3.1 Battery Topology

This page explains how to tell the BMS which modules and how many cells are connected, using the settings on the LiMon™ Battery tab. The BMS compares these settings with what it finds on the cells and on the daisy chain, and it switches off both power paths when they differ. Set them before any other setting. 1.2 System Concepts introduces cells, modules, packs and the board roles.

Applies to

Boards: BMS1820, BMS3610, BCU2002, BMS1601, BMS1810. Slave modules: BMS1802, BMS1202.

3.1.1 How It Works

Each board that measures cells is one module with a number, its module ID (a BMS3610 is two modules). The BMS adds the modules in series into one pack. For each ID you enter the number of cells that the module should have.

Fig.3.1.1: Module IDs, Cell Counts and Parallel Groups

1 · Modules in series: one pack Slave Module Type Selection = BMS1802 Module ID#n = 18 Master: ID#0 its own cell inputs 18 cells in series Module ID#n = 18 Slave: ID#1 first in the chain 18 cells in series Module ID#n = 18 Slave: ID#2 second in the chain 18 cells in series Next IDs up to the board limit isolated daisy chain Result Total: 54 cells Pack voltage: the sum of all detected cells 2 · BMS1810 only: parallel groups Parallel Group = PG1 ID#0 Module ID#1 Module Parallel Group = PG2 ID#2 Module ID#3 Module Result Pack voltage: the group with the lowest voltage Voltage difference between the groups: checked against Pack Delta Voltage Protection Balancing and cell delta still compare all cells together. The cell counts are chosen values. On BCU2002 the first slave is ID#0; on BMS3610 the slaves start at ID#2; BMS1601 has ID#0 only. Blue labels are LiMon™ settings. Parallel groups on BMS1820, BMS3610, BCU2002 and BMS1601 are TBD.

  • Modules are found at start-up. The BMS looks for the modules on the chain only when it starts. After you add or remove a module, restart the BMS.
  • Cells are detected by voltage. A cell input that reads about 0 V does not count as a cell. The setting does not select the inputs; it is the number the BMS expects.
  • The two numbers must match. When the detected and the set cell count of a module differ, the BMS sets Cell Connection Error.
  • Pack voltage. The pack voltage is the sum of all detected cells of all modules, so the total cell count of the pack is the sum of the module cell counts.
  • Lost module. When the BMS can no longer read a module that has cells set, it sets Slave Module Communication Error.

Both errors are CRITICAL: they switch off both paths and set User Attention Required until the BMS restarts; see 4.1 Protection and Recovery.

Table.3.1.1: Module IDs per Board

Board Module IDs The board's own cells First slave Slave modules Maximum slaves
BMS1820 ID#0 to ID#14 ID#0, 5 to 18 cells ID#1 BMS1802 or BMS1202 14
BMS3610 ID#0 to ID#9 ID#0 and ID#1, 5 to 36 cells in total ID#2 BMS1802 or BMS1202 8
BMS1810 ID#0 to ID#9 ID#0, 5 to 18 cells ID#1 BMS1202 9
BCU2002 ID#0 to ID#15 none ID#0 BMS1802 or BMS1202 16
BMS1601 ID#0 ID#0, 6 to 16 cells no slaves — 0

A slave module measures 5 to 18 cells on BMS1802 and 5 to 12 cells on BMS1202, and needs at least 11 V from its cells. LiMon™ shows ID#0 to ID#15 on every board; an ID above the limit of the board has no effect.

3.1.2 Module Type

Slave Module Type Selection tells the BMS which board the slave modules are. It applies to all slaves of the master, so BMS1802 and BMS1202 cannot be mixed on one chain. The BMS takes this setting when it starts, so restart it after a change.

  • ---: no slave modules. Use it on a master that only measures its own cells.
  • BMS1802: the module ID follows the position in the chain. The first board after the master is the first slave of Table.3.1.1, the next board is the next ID.
  • BMS1202: the module ID follows the Board ID pins A0 to A3 of the board, so Board ID n is Module ID#n. Number the boards from the first slave of Table.3.1.1 upwards, without gaps. See the BMS1202 pinout.

BMS1810 works with BMS1202 slaves and BMS1601 has no slaves; on these two boards the setting changes nothing.

3.1.3 Cell Count per Module

Each row Module ID#n on the Battery tab has two boxes. The right box is the cell count of that module. Enter the number of cells that are really connected, and --- for an ID that has no board. A change is used as soon as it is saved.

  • Split the cells evenly. Each board draws its supply from its own cells, so give all modules the same number of cells, or as close as possible. 40 cells on three boards are 14, 13 and 13.
  • BMS3610. ID#0 and ID#1 are the two sides of the board. The same rule applies: 30 cells are 15 on ID#0 and 15 on ID#1. A small pack can stay on one side: all cells on ID#0, and ID#1 at ---.
  • Unused inputs. Short the unused cell inputs to the highest cell, on BMS1820, BMS3610, BMS1810, BMS1601, BMS1802 and BMS1202; the connection pages of BMS1202 and BMS1810 show it. An open input can read a voltage and count as a cell. On BMS1601 the datasheet of the cell monitor asks for the same.

