What is a Battery Management System or BMS?
Battery Management System (BMS) is the safety system of any battery and is responsible for keeping battery conditions (voltage, current & temperature) within safe limits.
The level of protection depends both on the requirements stemming from the chemistry of the battery and the complexity level of the BMS itself. Ancient chemistries like lead – acid and nickel cadmium batteries are durable in over current and over voltage conditions due to their high internal resistance and high self-discharge rates. Most of the time, no protection is required other than a fuse.
However, newer sealed lead acid (SLA) batteries, valve regulated lead acid (VRLA) batteries and nickel metal hydride (Ni-MH) batteries provide higher efficiency and are also vulnerable in over voltage, under voltage, over current and over temperature conditions. Therefore, it is important to monitor these batteries during operation for stated conditions.
Battery Management Systems for Lithium Ion Batteries
Lithium based batteries are much more sensitive to operating conditions with respect to all other batteries. Although previously stated batteries are vulnerable, most of the time, they can handle short term violations and self-regulate. Lithium based batteries on the other hand are very strict in terms of voltage & temperature limits. For instance, if you exceed maximum charging voltage of an SLA, VRLA or Ni-MH cell by 100 milli-volts, the cell will self-discharge, put out some heat and will operate, may be along with a slight capacity loss. If you try to over-charge a lithium-based cell by the same amount, the cell will experience a destructive reaction, heat up and rupture eventually.
The battery management system is responsible for “keeping up” the battery within safe conditions. Since each battery type has distinct limits and tolerances, the battery management system should conform to the same limits and tolerances.
At this point, it is important to define the term “keeping up”. The capabilities of the BMS define the technique and extent of protection.
The simplest form of a BMS, which is also called “Battery protector”, takes cell voltages and battery current and compares these values to pre-defined limits. Any exceeding results in disconnection of battery. The function is provided generally by means of simple analog comparator IC’s. A group of MOSFET’s are employed to provide disconnection. Picture 1 shows an example of this system for 4 lithium cells.
The exemplary system employs analog comparators for over voltage and under voltage conditions for each cell voltage and over current comparators for charging & discharging current. The big MOSFET’s control the output of battery pack and disconnects for any alarm conditions. The circuit also has “bleeding circuits”, which are resistors controlled by small MOSFET’s. Whenever a cell voltage exceeds defined limit, the corresponding resistor is connected to cell terminals and bleeds that cell up to the point the over voltage condition is removed.
Some protectors also provide temperature protection using the same comparison method. High current protectors also include an on-board sensor to measure BMS temperature so that the MOSFET’s does not break up due to high temperature.
Figure 1 – 4S Lithium Battery Protector
Battery protectors are used in a wide range of applications from small home appliances and hand tools to small vehicles like e-bikes & e-scooters. They provide basics for battery protection. However, they lack following features;
- Balancing cells to the same charge level
- Calculating battery’s state of charge (SOC) and state of health (SOH)
- Providing battery information via a communication port
Furthermore, these protectors bleeding capacity is small compared to battery current and does not provide an effective protection. Most of the time a battery hits an over voltage condition, the disconnection circuit kicks in. If large imbalance occurs among the cells, the rest of the cells left uncharged and usable capacity of the drops substantially.
In the next part, we will examine the functions of advanced battery management systems.