I was looking for the simplest charger circuit for 3.7V, 350 mAh pouch battery.
I came up with this circuit. It uses a TL431 with an NPN transistor to cut off. It seems to work well, but I have some concerns about the output as the output will be at the same point where battery is placed.
Is it ok?
I already designed a TP4056 PCB. It works but it is too big for my project so I want something smaller.
Don't do this. Proper LiIon charger ICs aren't any harder to get, need less or the same amount of components and unlike your circuit are likely to actually provide safe charging.
The shown schematic seems very questionable, seeing there's a diode (the blue LED) that limits the difference between the transistor base and the TL341. This never shuts off charging. That will damage the battery! So, this is anything but a "smart" charger.
Also, this is 2023; a charger without input for a small thermal sensor feels unnecessarily dangerous. Be nice to your batteries!
So,
I already designed a tp4056 pcd and working but it is big for my project so I want something smaller.
Then use something that's actually smaller! The MJE13005G from your schematic alone is about the size of a TP4056.
There's really many Lithium Ion charger ICs, just pick one. If you want small, but still easy to solder, TI's BQ21040. You can ask them to send you a free sample.
I threw together a bq21040 charger board. Nothing here was tested, just for your enjoyment. Here's the design files, go wild, it's open source hardware (see license). You can probably directly drop the .kicad_pcb file into Aisler's website (or oshpark's, or you can generate gerbers and put them through jlcpcb.com) and get the board manufactured for probably less than what a handful TP4056 would have cost you.
The whole board is 18 mm × 15mm, and as you can see, mostly unused space, mounting holes, and connectors. I'm sure you can easily schrink it down to 12×10mm or less.
The temperature sensor you want is a 10 kΩ NTC (103AT).
It depends what you mean by "work".
This so called "smart auto cut-off" circuit is not smart but dumb and it does not have auto cut-off off as it will keep battery with float charging.
It will start by pushing multiple amps of current into the discharged battery, while gradually current reducing until battery voltage hits about 3.7V.
It will also never turn off as it keep the battery at floating voltage of about 3.7V indefinitely until the battery is either disconnected or power turned off, in which case the battery is being discharged through the resistors at about 2.3mA.
Now, that's not a correct way to charge lithium ion batteries.
This will degrade and damage batteries, and damaged lithium ion batteries can burst into flames or explode, causing fires.
Correct way to charge lithium ion batteries is to use constant current limiting until voltage is at 4.2V and then after current has dropped to e.g. 10% of initial charging limit current the battery must be disconnected from charging circuit so it is not kept floating indefinitely.
Do not build and use this circuit or any other random circuits you find from random web pages for charging lithium ion batteries.
Is the green charge LED supposed to turn off when full, and it switches to the blue full charge one?
The conventional current output of the green LED interferes with the resistor divider that provides the reference Voltage for the TL431. That seems bad. It's probably relying on the breakdown Voltage of the LED to not supply current and not interfere with the TL431 reference Voltage when the lithium cell is near full charge. The green LED may only come on if the lithium cell is at a very low Voltage, which it probably shouldn't ever get to anyway. It pulls current though the blue LED to turn off the transistor.
It seems like an unreliable circuit that was done to see how few components could be used. Some changes to what LED is used, not having the exact same supply Voltage, and what what temperature it is used will make it not work properly, if it ever did.
I believe you can make this work well with fewer components if you get rid of the dual LED. You should be able to fit one charge only LED in there by connecting it and its resistor to the base of the transistor so it comes on only when the transistor is on. It won't interfere with the resistor divider nor will current have to go through it for the circuit to work.
As someone else said, it will float the lithium cell. If the Voltage is low enough, such as 4 Volts, you can do that. If you want to get the maximum capacity out and charge to 4.2V, floating the cell is bad for it. The charger needs to shut off completely. See what Voltage a fully charged lithium cell sits at to determine what what float Voltage is safe.
It might be relying on the BMS within the lithium cell to provide additional over charge protection.
