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

Question

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.

Answer 1

Your update asks a different question, so I must add a different answer.

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.

Answer 2

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.

---

To your question:

Why aren't there BMS ICs

There are: those few BMSs that can handle multiple strings use standard Li-ion BMS ICs. Each IC handles a bank of cells (e.g., 12 cells for the LT68xx ICs). Each bank is isolated from the other banks and from the low voltage power supply and communication lines. The banks are wired to the cells. For example, for a battery with 3 strings of 12 cells, the BMS uses 3 banks, one for each string, and each string uses a standard Li-ion BMS IC.

What's special about the BMS is not the Li-ion IC, it's the software and the isolation between the ICs.