1.2 System Concepts

This page introduces the terms used in the rest of the manual: how LiBat™ boards are combined into a battery system, which power paths the BMS switches, what it measures and how it reports and handles events.

1.2.1 Building Blocks

Fig.1.2.1: A LiBat™ Battery System

Pack 1 Pack 2 (parallel pack) Master BMS reads the cells, controls the power path Slave module BMS1802 / BMS1202 Slave module BMS1802 / BMS1202 isolated daisy chain Power path charge, discharge and precharge contactors controls Cells modules of cells in series; each board measures and balances its own cells Master BMS reads the cells, controls the power path Slave module BMS1802 / BMS1202 Power path contactors Cells modules of cells in series CAN · Multi-Master (parallel packs only) Masters: BMS1820, BMS3610 and BMS1810 (with their own cell inputs) or the BCU2002 control unit. Each master connects to LiMon™ and to chargers, inverters or a vehicle. Standalone: BMS1601 measures its cells and switches the power path on one board, without slave modules.

Table.1.2.1: Terms

Term Meaning
Cell One battery cell, or cells in parallel that act as one cell. The BMS measures and balances each cell.
Module A group of cells in series measured by one board.
Pack All modules in series with one power path, managed by one master or standalone BMS.
Parallel packs Several packs on the same DC bus, each with its own master, working together in Multi-Master mode.

1.2.2 Board Roles

Table.1.2.2: Board Roles

Role Boards What it does
Standalone BMS BMS1601 Measures its own cells and switches the power path on one board, without slave modules.
Master BMS BMS1820, BMS3610, BMS1810 Measures its own cells, reads the slave modules over an isolated daisy chain and controls the power path. The board page gives the number of slave modules.
Control unit BCU2002 Battery control unit for multi-master systems; reads the slave modules over an isolated daisy chain and controls the power path.
Slave module BMS1802, BMS1202 Measures and balances the cells of one module and its temperature sensors; reports to the master. It does not switch the power path.

In a system with parallel packs, one BMS is the Multi-Master Main Controller and the masters of the other packs follow it over CAN. The board pages under BMS Hardware give the cell range, inputs and outputs of each board, for example BMS1820.

1.2.3 Power Paths

The BMS connects the battery to the load and the charger through switched power paths. It switches them on and off through its outputs.

  • Charge path: lets current into the battery. Protections against charging (for example a full cell or a cold battery) switch it off.
  • Discharge path: lets current out of the battery. Protections against discharging (for example an empty cell or too much current) switch it off.
  • Precharge: charges the input capacitance of the load through a resistor before the discharge path closes.
  • Main contactor: one contactor for both directions; it is on only while both the charge and the discharge path are on.

See 2.6 Power Path for the outputs and wiring layouts, and 2.7 Precharge for the precharge.

1.2.4 What the BMS Measures

  • Cell voltages of every cell, through the master and the slave modules.
  • Temperatures of the external sensors and of the boards.
  • Pack current through a current sensor; + is charge, − is discharge (2.4 Current Measurement).
  • State of charge (SoC), counted from the pack current and the battery capacity.
  • Balancing: the boards bleed charge from the highest cells so that all cells reach the same voltage.

1.2.5 Events and What They Do

The BMS compares its measurements with the limits you set in LiMon™. Each event belongs to a class, and the class decides what happens.

Table.1.2.3: Event Classes

Class What happens Example
WARNING Reported only; no path is switched off. It clears when the value is back inside the limit. Cell Over Voltage Warning
PROTECT Switches off the affected path and switches it on again by itself when the condition clears. Discharge Over Current Protection
CRITICAL Switches off both paths and sets User Attention Required; the paths stay off until the BMS restarts. Pre-Charge Fault
STATUS Reports a state of the BMS; no path is switched off. System Power On
MULTI-MASTER In parallel packs only: acts on the power output of another pack. Multi-Master Communication Timeout

A PROTECT event that repeats is counted. When it has tripped as many times as Error Recovery Attempts (LiMon™ Peripheral tab, default 5), the path stays off and the BMS sets User Attention Required until it restarts. The full list of events is in the error bit table; 3.1 Protection and Recovery explains what to do when the BMS does not recover by itself.

1.2.6 Settings

All settings are stored in the BMS and set with LiMon™ on the tabs Battery, Cell, Temperature, Current, Peripheral, Interface and Integration. A saved setting takes effect at once; LiMon™ can also export the settings to a file and import them again. The LiMon™ settings pages describe every field; this manual explains how to choose the values.