Most popular use of active battery balancing techn

  • Detail

Using active battery balancing technology to increase the energy supply of large lithium-ion battery packs

some consumer applications require single lithium-ion batteries (such as), or three batteries in series with audible and visual alarm devices; Torque direction identification; Multiple peak holding; Power level display and low battery prompt and two parallel batteries (such as laptop). This leads to the demand for higher power, higher capacity and more robust battery packs. Batteries installed in series can increase the voltage, while batteries installed in parallel can increase the capacity. The number of these battery packs varies, from six batteries used in notebook computers to hundreds of batteries used in electric vehicles, which brings many new design difficulties to battery designers

these high-capacity batteries need advanced management to ensure high-quality design. We must consider appropriate temperature, voltage and current measurements. As lithium-ion battery packs become larger and larger, more attention is required to heat dissipation management, battery pack reliability, battery life and battery balance. In fact, with the increase of the number of batteries required in the battery pack, the difference in temperature, capacity and series impedance between battery cells has become an important problem. This article will mainly discuss the impact of these differences and how to control these differences in battery design

problem: battery state mismatch

the function of battery is to store and provide energy for its host. We want to store and get as much energy from the battery pack as possible. The main aspect that prevents multiple battery packs from completing this work is battery impedance. Let's take a look at how it affects the power supply to the battery host

in lithium-ion battery packs, there are some predefined minimum and maximum voltage values that each battery in series is allowed to reach. This is a safety feature controlled by the IC in the battery pack, see Figure 1a. As long as each battery is kept between overvoltage and undervoltage disconnection range, the battery pack can be discharged and charged. If a battery reaches any of the above thresholds, the entire battery pack will turn off (undervoltage), leaving the battery pack that should be available to the host in an uncharged state (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 the use of battery capacity

there are many reasons for battery imbalance:

* uneven thermal stress

* impedance variable

* low battery capacity matching

* chemical difference

some of these reasons can be minimized through battery selection and better battery pack design. Even so, in all the preliminary design work, the main reason for the imbalance of the battery is the non-uniform thermal stress. The temperature difference between batteries can cause changes in impedance variables and chemical reactions. This creates a temperature difference, and the battery is exposed to this difference for a long time (see figure 2*). This is a FLIR diagram of a laptop, which shows the degree of temperature difference, even in consumer electronics applications. The self discharge rate of a lithium-ion battery doubles for every 10 ℃ rise in temperature. A characteristic of lithium-ion batteries is that the internal impedance is a function of temperature. Lower temperature batteries show high impedance, so the IR voltage drop is greater during charging or discharging. This resistance also increases with the increase of the duration of exposure to high charging state and high temperature and the extension of charging cycle time

solution: battery balance technology

due to the impact on energy supply and the danger of overcharging lithium-ion batteries in series battery applications, battery balance technology must be used to correct the imbalance. There are two types of battery balancing technology: passive battery balancing technology and active battery balancing technology

passive battery balancing technology

the passive battery balancing method known as "resistance leakage" balancing uses a simple battery discharge path to discharge the high-voltage battery until all battery voltages are equal. In addition to other battery management functions, many devices have battery balancing functions

lithium ion battery pack protectors such as BQ77PL900 are mainly used in many cordless battery powered equipment, power assisted bicycles and mopeds, uninterruptible power supplies and medical equipment. The circuit mainly acts as an independent battery protection system, using 5 ~ 10 batteries in series. In addition to many battery management functions controlled through the I2C port, the battery voltage can also be compared with a programmable threshold to determine whether 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 the charging continues

Figure 3

Figure 4

the battery balance algorithm only uses voltage divergence as the balance standard, which has the disadvantage of overbalance (or underbalance), which is due to the influence of impedance imbalance (see Figure 3 and Figure 4). The problem is that the battery impedance also causes voltage differences (vdiff_start and vdiff_end) during charging. Simple voltage battery balance does not distinguish between power imbalance and impedance imbalance. Therefore, this balance cannot guarantee that all batteries will get 100% power after full charging

one solution is to use a battery power monitor, such as bq2084, etc. They all have improved voltage balancing technology. Because the impedance difference between batteries will mislead the algorithm, it only balances near the end of the charging cycle. This method minimizes the effect of impedance differences because the irbat voltage drop becomes smaller when the charging current gradually weakens to the termination threshold. In addition, this IC also makes the balance judgment based on all battery voltages, so it is a more efficient implementation method. Despite many improvements, the need to rely solely on the voltage level limits the balancing operation to the high state of charge (SOC) region and operates only during charging

another example is the bq20zxx battery power monitoring meter product series, which uses the impedance tracking balance method. