Research

A battery management system (or BMS) is a system that manages a rechargeable battery (could be a single cell or multiple cells as a battery pack). It's main goal is to keep the battery in its safe operating area, which is usually defined by a temperature range, a voltage range and a current range that cannot be exceeded. Additionally, it might measure data (voltage, current, state of charge, etc.) and report it externally, and oftentimes it also makes sure that a battery pack remains balanced (difference in cell voltages should be as close to 0 as possible).

Important Properties

Voltage: minimum and maximum cell voltage
State of charge (SoC) or depth of discharge (DoD), to indicate the charge level of the battery
State of health (SoH), a variously defined measurement of the remaining capacity of the battery as % of the original capacity
State of power (SoP), the amount of power available for a defined time interval given the current power usage, temperature and other conditions
State of Safety (SOS)
Maximum charge current as a charge current limit (CCL)
Maximum discharge current as a discharge current limit (DCL)
Energy [kWh] delivered since last charge or charge cycle
Internal impedance of a cell (to determine open circuit voltage)
Charge [Ah] delivered or stored (sometimes this feature is called
Total operating time since first use
Total number of cycles
Temperature Monitoring
Coolant flow for air or liquid cooled batteries

Development

A great overview on the important characteristics when designing a BMS can be found on monolithic power's website. The system consists of an analog front end (AFE) and a fuel gauge section.

Analog Frontend

The AFE handles the following:

cell balancing
main low-side sense resistor for current measurements
main high-side mosfet control to connect/disconnect the battery. Can we use this as an on/off switch?

Fuel Gauging

Article used: chapter 3 This expression comes from the car industry, where they tried to measure the remaining fuel available in the tank. In modern times, we use much more batteries as power sources and the term fuel gauging survived the ongoing transition. For batteries it means how much energy is left that we can safely take out of the battery?

Texas Instruments has a technology called Impedance Track (IT) that models the battery and estimates the remaining state of charge. The main factors are:

measuring Qmax
measuring cell impedance
calculating capacities

Factors

Aging: every cell has aging effects. Qmax and cell impedances can account for ageing effect as the cell is cycled. Temperature: the temperature is an important factor of the available charge left in the battery

Glossary

Definition	Meaning
Qmax	amount of charge available in fully charged cell
SoC	State of Charge (in %)
OCV	Open circuit voltage
DOD	depth of discharge: during no load condition

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