why bms is important?
A battery management system (BMS) is any electronic system that manages a rechargeable battery (cell or battery pack), such as by protecting the battery from operating outside its safe operating area, monitoring its state, calculating secondary data, reporting that data, controlling its environment, authenticating it and / or balancing it.
A battery pack built together with a battery management system with an external communication data bus is a smart battery pack. A smart battery pack must be charged by a smart battery charger.
A BMS may monitor the state of the battery as represented by various items, such as:
Battery thermal management systems can be either passive or active, and the cooling medium can either be air, liquid, or some form of phase change. Air cooling is advantageous in its simplicity. Such systems can be passive, relying only on the convection of the surrounding air, or active, using fans for airflow. Commercially, the Honda Insight and Toyota Prius both use active air cooling of their battery systems. The major disadvantage of air cooling is its inefficiency. Large amounts of power must be used to operate the cooling mechanism, far more than active liquid cooling. The additional components of the cooling mechanism also add weight to the BMS, reducing the efficiency of batteries used for transportation.
Liquid cooling has a higher natural cooling potential than air cooling as liquid coolants tend to have higher thermal conductivities than air. The batteries can either be directly submerged in the coolant or coolant can flow through the BMS without directly contacting the battery. Indirect cooling has the potential to create large thermal gradients across the BMS due to the increased length of the cooling channels. This can be reduced by pumping the coolant faster through the system, creating a tradeoff between pumping speed and thermal consistency.
Additionally, a BMS may calculate values based on the below items, such as:
The central controller of a BMS communicates internally with its hardware operating at a cell level, or externally with high level hardware such as laptops or an HMI.
High level external communication are simple and use several methods:
Low voltage centralized BMSes mostly do not have any internal communications.
Distributed or modular BMSes must use some low level internal cell-controller (Modular architecture) or controller-controller (Distributed architecture) communication. These types of communications are difficult, especially for high voltage systems. The problem is voltage shift between cells. The first cell ground signal may be hundreds of volts higher than the other cell ground signal. Apart from software protocols, there are two known ways of hardware communication for voltage shifting systems, optical-isolator and wireless communication. Another restriction for internal communications is the maximum number of cells. For modular architecture most hardware is limited to maximum 255 nodes. For high voltage systems the seeking time of all cells is another restriction, limiting minimum bus speeds and losing some hardware options. Cost of modular systems is important, because it may be comparable to the cell price. Combination of hardware and software restrictions results in a few options for internal communication:
To bypass power limitations of existing USB cables due to heat from electrical current, communication protocols implemented in mobile phone chargers for negotiating an elevated voltage have been developed, the most widely used of which are Qualcomm Quick Charge and MediaTek Pump Express. "VOOC" by Oppo (also branded as "Dash Charge" with "OnePlus") increases the current instead of voltage with the aim to reduce heat produced in the device from internally converting an elevated voltage down to the battery's terminal charging voltage, which however makes it incompatible with existing USB cables and relies on special high-current USB cables with accordingly thicker copper wires. More recently, the USB Power Delivery standard aims for an universal negotiation protocol across devices of up to 240 watts.
A BMS may protect its battery by preventing it from operating outside its safe operating area, such as:
The BMS may prevent operation outside the battery's safe operating area by:
A BMS may also feature a precharge system allowing a safe way to connect the battery to different loads and eliminating the excessive inrush currents to load capacitors.
The connection to loads is normally controlled through electromagnetic relays called contactors. The precharge circuit can be either power resistors connected in series with the loads until the capacitors are charged. Alternatively, a switched mode power supply connected in parallel to loads can be used to charge the voltage of the load circuit up to a level close enough to battery voltage in order to allow closing the contactors between battery and load circuit. A BMS may have a circuit that can check whether a relay is already closed before precharging (due to welding for example) to prevent inrush currents to occur.
