Instrumentation & Measurement Magazine 25-7 - 15

SOC information of a particular phone to devise smart charging
protocols that preserve battery health; for example, if a
user is found to be using only 40% of the battery on a daily basis,
then the battery is only charged up to 70%.
The SOC cannot be directly measured on a battery, rather, it
must be computed based on other direct measurements: voltage,
current and temperature. SOC estimation remains one of
the active research topics. Two prominent approaches to SOC
estimation are Coulomb counting and voltage-based lookup
[3]; both of these approaches suffer from numerous sources
of error; a fusion-based approach is developed to combine
the benefits of both the voltage-based and current-based approaches.
The fusion-based approach is often implemented
using the extended Kalman filter [3]. The extended Kalman
filter-based approach to SOC estimation has received significant
attention in the literature; however, model uncertainty
remains a significant problem [1], [2], [11].
Time to Shut Down
Rechargeable batteries in general are extremely sensitive to
voltage breach [5]. Subjecting a Li-ion battery to over-voltage
(or over-charging) will trigger thermal runaway which is an irreversible
process causing the battery to melt down and catch
fire. Most rechargeable batteries also suffer from low-voltage
breach. For example, when the terminal-voltage of the battery
drops below certain threshold-this may happen due to overdischarging-the
battery might be permanently damaged;
consequently, the damaged cell might become a short-circuit
and cause damage to adjacent battery cells and eventually
to the entire battery pack.
Hence, it is important for
a BMS to compute the time
to shut down (TTS) during
charging as well as during
discharging so that battery
can be electronically
isolated from the charger/
load to prevent damage to
the battery-pack.
Fig. 2 illustrates the TTS
requirement while charging
or discharging a battery
under two different scenarios.
In one scenario,
indicated in purple, the
magnitude of the current
is high and causes a voltage
drop of approximately
0.1 V; that is, the terminal
voltage will be 0.1 V above
the OCV during charging,
and it will be 0.1 V below
the OCV during discharging.
At this rate, the battery
needs to be shut down
when the SOC is just below
October 2022
80% during charging and when the SOC is about 15% during
discharging. Here, the safe voltage range is assumed to be
between 3.5 V (discharge threshold) and 4.2 V (charge threshold).
In the second scenario, shown in green, the magnitude
of the current is low causing a voltage drop of about 0.03 V. At
this rate, the battery can be charged to about 95% SOC and discharged
to about 5% under the same voltage thresholds.
Fig. 2 also shows why a battery cannot be fully charged
(when its SOC reaches 100%) using high current; indeed, any
current more than zero will not fully charge the battery- this
is due to the strict adherence to the voltage protection mechanism
that is an integral part of modern battery cells. To fully
charge a battery, the current must be reduced; this can be
achieved by a constant voltage (CV) charging topology. During
CV charging, current gradually reduces to zero. Typical fast
charging algorithms employ a constant (high) current at the
start and then switch to CV charging; i.e., fast chargers employ
constant-current constant-voltage (CC-CV) charging strategy.
Finally, Fig. 2 also illustrates that the discharge capacity of a battery
decreases with increasing discharge current.
State of Health
Two major indicators of the SOH of a battery are power fade
(PF) and capacity fade (CF). PF indicates the percentage
increase of the internal impedance of the battery, and CF indicates
the percentage decrease of the battery capacity. Both PF
and CF need to be computed at a fixed temperature.
Due to solid electrolyte interface (SEI) formation (and
growth) and other internal chemical reactions within a battery,
Fig. 2. Time to shut down (TTS). A Li-ion battery needs to be kept within strict upper and lower voltage limits. During
charging and discharging the terminal voltage cannot exceed these voltage limits. The higher the amplitude of the
charging/discharging current, the shorter the TTS.
IEEE Instrumentation & Measurement Magazine
15

Instrumentation & Measurement Magazine 25-7

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