Application of smart battery system

Lithium-ion batteries have become the preferred energy source (power source) for laptops and handheld systems. As the demand for power from CPUs, displays, and DVD drives continues to grow, high-energy-density battery packs are also developing. At the same time, high-volume manufacturing processes ensure that high-energy-density battery packs have a reasonable price level.

Many new technologies increase the power consumption of the system while improving performance. For chemical companies that produce batteries, substantial progress in battery production technology is difficult, time-consuming and costly. Therefore, it is necessary to find ways to optimize power conservation. Smart battery system (SBS) is the most promising technology that has emerged and can greatly improve the performance of battery packs.

In the computer industry, lithium-ion batteries are both loved and feared. The accidents that occurred in the early days of lithium-ion battery applications are still fresh in the minds of the companies involved. They have learned a profound lesson: in any case, the rated parameters of lithium-ion batteries must not be exceeded, otherwise they will definitely cause explosions or fires.

In addition to parameters such as the battery's chemical composition or electrodes, there are several parameters that are determined for lithium-ion batteries that, if exceeded, can cause the battery to enter a runaway state. In the graphs that explain these parameters (see Li-ion Parameters Graph), any point outside the corresponding threshold curve is a runaway state. As the battery voltage increases, the temperature threshold decreases. On the other hand, any action that causes the battery voltage to exceed its designed value will cause the battery to overheat.

Beware of the dangers caused by chargers

Battery pack manufacturers build in several layers of cell and packaging protection to prevent dangerous overheating conditions. But there is one component in battery use that can cause these measures to fail and cause harm: the charger.

There are three ways that charging lithium-ion batteries can cause harm: the battery voltage is too high (the most dangerous situation); the charging current is too large (excessive charging current causes lithium plating effect, which causes heating); the charging process cannot be terminated correctly, or charging at too low a temperature.

Designers of lithium-ion battery chargers take extra precautions to avoid exceeding the allowable range of these parameters, to absolutely ensure that the system parameters operate within a safe range.

For example, the smart battery charger specification allows a negative voltage deviation of -9%, but emphasizes that the positive deviation must not exceed 1%. This ensures compliance with the smart battery safety standard. Of course, in actual design, the positive and negative deviations are random. Therefore, designs that meet this specification often set the target voltage value of the charger to around -4% of the rated value.

Due to the inaccuracy of charging voltage (whether -4% or -9%), the battery is always undercharged. The fear of potential dangers of lithium-ion batteries leads to low utilization of battery pack capacity. According to the experience of industry experts, even if the voltage after charging is only 0.05% lower than the rated value, the capacity drop is as high as 15%.

Battery built into the computer

The principle of smart battery technology is very simple. A small computer is built into the battery to monitor and analyze all battery data to accurately predict the remaining battery capacity. The remaining battery capacity can be directly converted into the remaining working time of the portable computer. Compared with the original capacity measurement method that only relies on voltage monitoring, the working time can be immediately extended by 35%.

Unfortunately, smart battery technologies can only go so far. Unless they can communicate with the charger circuitry, they cannot determine their operating environment or exert control over the charging process.

In the context of a "smart battery system", the battery requests the smart charger to charge it under certain voltage and current conditions. The smart charger is then responsible for charging the battery according to the requested voltage and current parameters.

The charger relies on its own internal voltage and current references to adjust its output to match the values ​​requested by the smart battery. Since these references can be inaccurate by as much as -9%, the charging process may end with the battery only partially charged.

A more detailed understanding of the charging environment can reveal more issues that affect the efficiency of lithium-ion battery charging. Even in the most ideal case, assuming that the charger is 100% accurate, the resistance elements between the batteries in the charging path introduce additional voltage drops, especially during the constant current charging stage. These additional voltage drops cause the charging process to enter the constant voltage stage prematurely from the constant current stage.

Since the voltage drop introduced by the resistor will gradually decrease as the current decreases, the charger will eventually complete the charging process. However, the charging time will be extended. The energy transfer efficiency is higher during constant current charging.