In order to maximize the battery's capacity, and to prevent localized under-charging or over-charging, the BMS may actively ensure that all the cells that compose the battery are kept at the same voltage or State of Charge, through balancing. The BMS can balance the cells by:
BMS technology varies in complexity and performance:
BMS topologies fall in three categories:
Centralized BMSs are most economical, least expandable, and are plagued by a multitude of wires. Distributed BMSs are the most expensive, simplest to install, and offer the cleanest assembly. Modular BMSes offer a compromise of the features and problems of the other two topologies.
The requirements for a BMS in mobile applications (such as electric vehicles) and stationary applications (like stand-by UPSes in a server room) are quite different, especially from the space and weight constraint requirements, so the hardware and software implementations must be tailored to the specific use. In the case of electric or hybrid vehicles, the BMS is only a subsystem and cannot work as a stand-alone device. It must communicate with at least a charger (or charging infrastructure), a load, thermal management and emergency shutdown subsystems. Therefore, in a good vehicle design the BMS is tightly integrated with those subsystems. Some small mobile applications (such as medical equipment carts, motorized wheelchairs, scooters, and fork lifts) often have external charging hardware, however the on-board BMS must still have tight design integration with the external charger.
Various battery balancing methods are in use, some of them based on state of charge theory.
A battery management system allows users to monitor individual cells within a battery pack. As cells work together to release energy to the load, it is crucial to maintain stability throughout the whole pack.
This is where a battery management system (BMS) comes into play. A BMS allows for constant monitoring, gathering, and communicating information to an external interface where users can observe the status of each cell and the health of the battery pack as a whole. The BMS monitors and manages a battery pack in order to protect it from damage, prolong its life, and keep the battery operating within its safety limits. These functions are key to efficiency, reliability, and safety.
What does a BMS measure?
A BMS can measure different figures such as current, voltage, temperature, and coulomb count. With these measurements the system can assess the health of the battery and readjust operations as needed to protect the pack.
For instance, a drop in cell voltage at a given load can indicate an increase in internal resistance. This then can point toward dry-out, corrosion, plate separation, or other diagnoses.
A sudden increase in the temperature of one cell could indicate the possibility of a thermal runaway event within the whole battery pack. The BMS could then stop the flow of energy and alert the user to a potential problem so that it can be contained before it gets out of control.
Coulomb counting can help to estimate the available capacity of a battery. This is done by measuring the amount of energy leaving and entering the battery during charge/discharge cycles. A decrease in coulomb count during a full cycle when compared to a new battery cell indicates a drop in battery capacity.
What indicators can be inferred with a BMS?
State of charge (SoC) and state of health (SoH) are important indicators for assessing the usability and capabilities of a battery. Together the SoC and SoH provide a state of function, an overview of the battery and the pack’s capabilities as a whole.
State of charge is probably the most straightforward and common measure that a person would come across. The battery percentages on phones or laptops are the states of charge. In electric vehicle batteries, the SoC is used to determine the remaining range left of the car before it needs to be recharged. This, however, by itself, is not indicative of the overall health of the battery. While SoC can show the short term capability of the battery — showing how much energy there is left — it cannot indicate the true capacity of the battery cell or pack. Cell capacity decreases with age so while SoC may read 100%, after a while the true capacity is likely less than that.
Nonetheless, SoC is still an important measure in managing batteries. For instance, the SoC of individual cells in the battery chain needs to be known in order to balance the load evenly across cells within the pack.
Complimentary to SoC, State of Health measures the long-term capabilities of the battery pack. Taking into account charge acceptance, internal resistance, voltage, and self discharge, SoH is an estimation of how much longer a battery can operate optimally. It is usually measured against a fresh battery cell in order to infer the cell’s position within its lifecycle.
There are no standard parameters to indicate SoH as it would depend on the function and applications of the battery cell. Different parameters such as cell resistance or self-discharge may be individually weighted to assess the overall SoH.
