# Adding power path to TP4056 solar battery charger and limit output voltage to 5.5 V or below

I have a 6 V solar panel and a bunch of TP4056 modules with the battery protection circuits lying around and I thought I'd use them to make a solar powered BLE circuit with the nRF52840 dongle for some future projects as also have a couple of those lying around.

The project was inspired by a solar charging circuit from one of Andreas Speiss's videos (https://www.youtube.com/watch?v=37kGva3NW8w) where he added a "power path" so that the load (in my case the nRF52840) runs directly on solar when there is sunlight instead of discharging the battery. However, I don't quite understand how the circuit works and I was hoping that someone here could provide an explanation.

If I have understood correctly P-channel MOSFET will only conduct when the voltage difference between the gate and the source is negative. So when there is full sun the solar panel will output 6 V and the source voltage will then be 6 V minus the drop across the Schottky diode. Vgate - Vsource will be positive and the P-channel FET will act as an open circuit.

However, I don't understand how the circuit works when there is no sun, i.e. Vgate = 0 V. Won't the source voltage also be 0 in that case, i.e. Vgate - Vsource = 0 V and still act as an open circuit? The source voltage would have to be 2 V in order to reach the gate threshold voltage. Will Vsource be the same as the battery voltage? If yes, can someone explain why? I am probably missing something fundamental here so please bear with me.

There is also a second issue that I would like to get some advice on. The nRF52840 has an internal regulator that has a minimum input voltage of 2.5 V and max 5.5 V. Now the 6 V solar panel will actually output ~6.25-6.3 V in full sun (based on measurements) so I was wondering what the best way to limit the voltage to max 5.5 V, while still being able to power the nRF52840 with solar down to 2.5 V.

I came a cross this What is the best way to limit voltage of a solar panel? question, which initially suggests using a shunt voltage clamp. Going through the comments it would seem that a LDO wont have to dissipate as much energy as the shunt voltage clamp. So do you think its better to go with a LDO with a minimum input voltage of 2.5 V, max 6.5 V and a 5 V output voltage e.g. the MAX8867 (https://eu.mouser.com/datasheet/2/256/MAX8868-1389206.pdf)

I made a quick schematic in Kicad, but the circuit/component to limit the voltage to 5.5 V or below is missing as you can see.

• Have you considered a different regulator which can accept your full input voltage as-is? Jun 17, 2021 at 22:11
• @winny: Do you mean choosing another LDO than the MAX8867 or the regulator on the nRF52840 side? If its the latter, then that's not possible as the regulator is inside nRF52840 chip, its not a component on the dongle PCB. Jun 18, 2021 at 6:41
• Your goal is to charge your li-ion cell from your 6 Vpeak solar cell, yes? Jun 18, 2021 at 8:48
• Yes, charge the li-ion battery and at the same time powering the nRF52840 with the solar panel when sun is available. Jun 18, 2021 at 11:14
• Crude but working would be to just put a few diodes in series to limit the max voltage to what you can tolerate but a better solution is to get a regulator which can accept the full input voltage and a separate one (if needed) from your battery to you nRF52840. Wait, scratch that. I was under impression that the TP4056 was the limiting factor but you have 8 V as max input. Why not just run your nRF52840 straight off the battery and let the TP4056 do its job? Jun 18, 2021 at 11:16