT built in) 4. Use Chinese Ultra Low Cost Charging IC JZ4054 ($0.06 @1k, $0.10 @10) http://www.szjingzhong.com/enproduct_show.html?id=26 Pros: Lowest cost IC for charging Li-Ion batteries Same footprint as MCP73831 Reliable in my experience Cons: Poor Documentation (No English) Not Available from US Distributors Low Input Voltage Range Bad Efficiency (No MPPT built in) 5. Bootleg MPPT onto option 3 or 4 This is an idea I have, not 100% sure that it will work, but its worth experimenting. The Wemos chip would monitor the Solar Cell voltage and adjust the charger current with PWM to maintain the maximum power point. Here's the basic circuit: [image: Inline image 1] Pros: Lower Cost (only requires a few extra components) Decent Efficiency Cons: Requires Wemos control (maybe updating once a second) Complexity time to test and implement Also, I recommend this chip for buck controverting (stepping down the Li-po voltage to 3.3V): PAM2305 ($20 @1k, $0.40 @10) https://www.diodes.com/products/power-management/dc-dc-converters/integrate… -Fitz