Li-Ion BMS which protects parallel cells in addition to series cells

Background

If li-ion cells are arrayed (series x parallel — S x P or P x S) to form a battery, it is generally recommended to manage parallel stacks of series cells (P x S) independently, such that each individual stack may be managed with an independent BMS, as depicted below.

This is because a typical BMS IC will assume that all parallel cells in a series stack will have equivalent voltages and equally distributed (equivalent) currents.

Given the assumption of equivalent voltages, these ICs will also thus assume that those equivalent currents do not flow between two parallel cells.

Issue

In reality, cells will not have identical internal resistances and these differences will further diverge over the lifespan of the cells, increasing the magnitude of cell voltage divergence for any cells excepting when the parallel cells have recently been fully energized to an actively-balanced value, before they again diverge during operational discharge.

Question

Why aren't there BMS ICs which are designed to consider the parallel connections of arrayed sets of cells (with an individual pin for every cell node). Such an IC would have global vision of the array.

Below is an example of how such an IC might manage parallel-cell nodes for 2P x 3S array.

IMPORTANT : Not shown are the typical paths for balancing and protection for series cells, (w.

B+          BM1          BM0          B-
•                                      •
|                                      |
*            *            *            *

*            *            *            *
|            |            |            |
/            /            /            /
|            |            |            |
•-----||--/--•-----||--/--•-----||-----•

*            *            *            *
|            |            |            |
/            /            /            /
|            |            |            |
•-----||--/--•-----||--/--•-----||-----•

• || : Li-Ion cell

• / : Switch

• • : IC pin.

• * : Shared net (when vertically aligned).

• B : Battery

• M : 'Middle' node

Such an IC could (dis)connect like-series-voltage nodes from a central node, allowing for isolation of any node.

This would open the door for temporarily paralleling (or totally bypassing) certain cells for algorithmic balancing purposes, and for bypassing certain cells for protection purposes.

• Depends how you do it. Individual strings of series connected cells connected in parallel (difficult due to the reasons you mention) or parallel cells connected in series, at which point the BMS will just see higher capacity if each cell. Please draw your exact configuration. Sep 17, 2023 at 14:57
• " these ICs also thus assume that those equivalent currents do not flow between two parallel cells." The IC doesn't know or care whether it's a single cell or multiple cells in parallel. In the second case, the IC doesn't know or care whether there's current flowing between cells. Cells in parallel are electrically identical to a single, large cell. Sep 17, 2023 at 15:31
• @DavideAndrea : Newly manufactured and quality assured — yes. But over time, isn't this increasingly less so? (And if so, doesn't the inability to account for a cell pouring current into an adjacent cell leave a significant hole in protection capability?) Sep 17, 2023 at 15:35
• Why would current transfer between parallel cells? All the cells rise and fall together. Some might want to give or take more current but they’ll do that during charge and discharge. And even if there is some current transfer between cells (I could see this after a transient where dynamic impedance may differ from static impedance between cells) it would rapidly balance and stop. Sep 18, 2023 at 1:23

Below is an example of a 3Sx2P array.

What you have there is 6 different batteries, not a single battery. Each battery has its own BMS. But each BMS has a single IC, a standard IC. There's no need for any special IC, because each IC only knows of its own cells.

What is special is only the BMS software (which is not in the Li-ion IC): it must prevent turning on a switch if its string voltage differs from the bus voltage.

Such an IC could (dis)connect like-series-voltage nodes from a central node, allowing for isolation of any node.

This would open the door for temporarily paralleling (or totally bypassing) certain cells for algorithmic balancing purposes, and for bypassing certain cells for protection purposes.

That's a "dynamic arrangement". Everyone who "invents" that, eventually realizes that it's a solution in search of a problem and gives up. Though technically possible, bypassing cells with contactors would double the size of the battery, increase the cost 10-fold, and reduce reliability considerably.

• If I'm not mistaken, I think I asked about arrayed cells, and the wording of my question was changed to batteries Sep 17, 2023 at 15:40
• No. Because each battery has its own switch, it's a battery, not a cell. Sep 17, 2023 at 15:41
• In this way, why is cell balancing not called battery balancing if switches are similarly used to, say, burn off individual cells? (No snark intended here by the way — I don't think this comment clearly presents tone..) Sep 17, 2023 at 15:43
• Each BMS performs cell balancing of the cells inside its battery. There is no battery balancing because each battery with an open switch is at a different SoC level, Therefore, two batteries with open switches are not balanced to each other and there are no mechanisms to balance them to each other. It is possible in theory to have an "array master" talk to all the batteries and coordinate them. In that case, it is possible for for a DC-DC converter to transfer charge in or out of a battery to bring it to the same voltage as the bus before it is turned on. That's almost "battery balancing". Sep 17, 2023 at 15:50
• Continued: But, again, that is not a function of the Li-ion BMS IC. So, my answer remains the same: there is no need for a special Li-ion BMS IC for this "battery balancing". Sep 17, 2023 at 15:51

There are two arrangements:

Series strings connected in parallel (Series first)

For example:

3S2P: (series strings connected in parallel)

*---||---*---||---*---||---*
|                          |
*---||---*---||---*---||---*


The notation indicates the number of cells in series, the number in parallel, and whether series or parallel came first.

In this example, "S" comes first, so we know that cells were connected in series first.

My utility lets you play with cell arrangement, and tells you the corresponding standard notation.

Parallel blocks connected in series (Parallel first)

For example:

2P3S: (parallel blocks connected in series)

*---||---*---||---*---||---*
|        |        |        |
*---||---*---||---*---||---*


The notation indicates the number of cells in series, the number in parallel, and whether series or parallel came first.

In this example, "P" comes first, so we know that cells were connected in parallel first.

With rare exceptions, the correct arrangement is parallel first:

• Simpler and cheaper BMS (because a block of cells in parallel is seen as a single cell)
• Better performance in case the cells are not perfectly identical (because the weak cell is supported by its peers in teh same block).

BMS for series first

If, for whatever reason, a series first arrangement is used, the battery requires one of the few sophisticated (= \$) BMSs that can be configured for such arrangement. It's expensive because the number of voltage taps increases almost linearly with the number of strings. Note that a single BMS handles multiple strings connected permanently in parallel.

1 battery = 1 BMS = 1 protection switch = 1 current = 1 SoC level


Battery arrays

A battery array is different from a single battery with multiple strings in parallel. In that case, each string is a single battery with its own BMS and its own protector switch.

N batteries = N BMSs = N protection switches =
= N different currents = N different SoC levels (for batteries that are disconnected)


Extremely few BMS are compatible with battery arrays. Regular BMS will blow up if used in an array because of the immense current between batteries at different SoC levels when a single battery is turned on at any random time.