Week 7 State of charge estimation
State of Charge Estimation
Aim:
* To know the functions of Battery management system and its importance.
* To know about coulomb counting method and how the BMS uses this method to estimate the state of the charge.
Battery Management System:
* A Battery management system (BMS) is any electronic system that manages Rechargable batteries, such as by protecting the battery from operating outside its safe operating Zone, monitoring its state, calculating secondary data, reporting that data, controlling its environment.
* A Battery pack built together with a battery management system with an external communication Data bus is a smart battery pack.
* A BMS may monitor the state of the battery as represented by various items such as,
1. Voltage:
* Total voltage, voltages of individual cells, minimum and maximum cell voltage, or voltage of periodic taps
2. Temperature:
* Average temperature, coolant intake temperature, coolant output temperature, or temperatures of individual cell
3. State of Charge (SOC) or Depth of Discharge (DOD):
* To indicate the charge level of the battery
4. State of Health (SOH):
* A variously-defined measurement of the remaining capacity of the battery as % of the original capacity
5. State of Power (SOP):
* The amount of power available for a defined time interval given the current power usage, temperature and other conditions
6. State of Safety (SOS):
* Monitors Over current charging, Discharging, over temperature and prevents the battery pack from going beyond the safe operating zone.
7. Coolant flow:
* For a typical type of air or fluid cooled batteries.
8. Current:
* The current levels going in or out of the battery is monitored.
* These are the key functions of a Battery management system in a battery pack.
State of charge:
* State of charge (SoC) is the level of charge of an electric battery relative to its capacity.
* The state of charge (SOC) of a cell denotes the capacity that is currently available as a function of the rated capacity.
* The value of the SOC varies between 0% and 100%. If the SOC is 100%, then the cell is said to be fully charged, whereas a SOC of 0% indicates that the cell is completely discharged.
* There are some ways to determine this state of charge and they are,
1. The battery voltage translation:
* In many systems, a simple voltmeter is used as a SOC indicator, the battery voltage decreases more or less linearly as the battery is discharged.
* In particular, the voltage in a Lead Acid battery decreases significantly as it is discharged. Knowing the relationship of battery voltage and SOC allows the voltmeter to be calibrated to report SOC.
* A major limitation with this technique is that the battery voltage is also affected by temperature and battery current. Knowing the way temperature and current affect the voltage allows one to compensate for those effects to a great extent.
2. The battery current integration
* This battery current integration methos is also known as Coulomb Counting mathod.
* The Coulomb counting method measures the discharging current of a battery and integrates the discharging current over time in order to estimate SOC. Coulomb counting method is done to estimate the SOC(t), which is estimated from the discharging current I(t), and previously estimated SOC values, SOC(t - 1). SOC is calculated by the following equation:
SOC(t) = SOC(t - 1) + (I(t) / Qn) * δt
* For example, a 2 A current into a battery, for 3 hours, will add 2 * 3 = 6 Ah to the battery charge. If the battery capacity is 24 Ah, that will increase its SOC by 6 / 24 = 25 %. That's 25 % more than it had been at the beginning; but, without knowing what the beginning SOC was, we do not know the final SOC.
* This is how BMS Implements the coulomb counting method to estimate the state of charge of a battery.
Limitations of Coulomb counting Method:
* Coulomb Counting doesn't work as well with Lead Acid batteries, because of the significant leakage current within Lead Acid batteries does not go through the battery current sensor and is therefore not taken into account.
* Not all the charge in a Lead Acid battery is converted into useful work: some of it is converted in waste heat, an effect that is characterized by the Peukerts exponent.
* On the other side, Coulomb Counting does work as well with Li-Ion batteries, because of their low leakage and good Peukert exponent.
* Another limitation of Coulomb Counting is drift. In any integration, any small, constant error in the variable being integrated results in a drift in the result. In the case of Coulomb Counting, any small offset in the measurement of battery current, will result in the SOC drifting up (or down) over time.
Methods employed by BMS for better acuracy of SOC:
1. Combination of Voltage and current Integration:
* Combining these two techniques results in a reasonable better way of estimating of SOC in a Li-Ion battery:
* The battery current is integrated ("Coulomb Counting") to get the relative charge in and out of the battery
* The battery voltage is monitored, to calibrate the SOC when the actual charge approaches either end.
* If the SOC estimated through Coulomb Counting is uncalibrated (i.e) it is not equal to the actual SOC, eventually the battery will be charged or disccharged so far. Uing that Voltage Translation we can estimate SOC, and cablibrate the estimate value.
* For example, if the actual SOC is 80% but the estimated SOC is 50%, and the Li-ion cell is charging, its voltage soon becomes higher than a threshold say, 3.4 V, which corresponds to an actual SOC say, 90 %. At that point, the BMS sets the estimated SOC to 90 %, to calibrate it.
* Going back to the issue of drift, in stand-by, the battery is kept full, meaning that Voltage Translation will be used, avoiding the drift of Coulomb Counting using the above mentioned method.
* This is a one of the methods which Battery management system uses to estimate the State Of Charge.Similarly there are also many methods involved to do this.
2. Modified Coulomb Counting Method:
* To improve the Coulomb counting method, a new technique called modified Coulomb counting method is proposed.
* The modified Coulomb counting method uses the corrected current to improve the accuracy of estimation.
* The corrected current is the function of discharging current. There is a quadratic relationship between the corrected current and discharging current of battery.
* By practice of experimental data, corrected current is calculated by the following form,
Ic(t) = K2 * I(t)^2 + K1 * I(t) + Ko
where k2, k1 and ko are constant values obtained from the practice experimental data
* In modified Coulomb counting method, SOC is calculated by the following equation:
SOC(t) = SOC(t - 1) + (Ic(t) / Qn) * δt
* The experimental results show that the accuracy of the modified Coulomb counting method is superior to the conventional Coulomb counting method.
* These are some of the methods which the battery management system employs to estimate the state of charge using the coulomb counting method.