Lipo Cell Charger Rev 2.0 – A Combination of various circuits

Lipo Cell Charger Rev 2.0 – A Combination of various circuits

A few months ago, I designed, what seemed at that time like an elegant solution to solve my 18650 Lipo cell charging issues. After publishing it online, I received quite a lot of extremely useful comments, most of which highlighted some of the shortcomings of that circuit.

Well, circuit is my attempt to address some of those, and turn the circuit into something that is actually useable…

What is on the PCB

To answer this, let us start at the core and work outwards. The 18650 Lipo Cell is charged using an MCP73832T chip. This chip will charge the cell up to a nominal voltage of 4.2v DC and accept an input of up to 6.0v DC…

This presented me with my first problem, as absent-minded me accidently fried quite a few of them when accidentally connecting 12v to a PCB…

My solution to this was to add a Buck converter, based on my previous 3A Buck converter circuit(s) using the MP9943GQ from MPS. The buck converter can accept between 7v to 12v DC and output a stable 5.3v DC, saving me from frying chips, and also making it useable with most of the power supplies lying around in my lab – and it can also be used in the car at a later stage — Future project —

While that keeps the MCP73832T happy, and the 18650 cell charged, the next problem that presented itself was the nominal voltage of the 18650 cell itself, which is usually between 3.2v and 4.2v. In order for this voltage to be universally useful to me I needed a boost converter to increase the output to 5.15v.

This boost converter was built around the MP3423GG, also from MPS. This tiny little 14 pin IC provides the 5.15v that I need with ease, at a rated current of up to 3A. Ideal for powering other peripherals, like LDO regulators to power an ESP32 etc…

Manufacturing the PCB

The PCB for this project was sponsored by PCBWay .

Clicking on the PCBWay link will take you to the PCBWay website. It will enable you to get a $5.00 USD voucher towards your first PCB order. (Only if you sign up for a free account).


The assembly of this PCB was quite challenging, to put it lightly. 3 QFN chips, with different pad spacing, and small sizes really tested my limits. Needless to say, proper equipment is a must, and even then, a very steady hand and good magnification will go a very long way towards ensuring success… Another option would definitely be to have the PCB professionally assembled…

A stencil is definitely required for this one, is only for that 14-pin boost converter IC, to make sure that the amount of solder paste is just right – and even then, it is still a challenge!


This was a challenging build that tested my assembly skills quite a bit. The circuit functions like intended. Now it is time to add some more features, like an ideal diode chip at the output, to allow for daisy chaining the lipo cells if required, as well as maybe a microcontroller to control power to the buck converter when the battery is not being charged.

Didn’t see a schematic so here is a uniformed question.

When the battery discharges to a point that the boost converter cannot provide the correct output is there something that shuts off the output?

All the chargers that I have tried will keep running and the output drops below the expected boost value. As an example this drives USB devices nuts.

Hi Don,

I am including the schematic, as an attachment. I decided to not include the schematic by default anymore unless someone specifically asks for it…

To answer your question on the boost converter and battery discharge…

At the moment, I have not implemented battery monitoring yet. That is part of my next phase, where I will attempt to control both the buck converter at the input, as well as the boost converter at the output and battery voltage… I am thinking about something tiny like an attiny202 or similar…

Since purely power related circuits is not exactly my strong point, I decided to do this interim revision, mainly to see if these 3 circuits will coexist peacefully together on a single PCB… The Next steps will also include the ideal diode circuit at the lipo output, and maybe a way to charge multiple cells - since the buck converter can definitely handle that with ease…

(Attachment is missing)

:smile: Thats what I was thinking… having seen the ideal diode board I was half expecting to find it used here too!

Nice clear pictures! Adding something for scale might help since the last picture with your finger in it emphasises that these are quite ‘dainty’ boards!

I like this ‘series’ of articles, good work!

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Thank you Owen,

To give you some perspective, the current Rev 2.0 PCB is 3" x 3.5" (75mm x 96mm)

Not exactly dainty on that aspect, but then, those chips are TINY … especially the boost converter… 2mmx 2mm with 14 pads :slight_smile:

Will keep you updated on the progress :slight_smile: