Active battery balancing technology to help increase the power capacity of large lithium-ion battery pack

Some consumer applications require a single lithium-ion battery (such as a cell phone) or three tandem and two parallel batteries (such as laptops). This raises the need for higher power, higher capacity, and more robust battery packs. Installing the battery in series can increase the voltage, while installing the battery in parallel increases the capacity. These battery packs vary in number, from six batteries used in laptops to hundreds of batteries used in electric vehicles, creating many new design challenges for battery designers.

These high-capacity batteries require advanced management to ensure a high-quality design. We must consider the right temperature, voltage and current measurements. As lithium-ion battery packs become larger, more attention is needed to thermal management, battery pack reliability, battery life, and battery balancing. In fact, as the number of batteries required in a battery pack increases, differences in temperature, capacity, and series resistance between the battery cells become an important issue. This article will focus on the impact of these differences and how to control these differences in battery design.

Problem: Battery status does not match

The role of the battery is to store and provide energy for its host. We want to store as much energy as we can from the battery pack. The main aspect that prevents multiple battery packs from doing this is battery impedance. Let's take a look at how it affects the power to the battery host.

In lithium-ion batteries, there are some minimum and maximum predefined voltage levels that each series cell can achieve. This is a safety feature controlled by the IC in the battery pack, see Figure 1A. As long as each cell is maintained between the overvoltage and undervoltage off ranges, the battery pack can discharge and charge. If a cell reaches any of the above thresholds, the entire battery pack turns off (undervoltage), leaving the host's battery pack, which should be available, to become fully charged (see Figure 1B). In addition, it does not allow the charger to charge the battery pack with a large amount of energy (see Figure 1C) (overvoltage).

Figure 1: The impact of battery imbalance on battery capacity usage.

Battery imbalance for many reasons:

* Non-uniform thermal stress

Impedance variable

* Low battery capacity to match

* Chemical differences

Some of these reasons can be minimized by battery selection and better battery pack design. Even so, all the previous design work, the main reason for battery imbalance is non-uniform thermal stress. The temperature difference between the battery and the battery can cause changes in the impedance variables and chemical reactions. This creates a temperature difference, and the battery is exposed to this difference for a long time. This is a laptop FLIR map that shows the degree of temperature differences, even in consumer electronics applications. For every 10 ° C increase in temperature, the self-discharge rate of a lithium-ion battery doubled. A characteristic of lithium-ion batteries is that the internal impedance is a function of temperature. Lower temperature batteries exhibit high impedance, so IR drop is greater during charging or discharging. This resistance also increases with increasing exposure to high charge and high temperature, as well as longer charge cycle times.

Solution: Battery balancing technology

Due to the impact on energy supply and the risk of lithium-ion battery overcharge in tandem battery applications, battery-balancing techniques must be used to correct the imbalance. There are two types of battery balancing technologies: passive battery balancing and active battery balancing.

Passive battery balancing technology

A passive battery balancing method known as "resistive leakage" balancing uses a simple battery discharge path that discharges the high voltage battery until all battery voltages are equal. In addition to other battery management features, many devices feature battery balancing.

Lithium-ion battery pack protectors such as the bq77PL900 are primarily used in many cordless battery-powered devices, power-assisted bicycles and mopeds, uninterruptible power supplies, and medical equipment. Its circuit mainly serves as an independent battery protection system, using 5 to 10 serial batteries. In addition to the many battery management features controlled through the I2C port, the battery voltage can be compared to a programmable threshold to determine if battery balancing is required. If any particular battery reaches this threshold, charging stops and an internal bypass is activated. When the high voltage battery drops to the recovery limit, the battery balance stops and charging continues.

Battery balancing algorithms only use voltage dispersion as a balancing standard, with the disadvantage of being over-balanced (or under-balanced) due to the presence of impedance imbalances. The problem is that the battery impedance can also cause voltage differences (VDiff_Start and VDiff_End) during charging. A simple voltage-battery balance does not distinguish between power imbalance and impedance imbalance. Therefore, this balance does not guarantee 100% charge on all batteries after full charge.