Eliminate resistance voltage drop

The ideal situation is that the output of the charger accurately eliminates the effect of the resistor voltage drop. Someone may propose such a solution, in all stages of the charging process, the smart charger uses the data of the smart battery internal monitoring circuit to monitor and correct its output. This is feasible for a single battery system, but it is not suitable for dual or multi-battery systems.

In a dual-battery system, it is best to charge and discharge both batteries at the same time if possible. Although the battery charging is in parallel, a typical charger with only one SMBUS port is not competent for this task. Because if there is only one SMBUS port, the charger or other SMBUS device can only communicate with one battery at a time. Therefore, the ideal system should provide two or more SMBUS ports, so that two batteries can communicate with the charger at the same time.

Smart Battery System (SBS) Manager

In addition to providing multiple SMBUS ports, SBS Manager technology can also significantly improve the performance of lithium-ion smart batteries. SBS Manager is part of SBS and is defined by the SBS1.1 specification. It replaces the Smart Selector defined in the previous version.

The SBS manager provides an interface with the driver and the control system on the one hand, and manages the smart battery and charger on the other hand. The driver can read and request information about the battery, the charger and the manager itself. The interface related to this information transmission is defined in the specification. In a multi-battery system, the SBS manager is responsible for selecting the system power source and deciding which battery to charge or discharge at a specific time. In short, the SBS manager determines which battery to charge, which to discharge, and when.

A well-implemented SBS management has several advantages: a more complete and faster charging process, efficient simultaneous charging and discharging, and the ability to detect and quickly react to dangerous situations such as potential voltage overruns.

The SBS manager, which can monitor the battery voltage itself, can charge the battery to its true capacity. This can avoid undercharging caused by the smart charger due to inaccurate monitoring voltage (as mentioned above, usually -4% to -9%). In addition, this process does not require a particularly accurate reference voltage (accurate voltage reference is very expensive).

The strategy to avoid using a precise voltage reference is to use the measurement circuit inside the smart battery to measure the battery voltage with an accuracy of 1%. In this way, the SBS manager can command the charger to increase the voltage appropriately until the monitored voltage reaches the appropriate value.

A well-implemented SBS manager can charge a battery 16% faster than a conventional charger. It safely increases the charger's output voltage above the battery's rated voltage to compensate for the voltage drop due to the battery's internal resistance and loop resistance. This is accomplished by monitoring the battery's internal voltage and quickly adjusting the charger voltage.

When and how to charge

The SBS manager can decide when to charge the battery packs simultaneously. Simultaneous charging allows for better utilization of the charger current for charging. In a single-battery system, when entering constant voltage charging mode, the charging current provided by the charger decreases as the battery becomes more full. Unused current is wasted. This is not the case in a dual-battery system using the SBS manager, where the current that is not used to charge one battery can be used to charge the other.

Furthermore, the SBS Manager can determine which battery is in a state that allows for faster energy transfer. The batteries that can add the most capacity to the system are charged first, and the batteries that can hold more energy are discharged quickly. This can speed up the charging process by up to 60%. The SBS Manager can also decide when to enable the simultaneous discharge feature. Appropriate simultaneous discharge can increase system capacity by as much as 16%.

Of course, all these improvements must be safe for the battery's performance. As discussed earlier, lithium-ion batteries have a rated voltage. When the voltage applied to the battery reaches its maximum value, the charging process switches from constant current to constant voltage mode. The detection of this switching point is the responsibility of the smart charger SBS manager, based on the measured battery voltage. But the great advantage of the SBS manager over smart chargers is that it can constantly monitor and correct the charger and battery voltages. This ensures safety while reaching the battery's maximum capacity.

As the performance of computers and other devices continues to improve, the energy demand is growing rapidly, and the improvement of chemical batteries cannot keep up with this growth rate. Although SBS technology is very helpful, there will always be a day when SBS technology alone cannot provide the power required by high-performance systems, and a more intelligent power management solution is needed.

If that OEM could make a laptop computer last six hours without noticeably affecting performance, it would quickly take over the market. SBS Manager is a big step toward that goal.