Intersil covers the basics of battery management systems and discusses the features of the ISL94203 battery management IC in this technical webinar.
ISL94203 Product Training
Intersil Battery Management Solutions
Welcome to the ISL94203 battery management IC product training. During this training, we will cover the basics of battery management systems and then discuss the features of the ISL94203 battery management IC.
Changes in Battery Chemistry
Battery chemistry has undergone significant changes in recent years to meet the demand of the marketplace. Market drivers include increased power requirements and the desire to have long battery life, coupled with the appeal of small size and weight. Another concern is reducing environmental impact.
Lithium-ion batteries are a family of rechargeable battery types with high energy density both by weight and size.
The casual battery user may think there is only one lithium‐ion battery. However, there are actually several types, and new chemistries are being researched. You will choose the ideal battery chemistry/characteristic depending on the end application. The different chemistries have varying specific energy, specific power, safety, cost, life span, and performance.
Lithium-Ion Battery Drawback
As with most things, there are also drawbacks to the lithium-ion batteries. The window of safe operation for a given battery cell is fairly narrow, and due to the high energy density of the battery (roughly 1/8 that of dynamite), battery failure can result in serious damage.
Thus, there is a need to closely monitor these batteries; hence, the need for a battery management system.
Battery Management System (BMS)
What is a battery management system (BMS)? It is a system that uses circuitry to monitor battery life and catastrophic conditions. The BMS circuitry can also extend the on time and lifetime of a battery.
A complete lithium-ion battery system consists of a battery, BMS circuitry, a load and a source, usually a charger. It is important to know the load and the source voltages and currents when designing the system.
The BMS circuitry consists of protection/monitors, balancing circuitry and system management.
The protection/monitors include cutoff FETs or drivers to disconnect the battery from the load or the charger, current sense circuitry to monitor battery usage and over current conditions, external temperature sensors to monitor cell temperature, voltage cell monitors to determine the state of charge of each cell, and regulators and references to power up peripheral circuitry.
The balancing circuitry consists of the cell balancing FET or cell balancing drivers for larger current bearing battery packs. The cell balancing circuitry extends the life of a battery pack and the on time of the battery per charge.
The system management block consists of an external MCU or internal state machine that maintains the battery in an optimal state for the given application. It makes decisions based on circuit measurements, has analog and digital comparators to acknowledge catastrophic alert conditions such as cell voltage being too large or the load current being too large, and memory for recording diagnostic data. What happened before the battery went bad?
The easiest conditions to monitor are the catastrophic—the battery is on fire, there is a short with the load, the charger is sourcing too much current, etc.
Other conditions may be more subtle—it’s too hot or cold to charge? Has the cell been permanently damaged by charging?
The FET drivers are functionally tied to the status of the cell voltage within a battery pack. The state at which the BMS is in (load state or a charge state) and the digital/analog comparators that monitor temperature, current and voltage windows control which FET, if any, are on.
The diagram on this slide shows the cut off FETs that are part of the cell FET driver. The circuit connection allows for the charge current and load current to be significantly different via the use of two sense resistors.
What is Cell Balancing?
Cell balancing is the process of equalizing the state of charge in all the cells of a lithium-ion battery pack. The more cells in a system, the more likely that balancing is needed. Cells are usually matched when a pack is assembled to be within 0.2% of each other, but can become unbalanced due to varying charge and discharge rates, temperature variations, and unbalanced loading, among other reasons.
Why is Cell Balancing Important?
Why is this important? Cell balancing is vital to maintain the maximum on time of the battery pack. Since each cell has a strict limit for the operating voltage range, the entire pack must stop charging or discharging as soon as any one cell reaches the upper limit or lower limit, respectively. The result is a loss in pack capacity.
Continuous Cell Unbalancing
The cells may become more unbalanced over time as the battery is cycled, and the battery pack will continue to lose capacity until eventually it becomes unusable.
Cell Balance Example
Conversely, the battery pack capacity can be increased when healthy cells are balanced during charge. This results in longer on time and a longer battery lifetime. Since the balancing is slow, it may take several cycles to balance.
Industrial BMS Applications
Common applications that utilize Li-ion batteries include E-bikes, power tools, portable medical equipment, and battery backup systems.
ISL94203 Battery Management
Now, let’s take a look at the ISL94203 and the solutions it provides for the challenges of managing lithium-ion battery packs.
ISL94203 3 to 8-Cell Li-ion Battery Pack Monitor
The ISL94203 represents countless hours of architecture definition, design creation and applications support capability refinement. The block diagram on this slide reviews a wide assortment of internal capability unmatched in BMS applications. It is capable of complete standalone operation and supports from 3 to 8 lithium-ion cells equaling 6V to 36V. The ISL94203 has voltage, temperature, current monitor and pack switching control for battery protection. The cell voltages can be from 0V to 4.8V and are measured with the internal 14-bit ADC. The ISL94203 also has five power states to increase the battery life of the system, supports high-side N-channel FETs, and includes a 1.8V reference and 2.5V regulator for external peripherals.
Diagnostic and Protections Features
The ISL94203 provides numerous checks for fault conditions over current, voltage and temperature. Current, voltage and temperature monitors all have programmable threshold levels for triggering a fault condition that are saved in EEPROM (or can be controlled by a microcontroller), making the device extremely versatile and fit for a plethora of applications.
Cell Measurement and Cell Balancing
The cell voltage measurements are accurate up to ±5mV (under 0.2%) when in ideal voltage and temperature ranges of 2.8V to 3.8V and 0 °C to 50 °C. All eight cells can be sampled in under 1ms, although the typical scan rate is set to 32ms. Current monitors checking for serious fault conditions use analog monitors for continuous monitoring. The ISL94203 automatically balances cells when a cell is greater than the threshold set by CBMINDV and there are no faults detected. The device can be set to balance while charging, discharging, or both. It can also be set to never balance. It is not recommended to balance while discharging, as this consumes power that could be used on the load. One other balancing condition is also available, which is the end of charge balancing. This is the only mode in which the device does not take current flow into account when checking whether to balance.
5 Power States for Longer Battery Life
To save battery power, the ISL94203 has five different power states. If current is not detected for a set period of time, the device will move into idle mode. From idle mode, it will further reduce its supply current after the given amount of time by switching to doze mode. If there is still not current activity after a third, longer, time period, (or if the voltage drops below the sleep threshold) the part enters sleep mode. Each step down increases the time between voltage scans, until there are no scans in sleep mode. However, all wake up circuitry and the 2.5V regulator will still remain on, so the presence of a load will send the device back into normal mode.
The ISL94203 is only in power down mode when Vdd is too low for proper operation.
ISL94203 Evaluation Boards
The ISL94203EVKIT1Z contains three boards (as shown). One board is the ISL94203 evaluation board, the MCB_PS_Z board is a multi-cell power supply test board, and the third board is an interface board for USB to I2C. It is also possible to buy the interface board separately. The user interface software allows access to all registers needed to set the threshold levels and other desired settings of the ISL94203. The evaluation board also includes jumpers to allow multiple layout options. For example, the user can select either a single path for charge and discharge or two separate paths. Further documentation is available via the ISL94203 product page under the Documents tab.
In summary, a battery management system is responsible with protecting the battery cells, prolonging the on time and lifetime of the battery, and maintaining the battery in an operable state for its application.
The ISL94203 is a standalone battery management IC that allows great flexibility while maintaining safe battery operation, thanks to programmable settings saved to EEPROM. It supports high-side N-channel external FETs, and saves battery life by entering lower power states when not in use.