Chapter 12 - Battery Management Systems

Question 1

What are the functions of a BMS?

Answer 1

• Maintain safety
• Communications
• SoC
• SoH
• Temperature control
• Rebalancing
• Individual cell control
• Double layer safety
• Voltage control
• Current control
• Precharging to meet system voltage

Question 2

What does BMU stand for?

Answer 2

Battery Management Unit

Question 3

What does BMM stand for?

Answer 3

Battery Management Master

Question 4

What does MSM stand for?

Answer 4

Multi String Manager

Question 5

What does CAN stand for?

Answer 5

Controlled Area Network

Question 6

What does CSC stand for?

Answer 6

Cell Supervising Circuit

Question 7

What does DSP stand for?

Answer 7

Digital Signal Processor

Question 8

What does EMS stand for?

Answer 8

Energy Management System

Question 9

What does HMI stand for?

Answer 9

Human Machine Interface

Question 10

What does PCS stand for?

Answer 10

Power Conversion System

Question 11

What does UPS stand for?

Answer 11

Uninterruptible Power Supply

Question 12

What does ECU stand for?

Answer 12

Electronic Control Unit

Question 13

What does SCU stand for?

Answer 13

Software Control Unit

Question 14

What does VCU stand for?

Answer 14

Vehicle Control Unit

Question 15

At what levels does a BMS function?

Answer 15

• Individual cell level
• Battery/module/pack level
• String (multiple packs stacked in series)
• Battery system (Multiple strings in parallel)
• Energy system (Multiple battery systems and other power sources)

Question 16

What does the simplest BMS used in cell phone batteries have?

Answer 16

Internal chip to monitor voltage, temperature etc. and physical overvoltage protection

Question 17

What does PCB stand for?

Answer 17

Protection Circuit Board

Question 18

What functions does a PCB have, and how does it solve it?

Answer 18

• Max voltage protection (diode)
• Max current variation (inductor coil)
• Max current (fuse)
• Max noise (capacitor)

Question 19

What levels of protection do we have in a BMS, and what kind?

Answer 19

Cell: bypass diode
Module: software switch based on thermal or voltage information
Pack: software switch in case of error

Hardware protection elements (fuse)

Question 20

What is the rated capacity?

Answer 20

The supplied defined capacity. For a specific set of test conditions (specific T and C-rate)

Can be voltage limited to increase lifetime.

Question 21

What is State of Charge?

Answer 21

The current % of rated capacity (how much energy is ramining on this charge?)

Question 22

What is State of Health?

Answer 22

The max capacity in % of rated capacity.

Some batteries use this information to change charging/discharging limits

Question 23

What is precharging and why is it done?

Answer 23

For large battery systems connected to other voltage sources it is required to bring voltage of the battery close to the system voltage before contacting. Precharging occurs on a limited-capacity circuit with resistor in series.

Question 24

What is the function of current supervision?

Answer 24

To measure current in individual cells, packs and strings.

Communicate measurement to higher level via CAN

Can then shut down in case of:

• overcurrent in discharge direction (overload/short circuit)
• overcurrent in charge direction (overcharge)
• charge current above temperature determined limit
• measured current not matching request from BMS
• too slow response from communications to other levels

Question 25

What are some different types of short circuit prevention?

Answer 25

Software-based in BMS
Diode (reversible)
Fuse (irreversible)

Question 26

How does temperature supervision function?

Answer 26

Individual monitoring of each cell. “Looks out” for an upper and lower temperature limit. Shuts down if not behaving as expected.

On module level also looking at temperature (could be increases resistance from corrosion, poor contact or something else).

Question 27

How does voltage supervision function?

Answer 27

Individual monitoring of each cell. Shuts down if cell voltage over a predefined limit. Shuts down everything if if system voltage is above a certain limit.

Communicate all measurements to high level BMS.

Question 28

How does a contactor work?

Answer 28

When switched on, current goes through a coil that induces a magnetic field and breaks the contact to the main thing. When current is off, a spring or gravity brings the contactor in place.

Question 29

What are some algorithms for SoC determiniation?

Answer 29

1) Track voltage (only suitable for sloped curves)

2) Track current consumed from full charge (only suitable for frequent full charge)

Question 30

What is rebalancing?

Answer 30

If there are two cells in a battery with different SoC, the “bad apple” may limit the function of the entire battery because one cell will be emptied first.

Rebalancing is done by emptying all relevant cells, fill all cells with a voltage limit, and allow bypass of current past all cells that are filled to requested level.

Question 31

How can SoH be determined?

Answer 31

1) Matching of current-based and voltage-based SoC algorithms (impedance increase, maximum charge decrease)
2) Overpotential as a function of current at given voltage informing abstract model
3) Calculation of accumlated ageing from time, temperature, charge and discharge.