3.1.4 Parallel Groups

The left box of each Module ID#n row is the Parallel Group, PG1 to PG6. It is not the same as parallel packs, where each pack has its own master and the masters work in Multi-Master mode (1.2 System Concepts).

  • BMS1820, BMS3610, BCU2002, BMS1601: TBD (not implemented in the firmware yet). Leave it at ---. The value is only sent in the module voltage frames on CAN.
  • BMS1810: a parallel group is a string of modules in series, and the strings are connected in parallel. Modules with the same group number form one string.

What changes on BMS1810 when groups are set:

  • All modules need a group. Give every module a group and start from PG1 without gaps. While one module stays at ---, the BMS treats all modules as one series string.
  • Pack voltage. The pack voltage is the total voltage of the group with the lowest voltage, not the sum of all modules.
  • Difference between groups. When the voltage difference between the groups reaches Pack Delta Voltage Protection (LiMon™ Interface tab, default 250 mV), the BMS sets Max. Parallel Group Delta Voltage Protection, which is CRITICAL. Pack Delta Protection Release does not apply to these groups.
  • Cells. Balancing and the cell delta checks compare all cells together, across the groups.

3.1.5 Settings

Table.3.1.2: Battery Topology Settings (LiMon™ Battery tab)

Setting What it does How to choose Unit Range Default
Slave Module Type Selection Sets the board type of all slave modules. The model of the connected slaves; --- without slaves. Restart after a change. — ---, BMS1202, BMS1802 ---; BMS1810: BMS1202
Module ID#n Sets the cell count that the BMS expects on this module. The cells really connected; --- for an ID without a board. LiMon™ offers 4; use at least the minimum of the board (Table.3.1.1). cells ---, 4…18 ID#0: 7; other IDs: ---
Parallel Group Puts the module in a parallel group. Leave at ---. Only on BMS1810 with strings in parallel: the group of the string. — ---, PG1…PG6 ---

The LiMon™ reference describes every field of the screen: 7.1 Battery Settings.

3.1.6 Example

Example: 16S pack on one BMS1820 (chosen values)

Slave Module Type Selection: ---. Module ID#n: ID#0 = 16, all other IDs ---. Parallel Group: ---. Live View shows one module with 16 / 16 cells; with 3.2 V cells the pack voltage is about 51.2 V.

Example: BMS1820 with two 18S BMS1802 slave modules (chosen values)

Slave Module Type Selection: BMS1802. Module ID#n: ID#0 = 18, ID#1 = 18, ID#2 = 18, all other IDs ---. Parallel Group: ---. The pack has 54 cells in series; with 3.2 V cells the pack voltage is about 172.8 V. Save, then restart the BMS so that it takes the module type and finds the slaves.

3.1.7 Checking the Result

  • Module list: the module selection in Live View lists the modules that the BMS has found. Check that every board is there.
  • Cell count: Live View shows the detected and the set cell count of the selected module, for example 18 / 18. The two numbers must be equal on every module.
  • Pack voltage: the total voltage of the modules adds up to the pack voltage; check it with a meter on the pack.
  • Events: no Cell Connection Error and no Slave Module Communication Error after a restart.

3.1.8 Common Mistakes

  • Wrong cell count on a module. The detected and the set count differ, so the BMS sets Cell Connection Error and both paths stay OFF. Correct the count, save and restart: the error stays until the next start.
  • ID#0 left at the default. A new BMS expects 7 cells on ID#0, so any other pack gives Cell Connection Error at the first start. Set the count, save and restart.
  • An ID set for a board that is not there. The BMS finds nothing for this ID and reports Cell Connection Error or Slave Module Communication Error. Set the ID to ---.
  • A board on the chain with its ID at ---. The BMS detects cells that it does not expect and sets Cell Connection Error. Enter the cell count of that board.
  • Wrong module type, or --- with slaves connected. The BMS cannot read the slaves: they are missing in Live View or their cell count does not match, and Cell Connection Error follows. Select the right type, save and restart.
  • Slave added without a restart. The BMS looks for modules only at start-up, so the new board stays unknown until the next start.
  • Two BMS1202 with the same Board ID, or a gap in the Board IDs. The BMS cannot read these boards correctly and reports Slave Module Communication Error or Cell Connection Error. Set the Board ID pins of every board as 3.1.2 describes.
  • More cells than the board has. LiMon™ offers up to 18 cells on every ID, but a BMS1202 has 12 inputs, so a higher count never matches.
  • Parallel Group set by mistake. On BMS1810, the pack voltage then shows the lowest group instead of the sum of all modules, which also affects every limit that uses the pack voltage. Set all modules back to ---.