In the past, high-voltage, battery-operated products depended upon large, heavy lead-acid type batteries to deliver the necessary power. The size and weight of lead-acid battery packs often limited the range of battery-operated products that could be developed. Thanks to advances in battery technology, high-voltage battery packs can now be found in cordless power tools, cordless home appliances, mobile medical equipment, electric bicycles and more.

These products present a unique set of challenges to today’s product designers. The three characteristics most important to end-users of battery-operated products tend to be portability (light weight), high performance, and low cost. By contrast, the most important concerns for product designers are usually safety, reliability, and usable product life. Each of these requirements, which are often in conflict, must be satisfied and balanced against each another.

To meet both user requirements and environmental regulations, for instance, designers are adopting the latest generations of lithium-ion battery technology. New generation Li-ion cells can meet the needs of systems with high-power requirements, as well as systems with high-energy requirements. These Li-ion battery packs all share the need for in-pack battery monitoring and protection circuitry that was not necessary for battery packs using nickel-cadmium or nickel-metal-hydride cells. Li-ion battery packs generally require safety features such as:

• Pack over-current monitoring and short-circuit monitoring.
• Cell over-voltage and under-voltage monitoring.
• Pack/cell temperature monitoring.

Enlarge this picture Fig. 1. Example of a Li-ion battery pack managed by an ISL9208 chip. The design supports control of both charge and discharge currents using a single path with two MOSFETs.

Additionally, cell balancing and pack-capacity monitoring may also be  desired to further enhance the user experience.

While low-voltage battery powered products such as notebook computers and camcorders present a fairly constant demand for power from the battery, high-voltage battery products such as power tools and various mobility products have a higher demand for instantaneous power that must be supplied from the battery. Because of the extreme demands that can come from these devices, the battery pack monitoring and protection electronics are more complicated and provide additional levels of protection not found in the typical constant-demand battery packs.

Well-designed multi-cell monitoring and protection integrated circuits offer multiple levels of detection for error conditions, as well as timing windows for these error conditions to clear. At the same time, these ICs also provide hard limits beyond which a hard fault condition is deemed to have occurred.

Fig. 1 illustrates a simple example of a Li-ion battery pack that might be seen in a cordless power tool or small home appliance application such as a robotic vacuum. This design supports control of both charge and discharge currents using a single path with two MOSFETs. It also provides pack-current monitoring for over-current and short-circuit events; individual cell-voltage monitoring; pack-temperature monitoring; and fast, cell balancing with up to 200 mA of balance current.

Enlarge this picture Fig. 2. The ISL9216 and ISL9217 chipset can be used to manage battery packs with more than seven cells in series.

In this example, an Intersil ISL9208, functions as an Analog Front End (AFE) and operates in conjunction with an external microcontroller. The AFE performs level-shifting of the cell voltages and outputs the actual cell voltage on the analog output pin (AO) to the microcontroller.  The microcontroller uses this information to monitor the status of each cell during charge and discharge, as well as for cell balancing.

Along with the analog voltage of each cell, the AFE also reports any error conditions to the microcontroller. The charge and discharge FETs may be controlled directly by the AFE and provide an automatic protection mechanism to minimize any possibility of delays in protection being introduced by the microcontroller when critical error states such as over-current or short-circuit conditions exist. It is possible to disable this automatic protection feature if designers have some proprietary battery-management algorithms they prefer to use. In cases where the automatic protection feature is disabled, the AFE  will continue to monitor the current and will report an error condition to the microcontroller, which will then direct the AFE  to disable the MOSFETs or execute the proprietary algorithms.

For battery packs that require more than seven cells in series, as shown in Fig. 2, a chipset approach that incorporates a single microcontroller and multiple AFE’s can be easily implemented using Intersil’s ISL9216 and ISL9217 chipset. 

Enlarge this picture Fig. 3. Illustration shows the impact of not balancing series cells within a battery pack over multiple charge and discharge cycles.

In a well-designed battery pack, error conditions will be qualified in time and amplitude to avoid spurious shutdowns. For example, the ISL9208 provides multiple voltage, current, and timing thresholds that are programmable by designers for their specific applications. These include:

• Four discharge over-current thresholds.
• Four short-circuit thresholds.
• Four charge over-current thresholds.
• Eight over-current delay times (Charge).
• Eight over-current delay times (Discharge).
• Two short-circuit delay times (Discharge).