As SoH is usually measured against a new cell, the BMS must keep a record of the initial conditions of the battery and a log of measurements throughout the lifecycle of the battery in order to provide a more accurate indication of battery health.
Why is a BMS important?
Not only is a BMS important in indicating the health of a battery, but it also functions to protect the battery while in operation.
Each battery cell and chemistry has voltage, temperature, and current range within which it can safely operate. When a cell drops below or exceeds these ranges, it can be detected and controlled by the BMS. For instance, lithium is a highly reactive substance; thus the BMS should monitor each lithium cell to ensure that it remains operating within predefined limits. This keeps the battery safe and preserves it in the long run.
Another important safety feature of a BMS is cell balancing. Individual cells within a battery pack do not operate equally. One cell may be weaker or stronger than the other, charging or discharging faster than others within the chain. Without proper compensation, this could degrade the health of the overall pack. If one cell short circuits or fails, this affects the stability of the whole pack. Cell balancing equalizes the charge between individual cells based on each cell’s capability. The BMS helps to monitor and control the charge demanded from each cell in the chain, ensuring that SoC remains evenly distributed.
The role of a BMS in battery testing
It prolongs the life of your battery by defining an optimized range of use for electrochemical cells. It estimates battery ageing via SOH “State Of Health” and can tell you the number of cycles performed. It allows you to know in real time the state of charge of your battery thanks to the SOC “State Of Charge”.
The move to decarbonise our grid is increasingly using Lithium-Ion batteries. Starting with the transportation sector, where Lithium-Ion batteries remain the option of choice to power EV’s. But even the Lithium battery is critically dependent on a sound Battery Management System (BMS) to deliver.
What Is the BMS?
The performance of electric vehicles depends on a lot of factors – cell voltage, battery life and health, safety, charging-discharging rates, etc. All these factors, one way or the other are linked to the rechargeable batteries in electric vehicles. Rechargeable battery packs are made of multiple cell modules arranged in a series and parallel. These battery packs produce several hundred volts of electricity. Various functions inside the car are dependent on them. That is why, it becomes a critical component of the vehicle that requires constant monitoring and control. This is where BMS comes into the picture.
A BMS ensures the complete tracking of all the functions performed by the battery, and so the vehicle. Hence, it is a system that manages lithium-ion battery packs through integrated firmware and hardware. When paired with telematics, it provides real-time data on the status and health of a forklift battery. A BMS design can become as complex as the purpose the battery seeks to serve.
A battery-management system (BMS) typically consists of several components. The most common components include cut-off field-effect transmitters (FETs), fuel-gauge monitors, cell voltage sensors, real-time clocks, temperature monitors, and microcontrollers (BMS algorithms).
FET is accountable for connection and isolation between load and charger of the battery pack while the fuel-gauge monitor keeps track of the charge entering and exiting the battery pack. Here, the charge flowing is calculated by multiplying current and time. Further, the cell voltage sensors carry out the function of cell voltage monitoring which is a standard function of the BMS. It is very crucial in determining the health of the battery.
The temperature monitoring is another important feature of BMS and the internal ADC voltage-powered thermistor performs this function. 0BMS also has a Real-time Clock (RTC) which acts as a black-box system for time-stamping and memory storage. RTC allows the user to know the battery pack’s behaviour and, thus, warns before any alarming event. There’s also one microcontroller with BMS algorithms that makes quick and effective decisions in real-time.
BMS can be categorized based on topology. Topology relates to how it is installed and operates upon the cells or modules across the battery pack. For electric or hybrid vehicles, the BMS is only a subsystem and cannot work as a standalone device. It must communicate with at least a charger (or charging infrastructure), a load, thermal management and emergency shutdown subsystems. Therefore, the BMS is tightly integrated with those subsystems.
While there are several reasons why Battery Management System matters, and with more advances in technology it may add new functions, here is a list of the most important functions of BMS in a Lithium-Ion Battery.
