Inside look of Lithium batteries and importance of Battery Management system
Nowadays Lithium-ion battery demand and use is increasing in the market. Lithium-ion batteries are used in Electric vehicles as well as energy storage applications. The development of a Lithium-ion battery for a specific application seems to be simple and easy than other chemistry batteries like Lead-Acid, Aluminum, etc. But behind this simplicity, many mysteries are very tricky and complex to be solved by battery manufacturers. These mistakes are responsible for most cases of battery failures. These mistakes can take the lithium batteries in a different direction and may lead to the major reason for customer dissatisfaction.
Today we are going to discuss these challenges and the role of different technologies to solve these problems.
Mainly Lithium battery consists of two ports :-
BMS(Battery Management System)
Lithium Cells :- Lithium cell is an rechargeable cell in cylindrical/box shape. There are different chemistry of Lithium cells i.e. NMC, LFP etc. Lithium cell have a working voltage range according to the cell chemistry. Different chemistries of Lithium have slight differences like energy density, working voltage range etc. We will take LFP cell as example in this article, so from now lithium cell means we are talking about LFP chemistry lithium cell.
LFP cell work in voltage range from 2.5v to 3.7v. When cell voltage is 2.5v then cell is fully discharged. If we take LFP cell below 2.5 voltage then it will degrade the cell cycle life as well as can permanently damage the cell.
If we overcharge the cell above 3.7 voltage then it will overheat the cell which may result in cell permanent damage and will degrade the cell cycle life as well.
A Lithium battery consists of many individual cells connected in parallel and series. The cells are connected in series to increase the voltage of battery according to requirement and connected in parallel to increase capacity of the battery. Let us take an example of a 24v 100Ah battery for home inverter. We have LFP cells having capacity of 6Ah.
LFP cell specifications :-
Maximum voltage — 3.7v
Nominal voltage — 3.2v
Minimum voltage — 2.5v
Cell capacity — 6Ah
So, We will need 8 cells in series and we will get nominal voltage of 3.2x8=25.6v (There is always a range of working voltage of system like inverter, motor etc. Nominal voltage is just a general term used to represent the average voltage of the system)
So, our battery working voltage will be 2.5x8 = 20v to 3.7x8= 29.6v. Which can work on all 24v systems.
Now we have to calculate the number of cells in parallel to make the battery 100Ah. So, one cell have 6Ah capacity then we will need 100/6 = 17 (Rounding off according to ceiling value).
Eight cell having 17 cells in parallel will be connected in serial to make a 24v 100Ah battery. Total number of cells required will be 17*8 = 136
Capacity = 17 * 6 = 102Ah
Nominal_voltage = 3.2 * 8 = 25.6v
Total energy battery can store = Capacity*nominal_voltage
= 102 * 25.6(nominal voltage 3.2*8)
= 2611 W
Many companies take 7 cells in series make 24 volt battery. That battery will be overcharged by the inverter, however BMS will protect the battery from overcharging but the net energy stored by the battery will be less i.e. 102*22.4(Nominal voltage 3.2*7) = 2284 W
As the battery have cells in series as well as parallel. Cells in parallel will be at same voltage but the cells in series may get disbalanced which will limit the maximum energy storage capability of the battery. Let us understand this with and example.
All cells can not be identical and have different behavior according to Temperature, Capacity and Internal resistance.
So, Cells are binned based on Capacity, IR and voltage. Cells having least difference between capacity, IR and voltage is selected to build a battery.
After connecting cells to make complete battery pack, this battery needs different kinds of protection, because Lithium battery is very vulnerable to Over charging, Under discharging, Over current, Over-temperature and under temperature. So, this battery needs a management system which can protect this battery as well as get this data for further analysis and improvement in battery efficiency and control.
So Battery Management system is very crucial part of the battery. Battery Management system protect battery from different faults as well as give important data to make Lithium battery a feasible solution in real world applications.
BMS Protections :-
As all cells in series are not ideally identical, so can not be at exactly same voltage during charging and discharging. So, individual cell protection is necessary either in charging or discharging. To understand this in more detail please refer to this article.
Over charging protection:- BMS disable the charging of battery if any cell in the battery crossed the Overcharging threshold. As if a cell in the battery in series have 1% lesser capacity than other cells then its voltage will cross the voltage threshold earlier than the other cells because this cell will charge earlier than the other cells. So, BMS will disable charging to protect this cell from overcharging, however this will limit the battery maximum storage capacity to 99%.
Over discharging protection :- BMS disable the discharging of battery if any cell in the battery crossed the Under-discharging threshold voltage. As if a cell in the battery in series have 1% lesser capacity than other cells then its voltage will decrease faster than the other cells. So, BMS will disable discharging to protect this cell from under discharging, however this will limit the battery maximum discharging capacity to 99%.
Over Current protection :- BMS protect the battery from withdrawing current above a defined threshold as well as charging current above a defined threshold. Discharging current threshold and charging current threshold can be configured separately. Different BMS provide discharging current protections in two or three stages for peak, continuous loads.
Inrush Current protection :- When connecting battery to vehicle or during switching of contactors/Mosfets, high current may flow for few microseconds. If this current flows for higher time duration(In case of any malfunction)then it may be dangerous for battery. BMS protects the battery from higher inrush current.
Temperature protection:- Thermal stability of Lihium battery is very important. It is very dangerous for battery operation in very high temperature as well as very low temperature. BMS protect the battery to charge/discharge in very high temperature and very low temperature. High temperature and low temperature threshold can be configured in BMS.
BMS algorithms and Communication :-
Energy estimation and predication :- This is the most important and complex application of the BMS to estimate the energy stored in the battery known as State of Charge(SOC) and maximum energy storage capability of the battery know as State of Health(SOH). As capacity of battery decreases with its usage, BMS calculate the SOH of battery after each cycle. There are many algorithms to calculate the SOC and SOH of the battery. Different BMS’s use hybrid/custom algorithms to calculate accurate SOC and SOH of the battery.
Cell Balancing:- The cells in the battery may be dis-balanced due to factors we have disscussed earlier. BMS balances the cells if cells match a defined condition. User can configure the minimum voltage above which BMS can perform the balancing and cell voltage different. If the a cell is above the defined voltage and have voltage difference greater then configure voltage difference then BMS will perform the balancing. Balancing can be of two type Passive cell balancing and Active cell balancing. Please refer to my this article to understand these topology in detail.
Battery data and Communication:- BMS has a key role of providing data using different communication protocols. Most common communication protocols used in BMS are CAN, UART, SM-Bus, and BLE. BMS provide the following data of battery :-
- Cell voltages
- Pack voltage
- Battery temperature
- Power board temperature
- State of Charge
- State of Health
- Battery cycles completed
- Any fault and alert status
- Power status
- Charging/Discharging current
- Charger/Load connection status
- Current configuration of BMS for different protections like Current, voltage, temperature etc.
BMS also provide control to different parameters over communication protocols as :-
- Protection configuration like Charging/Discharging current, voltage protection, temperature protection, Battery capacity, Balancing etc.
- Power control (Enable/Disable)
- Firmware upgrade
BMS is the brain of the Lithium Battery. So, selection of the BMS is very crucial and important for a lithium battery. There are many factors which should be considered to select a BMS for your battery. Some of important factors are :-
Cell measurement accuracy
Current measurement accuracy
Analog protection response time
Balancing current and topology
Peak, Inrush and continuous current capability
SOC and SOH accuracy
Communication protocols and data points
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