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Solar Dual Li-ion charger

An old, broken solar water fountain upcycled to charge 18650 li-ion batteries.

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This is a simple and fun project that can be built in a day, provided you have all the necessary parts. 

I have inherited from some relatives the broken remains of a solar water fountain that floats on water and powers a small pump only with the help of the sun. These have no use besides the decorative one and are meant for garden ponds or small bodies of water. 

Since a dog ate this one, its primary function of pumping water and being watertight was lost forever. So I decided to harvest the solar panel, and what was left of the old motor.

I have a small collection of diverse pairs of 18650 cells, and a slew of devices that require one or two as primary power source. Some devices decided it was better to leave the charging in the hands of the user, and other personal projects do not have the luxury of built in charging. 

This leaves me with a problem. To charge a single 18650 is doable, but when I need two, and preferably balanced as well, things start to complicate. I have bought a dual charger that plugs directly into the wall, but after internal analysis, it seems unnecessarily dangerous and shoddy in construction.

So I decided to build my own. And what better way to do it than in the old solar panel, giving it added functionality and versarility.

  • 2 × TP4056 li-ion usb charger module
  • 2 × 18650 holder Electronic Components / Misc. Electronic Components
  • 1 × 8v solar panel
  • 1 × Voltmeter 7-segmenr module
  • 1 × Momentary switch

View all 6 components

  • TP4056 charge modules

    [ E C C 0 ] • 07/05/2022 at 10:12 • 0 comments

    A very popular charging module for li-ion and li-po batteries, this module can be had for cheap and is available almost everywhere. 

    This module charges and monitors the charging of one 18650 cell very well, and I have not had any problems with it yet. 

    It is intended for USB charging, and I have retained this functionality, since this is a charger first and solar harvester second. But, if we were to check the input range of the TP8056 in the datasheet, it can take up to 8 volts as input. This is great, since our panel, at maximum tilt outputs around 8 volts.

    Now, in a proper device, this would be a controversial design choice, because devices should not be run at the maximum allowable voltage, and some leeway should be created by derating parts. (Eg: you have a 20 volt rail, you should use a 35 volt capacitor etc.) 

    However, after testing, the chip seems to behave fine at 8 volts or even a bit above.  It seems the derating has already been baked in, or this chip is more resilient than it should be. 

    This is great, since we can hook it up directly to the solar panel, without any other regulation or anything else.

    ⚠️ WARNING ⚠️

    THIS IS CERTAINLY A HACK. I AM NOT LIABLE FOR ANY CONSEQUENCES THAT ARISE FROM USING DEVICES ABOVE THEIR RATED VOLTAGE LEVEL. 

    This should not work, but it does. And in my testing, it has not created any problems. If you want to improve on this, I would reccomend a swich mode power regulator, or a specialised solar harvesting chip/module.

    The two modules are soldered in parralel, so both get the same voltage, regardless of power source (USB or solar), but USB charging seems to be way faster, since it can pull more current than the solar panel can output.

    Some modules also include a DW-01 chip with the relevant mosfets attached, that acts like a overcharge/overdischarge protection module, and also overcurent/short prevention. This project can certainly do without it because it is only meant to charge the cells, but I have future plans and might use this as a solar sensor platform, and discharge control/protection is very welcome. 

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