Modifing a crude BMS circuit based on TL431 - Page 1
Forum section: electronics / beginners

author: Adhith
author_rank: Frequent Contributor
timestamp: April 15, 2021, 06:26:53 am
content: Hello there,I'm currently working on a college project on Al-ion cell. We are planning to build a 4S battery pack as our first trial test and also hoping to attach a BMS for additional safety. since the voltage & current limit for charging the Al-ion cell is totally different from Li-ion cells, we are left with the option of building a custom BMS for our application.On looking into the web, I found some BMS circuits for Li-ion using TL431 (pic attached) which does the balancing function. Also, found few youtube tutorials on it as well. I'm hoping to modify this circuit for making our custom BMS.for our project we need to balance the cell at 1.6V rather than the usual 4.2V range for Li-ion. Since the circuit comes with a potentiometer for adjusting the balancing voltage, I believe I could use that for the intended 1.6V but need to confirm with you guys.plus, I like to have under voltage protection as well; at 0.6V. Could I use the separate TL431 circuit for that? where the cell once reaching 0.6V triggers a transistor and cut-off the battery pack discharge line.would love to hear your thoughts on this. Thank youEDIT: I found the below youtube video which explains a circuit using the Lm339 IC. It have overcharge, undercharge, temperature & over current protection. It looks promising but still not suitable for my lower voltage use case. could anyone suggests some modification to this circuit for my application.

author: Capernicus
author_rank: Frequent Contributor
timestamp: April 15, 2021, 06:36:22 am
content: Why cant u just reduce the input voltage to do it?Instead of 12v use 3v?

author: capt bullshot
author_rank: Super Contributor
timestamp: April 15, 2021, 06:36:47 am
content: Quote from: Adhith on April 15, 2021, 06:26:53 amNo, that won't work. TL431 has a reference voltage of 2.5V, you can't get any lower than that with that circuit.

author: Adhith
author_rank: Frequent Contributor
timestamp: April 15, 2021, 07:00:06 am
content: Quote from: Capernicus on April 15, 2021, 06:36:22 amI didn't get you. the circuit that I attached is for 3 Li-ion cells in series.

author: Capernicus
author_rank: Frequent Contributor
timestamp: April 15, 2021, 07:27:59 am
content: So does that mean u have to change the resistors or it wont work, If thats all it is that shouldnt be too hard. (I could do it just about.)SO quarter the resistance to the bases of the transistors, but then more would go to the cells to be charged. so u have to balance it?

author: ledtester
author_rank: Super Contributor
timestamp: April 15, 2021, 08:01:35 am
content: Quote from: Capernicus on April 15, 2021, 07:27:59 amYou can only use the TL431 as a comparator for voltages greater than 2.5V so trying to detect a voltage of 1.6V is not possible.Also - in your diagram you have the + and - ends of the batteries mis-labeled (as well as CN).

author: Adhith
author_rank: Frequent Contributor
timestamp: April 15, 2021, 10:30:02 am
content: Quote from: Capernicus on April 15, 2021, 07:27:59 amI was under the assumption that I could use the same resistor but not sure now. seems like I need to change it right?

author: Adhith
author_rank: Frequent Contributor
timestamp: April 15, 2021, 11:09:06 am
content: Quote from: ledtester on April 15, 2021, 08:01:35 amunderstood. is there any other easily available comparator for a much lower voltage detection?Thnx for pointing out the mislabelling. Have edited that one now.

author: Zero999
author_rank: Super Contributor
timestamp: April 15, 2021, 01:51:21 pm
content: Quote from: Adhith on April 15, 2021, 11:09:06 amUse the TLV431 instead, which a reference voltage of 1.24V.

author: ledtester
author_rank: Super Contributor
timestamp: April 15, 2021, 05:54:07 pm
content: QuoteSince you also want to detect 0.6V I would just use a real comparator like a LM339.

author: Zero999
author_rank: Super Contributor
timestamp: April 16, 2021, 07:49:33 am
content: The circuit attached to the original post does not use the TL431 as a comparator, but as a linear shunt regulator. It doesn't ballance the cell voltages, but limits the maximum voltage across each cell.Yes, for undervoltage protection, a comparator circuit is required.

author: horvat.kamca
author_rank: Contributor
timestamp: April 16, 2021, 10:18:05 am
content: You could also use comparator with internal reference, for example TPS3700, to simplify circuit a bit and also to reduce current drawn by BMS.TLV431 needs about 100uA, LM339 about 1 mA. If total current drawn by BMS will be 2mA, battery capacity 3000mAh, the battery will be flat after 3000/2/24 = 62 days.