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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@Robin
Wed 2019.05.22
To obtain sufficient charge current, especially when the clouds block the sun, or sun low on the horizon, requires quite a large cell, std 60x110mm ~2x4" at 1W and the smallest I've been able to locate 30x60mm ~1x2" at 0.18W with output charge voltage above battery voltage. Simplest circuit, or maybe a voltage doubler or buck step up converter.
P=IE I=P/E 1/5=200ma
and 0.18/5 = 36ma
All manufacturers recommend as @allObjects points out, NiMH/NiCad and trickle charge as Li/LiPo batteries require a constant voltage/current after startup conditioning FOR THE DURATION of the charge cycle. You need a special board for LiPo charging and a guarantee the sun wont be covered by clouds during that process. Failing this, may/most likely result in fire (see numerous YouTube vids - Google solar LiPo charger or drone quad copter fires) as fluctuating V and I as when sun behind clouds, will change chemical makeup of Li, effectively lowering internal resistance or shorting out the cell.
There are many articles that suggest using a TP4056 module to build a solar charger. Yes, they may work for a few charge cycles, but then you will be calling your insurance company for a pay out.
I'm on the fence post with LiIon as I haven't found a yea article for using the TP4056 for them, or whether trickle charging via solar is feasible.
Lithium Polymer Charging/Discharging & Safety Information