Stacking 1S BMS in series or parallel, is it possible?

[Caspianlake]

I really cant find solid answers on this anywhere really, im trying to make a modular 18650/21700 battery system intended for quick and safe prototyping.
The original idea is to have modules with 1s BMS, 1x cell and a 3d printed enclosure with a power socket.


The issue here is connecting/stacking BMS circuits in series or in parallel
I have been both advised against for it, or been told that it is fine. I really do not know which one is the case.
I have been told that existing boards already have the capability, I have been told that it would be possible to design a custom board that has the capability, and I have been told it isnt possible. Really would appreciate a solid answer on this.

[RoGeorge]

- Not safe to put them in parallel, because when you put them in parallel, the current between cells is not controlled, and one strong cell can feed too much current into a smaller/discharged cell.
- Not safe to put them in series, because the weakest cell will discharge first, and will become reversed polarized.

Proper way to reuse them will be to use a BMS designed to support many cells, one that can balance and protect the cells.  If you improvise, might apparently work, but you risk to destroy the cells, or even worst, you risk to start a fire.

[Caspianlake]

Thank you very much for the info. I will not do it then and use appropriate BMS, safety first. I have 2 questions however;

Doesnt the BMS already protect against overcurrent/overcharge, avoiding the rapid uncontrolled balancing issue?
Could you elaborate on the reverse polarized thing?

[RoGeorge]

The protections I know for typical 1 standalone cell are against overcharge and overdischarge situations, overcurrent protection might be just a fuse.

For the negative voltage, imagine 3 batteries in series with a 10 ohms load (a light bulb).  2 batteries are fresh charged, the 3rd is almost empty.  At first, all 3 will generate voltage, but when the 3rd one has no energy left, it will be just like a resistor.  Draw them on paper, and replace the 3rd battery with a 100 ohms resistor.  Note that the voltage on the 100 ohm is about the same voltage as the 2 series cells, but with the + where it was the - of the battery.

If the description is not clear, try with 2 alkaline batteries (with 1.5V single use batteries, do NOT try that with Li-Ion cells).  Take 2 alkalines, 1 new amd another one discharged, put them in series and connect a light bulb.  Wait until the light bulb fades.  Do not disconnect the light bulb.  Now,  measure the voltage on the good battery, and you'll find it can still deliver voltage, and the polarity is correct.  Measure the voltage on the discharged battery, and you'll find it reverse polarized.

[Caspianlake]

Thank you very much for the explanation! I understand very clearly now. As I said I won't mess around with Li Ion batteries, safety first.

[Siwastaja]

Paralleling with permanent factory connections:
OK, carefully check voltages are close within some tens of mV before making the connections - but on the other hand, you only need one BMS cell unit / connection per group of paralleled cells. I.e., paralleled cells act like one bigger cell.

Paralleling with connections that can be made by user:
Possible, but pain in the ass to design right, avoid. You need extra disconnect switches and a process to make gross balancing (charging or discharging) before applying the connection.

Series
Normal way to do it. If by BMS you mean one with integrated disconnection switches (e.g. back-to-back MOSFETs), it will Just Work, but make sure these switches can handle the TOTAL pack voltage. Easy solution for a few cells, but if you have a long chain, finally the cost of replicating these disconnect switches (which need to handle the total voltage) becomes prohibitive, and you'd rather have a BMS which can drive one central disconnection switch.

[Siwastaja]

- Not safe to put them in series, because the weakest cell will discharge first, and will become reversed polarized.

This is literally the One Job BMS has. If it does not prevent this, I don't think it deserves to be called a BMS.

[RoGeorge]

- Not safe to put them in series, because the weakest cell will discharge first, and will become reversed polarized.

This is literally the One Job BMS has. If it does not prevent this, I don't think it deserves to be called a BMS.

That is the case for a BMS designed for many series cells.  For each cell with it's own protection (as the OP seems to have), and of unknown provenience, I don't think it is specified what will happen when the outside voltage became negative, and up to what negative voltage is suppose to withstand.  Unless it is specified by the manufacturer (in the battery datasheet) that those single cells batteries (each with its own protection circuit embedded in the battery) can be used in series, I'll consider they were intended to be used as a single cell, and unable to cope with negative voltage.

Without knowing the exact model of each battery (the battery datasheet), and the exact type of battery protection circuit each battery has (if it really has any), the answers varies.  That is why some people will say it's OK, others will say it's not OK.

When a question mentions "safe use", yet the exact situation is not well defined (what battery model, are they all the same type, what current they suppose to handle, what protection do they have inside, etc.) it is better to consider the worst case scenario.