Without a doubt, a safety assurance is the most crucial function for anything that may go unsafe at some point. For an electric vehicle’s lithium-ion battery, the BMS captures crucial data such as voltage, temperature and current for its various functions, including the safety.
BMS ensures thermal management of the battery and monitors its temperature continuously. Apart from adjusting cooling, it triggers other safety mechanisms to cease operations and minimize the risk.
Thermal Runaway is a condition where the current flowing through the battery on charging or overcharging causes the cell temperature to rise. Conditions like this may seriously harm the lifespan or the capacity of the battery. Overcharging lithium-ion cells may also lead to thermal runaway and, in the worst case, an explosion. BMS controls the current supply as well to avoid overcharging by enforcing the limits of maximum charge or discharge current according to the temperature.
Protecting user from an electric shock, the BMS also makes sure the vehicle chassis is completely isolated from a high voltage battery pack.
The battery cells need monitoring round the clock – charging or discharging. Any extraordinary situation needs to be identified and reported, in addition with triggering safety mechanism mentioned above. BMS does this function using integrated circuits with cell monitoring algorithms. A chain of command passes the voltage and temperature data to a cell management controller.
Apparently, these algorithms calculate the state of charge (SOC) and state of health (SOH). SOC helps to ensure that the battery is never over or undercharged. Playing the role of a fuel indicator of an electric vehicle, it indicates the energy remaining in the battery along with the distance the EV would cover before the battery needs a recharge. SOH, on the other hand, indicates the overall health of the battery. It provides an insight into the operating conditions of the battery. The information is crucial to project battery lifespan and maintenance schedule. Both SOC and SOH are generally in percentages.
This can be considered a direct outcome of the aforementioned cell monitoring system. The EV BMS tries to keep the SOC and SOH parameters within the limits as per specifications. It works like a guardian that determines the extent of current in the individual cells and communicates the same with the EV Supply Equipment (EVSE), or the charger. As the EV functions and discharges, the BMS also communicates with the motor controller to ensure that the voltage level does not get too low.
Thus, the vehicle shows an alert to the user to charge the battery pack. The BMS controls the recharging of the battery pack by energy generated through regenerative braking.
That’s not where the duty of Battery Management System ends. Individual cells of a battery pack develop differences in capacity with time. This amplifies the charge/discharge cycle for each of them creating non-uniformity and thus limits the amount of energy from the battery as a whole, and the extent of its charge. Cell balancing by BMS helps maintain the cell at equal voltage levels and maximize battery pack’s capacity utilization. BMS performs cell balancing by draining excess energy from cells that are more charged than others – through active and passive balancing techniques.
With time, battery cells deteriorate. BMS intelligently takes this deterioration in account which results in change on battery parameters such as voltage, current, etc. For instance, consider that a cell gets damaged by heat and starts getting charged at a lower voltage than the rest of the cells. BMS identifies this fault and optimizes the charging process for all cells now to charge on lower voltage. This reduces the stress on the overall battery pack and enhances its life overall.
But if you’re reading this article, you have probably already considered these options and have decided to not use a BMS. In that case, you’re going to need to wire in a balance connector so that you can balance charge your battery when necessary.
Balance charging requires, wait for it…. a balance charger. (Yea, this is tough stuff, I know.)
Cheap balance chargers up to 6s (6 cell groups in series) are readily available. The iMAX B6 charger is the common example, and clones of this common charger are available all over the place for as little as $30.
Balance chargers up to 8s are a bit pricier, and models designed for 10s or 12s can really start to break the bank, depending on your budget. So keep that in mind.
To take advantage of a cheap 6s charger, I’ll build a 4s battery in this example, which will have a nominal voltage of 14.8V and a charge voltage of 16.8V, which is 4.2V per cell.
The normal battery construction is the same as always. Snap together your VRUZEND caps, insert your cells and bolt on the bus bars. You can find other articles and videos on those exact steps here.
