A self powered 18650 Lithium-Ion battery discharge board
The charging circuits that I design need discharged batteries. Power resistors alone are not enough because when you forget the time they will discharge to all the way to 0V and damage the batteries. I also have an electronic load but that requires making the right settings over USB serial and it needs a external power supply, or laptop, I wanted something simpler.
It is simple, cheap and made from well available components. Just plug in the battery and it will discharge until a certain voltage and when done the LED will turn off and the board goes in very low power mode.
The first PCB version failed, the second worked well, and the third should be great (in progress)
Biggest change is the added momentary tactile switch between GND and CS pin, to reset the DW01 IC after a undervoltage lockout. That is needed because the ICs is in so low power state (0.1uA) that the 100nF capacitor barely discharges when the battery is removed.
In addition I have added two more connectors, one JST PH 2.0mm 2pins connector that will connect to many Li-Po batteries, and a 2p header connector for connecting a multimeter.
Finally I have made a hole in the PCB for pushing out the battery from bottom.
The boards were ordered from JLCPCB. It was my frst order. PCB was 4.50USD and express shipping 19.50USD, but I got the PCB for free because it was my first order at JLCPCB. That is so super cheap.
In version 1 of the board I used a comparator to disable the discharge, but now I have simplified the design. It uses a cheap battery protection circuit as the basis. You may find these Li-ion protection modules aliexpress for $0.15 each.
They have DW01 protection IC and the FS8205A dual N-channel mosfet both from Fortune Semiconductor. I have designed this circuit in components so I could replace DW01 which has 2.40V undervoltage level with another IC with higher undervoltage level like the FS312 which is at 2.90V.
I have ordered the PCBs from Elecrow and I expect they arrive 22 May 2020. Components were ordered from Aliexpress.
I have just received the boards and assembled two of them. One is equiped with the DW01+G and another with the DW01A-G.
It seems to work. The LED turns on and the resistors get hot, too hot to touch, so in the next design I reduce the discharge current from 1C (~3A) to 0.5C (~1.5A).
DW01A-G discharged until 2.40V and when voltage exceeded 3.00V it started discharging again in an endless loop, because it has "standby function release". When discharging is terminated this IC continued to use 1.5uA current from the battery.
DW01+G discharged until 2.40V and then it went into 0.1uA power down mode. It never reconnected the load even when battery voltage exceeded the re-enable voltage of 3V. This IC does not have "standy function release", it has a sort of one-shot protection. It can be restored to normal operation by shorting the CS pin to GND (or by connecting a charger), so in next design I need to add a momentary switch there. After the load was disconnected the battery regained voltage to slightly above 3.00V, which is perfect because most battery chargers will start the fast charging above 3.0V.
I also learned the batteries are not easy to remove from the holders and you need a string to remove them.
In near future I wan to test with FS312F-G that disconnects the load at 2.9V, and it will recover to a voltage much higher than 3.00V, like 3.3V or something.
I named the board BaDiTo after Battery Discharge tool. The first version of the board basically consist of power resistors, a mosfet and a comparator and a LED. Using one rocker switch you can choose between 1.5A load (2 resistors in parallel) or 3A load (4 resistors in parallel) which equals about 0.5C and 1C discharge current. The other rocker switch is to choose between two discharge voltages, 3.5V or 3.0V. The LED is on during discharging and it goes off when the dicharge voltage is reached. I have selected a low power comparator (TLV7011) and used a 2.5V LDO to make a stable reference votage. The PCB was just 4.90 USD and components about 2 USD.
When I received the boards I was happy. Elecrow did a great job.
When I tested the board it didn't work well. BaDiTo V1 FAILED. The discharge current was lower than expected and when connected to a oscilloscope I found oscillation. I realised I made a stupid mistake. The comparator should have been designed with a large hysteresis. When the voltage falls below the threshold, the load is removed, the voltage jumps back up and the load is re-enabled, then the voltage falls again and the load is removed, etc. in an endless loop, which is oscillation.
Then I remebered this board https://easyeda.com/Iam1CM69/18650-discharge-board designed by https://twitter.com/AstroTogger First I thought it was a super dumb design, but the longer I tought about it the more brilliant it became. It consist of just power resistors, a $0.10 battery protection module, a LED, and switch. The protection circuit with a DW01 will disconnect the battery when the undervoltage level (2.5V by default) is reached. The DW01 has anti-oscillation built in which is great. The DW01 will only restarted discharging when the load is removed, that is why the switch is there. And the LED is powered from the battery and lit during discharging. When discharging is done the LED turns off and the circuit uses 0.1uA which is super low. I contacted the designer and he said that after the load is disconnected the battery voltage jumps back to almost 2.9V, which is pretty good.