Instrumentation & Measurement Magazine 25-7 - 14

Battery Fuel Gauge:
A Crucial Element in Battery
Management Systems
Balakumar Balasingam, Pradeep Kumar, and Gary Rankin
R
echargeable batteries are becoming ubiquitous in
wide ranging applications including electric vehicles
(EVs), consumer electronics, communication
devices, aerospace equipment, household equipment, and
power tools. A battery management system (BMS) is an integral
part of every battery pack, tasked with ensuring the safety,
efficiency, and reliability of its performance. When a battery is
connected to a power supply for charging, it is the BMS that decides
how much current is allowed into the battery according
to various criteria, such as the reduced charging time and extended
state of health (SOH). Li-ion rechargeable batteries are
notorious for their susceptibility to over-voltage and undervoltages
exposure; consequently, it is the responsibility of a
BMS to protect the battery cell from exposure to extreme conditions,
thus preserving its quality and reliability for longer time.
A battery fuel gauge (BFG) [1], is the most important part
in a BMS. The BFG (Fig. 1) estimates the state of charge (SOC)
[3], [4], SOH, time to shut down (TTS) and remaining useful
life (RUL) of a battery based on three continuous measurements:
voltage across the battery terminals (terminal voltage),
current through the battery, and the temperature measured
at various points on the surface of the battery. The BFG estimates
are used in other important battery operations, such as
optimized charging [5], charge balancing [7], and thermal balancing
[8], [9].
The challenge in BFG is that there is no direct way to measure
the desired parameters, namely the SOC, TTS, SOH, and
RUL. These quantities need to be estimated based on three direct
measurements: voltage, current, and temperature. There
is a non-linear relationship between the open circuit voltage
(OCV) and the SOC of the battery, and the parameters of this
OCV-SOC curve need to be estimated for each battery chemistry
through experimental and empirical approaches [10].
The SOH of a battery has no direct relationship to any of the
three direct measurements; consequently, the SOH must be
computed for each individual battery using cumulative approaches.
The internal impedance of the battery is needed to
compute the available power; however, impedance changes
due to battery age, temperature and usage and need to be continuously
re-estimated for accurate BFG output. Similarly, the
battery capacity also changes with age, temperature and usage
and needs to be re-estimated. Finally, measurement noise
is a significant factor affecting the performance of a BFG [11].
State of Charge
The SOC is the remaining coulombs in a battery as a ratio of the
battery capacity:
SOC 
Remaining charge
Battery capacity
(1)
where SOC ϵ [0, 1]. It must be noted that battery capacity could
vary based on many factors, such as the temperature, age, and
discharge rate. The SOC is usually defined as a ratio of the
nominal capacity, which is the available coulombs at the nominal
temperature of the battery [10].
Fig. 1. Battery fuel gauge (BFG). A BFG estimates the state of charge (SOC),
state of charge (SOH), time to shut down (TTS) and the remaining useful life
(RUL) of a battery.
14
SOC is the most important BFG output that is required for
important battery management operations such as charging,
cell balancing, and safety enforcement. In an EV, the knowledge
of SOC is required to continuously compute and update
the remaining mileage. Accurate estimation of the remaining
mileage is crucial to the safety and comfort of the occupants.
Further, smart charging algorithms [5], make use of the SOC
estimate to reduce charging time without affecting the SOH of
the battery. In electronic devices, such as smart phones, tablets
and laptop computers, SOC information is continuously updated
and displayed. The BMS in smart phones exploit historic
IEEE Instrumentation & Measurement Magazine
1094-6969/22/$25.00©2022IEEE
October 2022
Instrumentation & Measurement Magazine 25-7

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