After you’ve got your battery assembled, it’s time to the wiring. Without a BMS, you’ll only have two thick gauge wires on your battery: one on the positive terminal and one on the negative terminal These will serve as both your charging AND your discharging connections.
Next, you’ll wire in your balance connector. The best way to find a balance connector is to look for a balance wire extension cable and just cut off the male-pin connector. Notice that there is one more wire than the number of cells that the cable is intended for. This is a 4s balance wire extension cable, so it has 5 wires. You’ll see why in a minute.
Next, find the red wire (or the wire that is intended to the be highest cell number wire if your wires are different colors) and connect it to the same location as your positive discharge wire. This will always be the positive end of the highest numbered cell in your pack. In my 4s pack, this is the positive end of my 4th cell group. I just slide it under the bus bar and tightened the nut, clamping the wire against the bus bar.
Now take the next wire down from the red wire, and connect it to the next cell group down’s positive terminal. This is likely on the other side of your battery. Continue moving down the balance connector wires, connecting each successive wire to the next lower cell group’s positive terminal. The second to last wire should connect to your first cell group’s positive terminal. The last wire will finally connect to the first cell groups negative terminal, which is the same location as your packs main discharge cable. That’s why you’ve got one more wire than cell groups – because the first cell group has a wire on the positive AND negative terminal.
To balance charge, you’ll plug your discharge/charge wires into your balance charger (you may have to use one of the adapters that comes with your charger). Then you’ll plug your balance wire connector into the appropriate spot on your charger.
Make sure to read your charger’s instruction manual carefully to select the proper balancing program for your battery. You want to match both the chemistry of the battery (usually marked li-ion or li-po in the charger’s settings) and the charge voltage, which is 4.2V for most li-ion cells. You also want to make sure you select the proper number of cells so that you don’t overcharge your battery.
If everything checks out on your charger, begin the charging process. Balance charging isn’t always necessary if you are using good quality cells. Most batteries will stay fairly well in balance after a few discharge cycles. You’ll need to check your cell groups to make sure they are staying fairly well balanced during discharging, and always balance charge if you see that the cells are becoming unbalanced. If you aren’t balance charging every time though, you can bulk charge.
Bulk charging is basically the same as charging a battery with a BMS, except that there is no BMS to watch the process. When lithium battery fires happen during charging, its usually because someone was bulk charging without a BMS and made a stupid mistake. Always perform bulk charging carefully when not using a BMS.
Bulk charging means that you aren’t balancing each cell group like with balance charging – instead you’re just charging the whole pack up together to a certain voltage, balance be damned. This is ok as long as two things don’t happen: 1) No single cell in the pack goes over the maximum voltage it is rated for, usually 4.2V for li-ion cells, and 2) The pack voltage shouldn’t go over the total proper voltage, which is essentially the # of cells in series multiplied by 4.2V for li-ion cells, and which basically means that the first situation has happened.
To make sure that no cell goes over it’s maximum voltage, bulk charging is normally done to a lower target voltage. For example, instead of trying to charge the pack to 4.2V x # cells in series, consider charging to 4.1V or 4.15V. For a 36V battery with 10 cells in series, that would mean charging to 41V or 41.5V instead of 42V. This adds a level of safety by giving you a buffer for a few cells to overcharge. Without balancing, cells being bulk charged will each charge to a slightly different voltage. If you charged to 42V in a 36V li-ion battery, some cells would reach 4.2V, but others might reach 4.18V, meaning others would have to reach 4.22V to allow the entire pack to reach 42V total. This isn’t terribly unbalanced, but as the imbalance grows, some cells could being reaching 4.3V or higher, which is a very dangerous situation.
This is why I almost never bulk charge without a BMS. If I’m not using a BMS, I always try to balance charge. And if I do bulk charge for a few cycles, I aim for a lower target voltage and I use a cell checker to watch my cell groups and ensure none are overcharging.
No matter what type of charging you are doing, there are some important safety tips that you should always follow:
