Week 7 State of charge estimation
State of Charge (SoC) is the level of charge of an electric battery relative to its capacity. ... In a battery electric vehicle (BEV), hybrid vehicle (HV), or plug-in hybrid electric vehicle (PHEV), SoC for the battery pack is the equivalent of a fuel gauge.
There are two methods for the State of Charge estimation, Coulomb counting, Kalman filter.
Coulomb counting:
Lithium-Ion State of Charge (SoC) measurement made by coulomb counting allows a measurement error of less than 1%, which allows a very accurate indication of the energy remaining in the battery. Laptops, medical equipment, and other professional portable devices use coulomb counting to estimate SoC by measuring the in-and-out-flowing current. Ampere-second (As) is used for both charge and discharge. losses reduce the total energy delivered, and what’s available at the end is always less than what had been put in. In spite of this, coulomb counting works well, especially with Li-ion that offer high coulomb efficiency and low self-discharge. Improvements have been made by also taking aging and temperature-based self-discharge into consideration but periodic calibration is still recommended to bring the “digital battery” in harmony with the “chemical battery.” To overcome calibration, modern fuel gauges use a “learn” function that estimates how much energy the battery delivered on the previous discharge. Some systems also observe the charge time because a faded battery charges more quickly than a good one.
Calculation of SOC time:
the estimated SOC time (t) is calculated by using the previous SOC time (t-1) and discharge current and discharging current.
`SOC(t) = SOC(t-1) + (I(t))/(Q_n) Deltat`
when this runs in a loop the 'I(t)' gets corrected after every loop
`I(t) = k_2I(t)^2+K_1I(t)+k_0`
Problems with the integration of coulomb counting with BMS
Makers of advanced BMS claim high accuracies but real-life often shows otherwise. Smartphones may show a 100 percent charge when the battery is only 90 percent charged. Design engineers say that the SoC readings on new EV batteries can be off by 15 percent. There are reported cases where EV drivers ran out of charge with a 25 percent SoC reading still on the fuel gauge.
the solution to this is a DC/DC converter that solves the primary cell system application
requirements, capably count coulombs accurately without significantly affecting IC quiescent
current (battery consumption), to determine the remaining battery state of charge