Week 7 State of charge estimation

State of Charge Estimation

1)What is Coulomb Counting?

Coulomb counting is a technique used to track the state of charge of a battery pack. It works by integrating the active flowing current(amps) over time to derive the

total sum of energy entering or leaving the battery pack. This produces a capacity that is measured in Amps.

Considering the equation-

SOC(t) = SOC(t₀) - ∫^t_t0 - ɳ(t)I(t)/C⁰_b dt

where I is current and ɳ is the efficiency during charging and discharging. Coulomb’s counting is easy to implement. But sometimes it becomes difficult to accurately estimate

battery SOC. Because battery capacity and efficiency are significantly influenced by operating temperature and current direction. Furthermore, it is necessary to calibrate the initial battery SOC.

In order to improve SOC estimation, coulomb counting and voltage-based method can be combined. A cumulative error counting integration can be removed by counter calibration based on

the open-circuit voltage characteristic.

2)Implementation of Coulombs counting for SOC estimation

Coulomb's counting based algorithm is often used as the core technology for SOC estimation in BMS. They express SOC as the ratio of available capacity to the nominal one.

The remaining capacity in the battery can be calculated by measuring the current flow rate.

The common equation to calculate SOC,

SOC = SOC₀ + ∫_t0^t0+τ I_bat Δt/Q_rated *100

where

SOC₀ represent initial SOC

I_bat represent current across battery

Q_rated is the nominal capacity

The estimation of SOC of Li-ion Battery is based on monitoring the voltage V_Bat the current I_bat and temperature. The operation mode of battery is recognized by the direction of the current

through the battery system. Coulomb counter is used to track the SOC when the battery is charging /discharging and self-discharging. The amount of charges in an operating period is obtained by integrating of measured charging/discharging current I_bat as expressed as

ΔQ = ∫_t0^t0+τ I_bat dt

Flow chart of the proposed algorithm

In the algorithm, we start by determining the SOC₀ from measuring the initial open-circuit voltage

SOC = f(OCV)

Charged Mode

The charging procedure of Li-ion battery consists of constant current-voltage (CC-CV).

The coulomb counter is represented by Q_gain which is represented buy charge accumulation during the operating period

Discharge Mode

when the current I_bat is negative then the battery is discharged mode. In this mode, the coulomb counter is presented by Q_lost which represent the number of charges lost

Q_lost(t + τ) = Q_lost(t) + ΔQ

The value of DOD is calculated as

DOD(t + τ) = DOD(t) + ΔDOD(t + τ)

where

ΔDOD(t + τ) = Q_lost(t + τ)/Q_rated

Self Discharge Mode

The amount of charges loss per hour is calculated, this amount is designed by the constant, Qper/hour / q. Then the quantity of charged dissipated in this phase

Q_oc is calculated as

Q_oc(h+1) = Q_oc(h) + qper /hour

h is an hour of storage

This value will be added to the number of charges lost. Q_lost during discharge mode and amount of charge accumulated

Q_lost = Q_lost + Q_oc

Q_tot = Q_tot - Q_oc