These multiple thresholds provide the designer a great deal of flexibility to deal with the charge and discharge profiles of various types of equipment, while using the same battery pack protection electronics.

Over-voltage and under-voltage conditions of individual cells within the battery pack are also important to monitor. If any cell voltage exceeds the manufacturer’s specified upper limit, charging must be disabled to prevent a potentially hazardous condition to exist. Similarly, if any cell voltage drops lower than the discharge cutoff limit specified by the manufacturer, discharging will need to be disabled. In some cases where the cell voltage becomes very low, alternate charging techniques may need to be employed, or the pack may need to be totally disabled for safety reasons. Individual cell voltages are read by the microcontroller and are therefore digitally filtered to eliminate noise and improve system accuracy.

As mentioned earlier, some packs may also have the ability to monitor and control current during the charge cycle as well as the discharge cycle. In a pack that can monitor current during both cycles, the charge cycle is suspended in the case of charge over current or short circuit events.  Using a separate set of charge limits, similar indications are provided from the AFE to the microcontroller, and the charge MOSFETs are then disabled by the AFE using either automatic protection or by reporting an error condition to the microcontroller and having it command the AFE to disable the MOSFETs after executing the proper error-handling firmware.

Pack and cell temperatures must be monitored for safety reasons. Most cell manufacturers have both upper and lower temperature limits on cells during the charge and discharge cycles. In densely populated battery packs there can be a significant differential temperature across the cells from the inner region to the outer region of the pack as it is cycled through charge and discharge phases. Pack designers must take this into consideration and place thermistors in locations that adequately represent the cell temperatures.

While safety is the single most important issue in battery pack design, well-designed products also take appropriate steps to help ensure a good user experience. New generations of  Li-ion packs typically have high cell counts, as well as the added cost of protection and monitoring electronics within the pack and system. Costs for additional/replacement packs can be quite high when compared to the previous generations of nickel-cadmium-based battery packs. Users of these new generations of packs want to experience increases in performance as well as extended run times and shorter charging times. One method to provide the improved user experience is to implement cell-balancing within the battery pack.

Cell-balancing is a method to maintain all cells within a pack at the same state of charge. The more cells that are connected in series, the greater the benefit that cell-balancing will provide to maximize the performance and usable life of the battery pack. Most cells from a manufacturer, especially within the same lot, are fairly well matched in terms of ability to accept, retain, and deliver charge. However, small variations between cells, as well as the differences in temperatures of cells during charge and discharge can lead to unbalanced conditions. These unbalanced conditions can dramatically reduce the usability of a battery pack.

In any battery pack, the charging process must be stopped once the cell voltages have reached the charge termination voltage specified by the manufacturer. Similarly, the discharge process must also be stopped once the cell voltages have reached the discharge termination voltage specified by the manufacturer. The termination of charge or discharge based on a single cell voltage reaching the termination point leads to an unbalanced pack. Some cells may have a tendency to charge and discharge faster than the other cells due to their physical location within the pack and/or perhaps small differences between the cells during the manufacturing process.

In a balanced pack, charge is transferred from series cells at higher states of charge to series cells at lower states of charge. (Parallel cells self balance.) This process can occur during charge, as well as discharge of the pack, although it is typically implemented during the charge cycle only for simplicity.

Fig. 3 shows the impact of not balancing series cells within a battery pack over multiple charge and discharge cycles. When originally assembled into the battery pack, the cells were all well matched and at the same state of charge, but over multiple charge and discharge cycles, they gradually become unbalanced. This results in a significant loss of capacity and significantly reduced usability of the pack.

Arguments against cell-balancing in the past were typically based on the longer charge times that were required for balancing or designs that were too complex to realize at a reasonable cost. That is no longer true. By using internal-balance FETs with the ability to handle up to 200 mA of balance current, the ISL9208 can perform cell-balancing quickly at a low cost, and it is simple for designers to implement.

Users of Li-ion battery packs are eager for the lighter weight, performance improvements, and other advantages these new devices offer. By using the techniques discussed, it is possible to design high-power Li-ion packs that meet all the safety requirements and still provide a rich user experience at a reasonable cost. Using an integrated AFE  provides a very robust design with a minimum of external components and a relatively low total-solution cost.