ISDT C4evo charger teardown

[umkumiut]

I was looking for a teardown of this charger before I bought it, but could not find any. Eventually I purchased one and here are my teardown pictures.
I've choosen this charger because it can handle 18650, 21700 and AA cells which I use frequently. This unit can charge many kinds of cells, can measure capacity and theoritically internal resistance, but this latter does not really work. My readings are on the high side and vary by just rotating the cells in their bays. This is due to the 2 wire measurement method. I built a 4 wire socket by myself which provides more accurate and repeatable results for me.
I'm not sponsored by ISDT, btw.

[umkumiut]

In order to disassemble the charger, you'll need to undo six screws in total.
Two screws are visible right away in the bottom left and right corners of the battery compartment. Then the display lens needs to be removed. It is fixed by double sided tape. I inserted a razor blade between the lens and the case so I was able to cut it around and remove it. Now two screws became accessible next to the buttons on the right. Then I removed the LCD because I thought there will be more screws under it - and there were 2. The LCD is fixed by a small piece of double sided tape. I inserted a plastic card from the right side of the display and pried it up. (Sorry I did not take pictures without the lens)
I unplugged the FFC of the LCD before I removed the screws. This type of connector requires its small locking bar to be turned up and back.

[umkumiut]

The board under the LCD holds the FFC connector and the buttons.
The second small board is for the spring loaded contacts of the 18650/21700 cells. Unfortunately this charger does not have separate contacts for the charge current and for voltage measurement.
There is a small fan (25*25*10mm) connected to the main PCBA. It is 5V rated and ISDT branded.

[umkumiut]

After disconnecting the fan, I could separate the top case and the main PCBA.
The main parts on the top side are the battery contacts, the B2B headers, the USB PD interface IC (CH224S), the inductors, capacitors and N-channel MOSFETs (8205S) for the individual charge circuits, a buzzer and 4 temperature sensors for the AA/AAA sockets. These 4 sensors happen to be approximately under the upper 18650 socket, but there is no sensor for the 2nd socket under that.

[umkumiut]

The bottom side is partially covered by a heatsink. It can be removed by 4 screws. There is a soft heat conductive pad between the heatsink and the PCBA. Under the heat sink, there are 12 discharge resistors (8x2R2 and 4x3R3), two TD2786 Synchronous Rectified Step-Down Converters with some passives (maybe for the 18650s) and 4 SOT23-6 devices with GM0N marking (I presume these are for the AA/AAA sockets). The uC is a 32 bit ARM HC32F460.
The negative side of the AA sockets is switched by 8205S MOSFETs and current is sensed via 0R1 resistors. An LM324 OPA amplifies this signal. The 18650 sockets have 2 8205S and a 20mOhm resistor for current sensing. The OPA here is an EG358 in each channel.
Additionally there is an HC4051 analog multiplexer and an HC595 8 bit shift registers present.
A CH442E dual analog switch can be found in the vicinity of the USB connector. I do not know the purpose of these parts.

[Fylim]

Hi, can you please advise how to build a 4 wire socket for more accurate IR result? Am I right you are also charging using the socket? Thanks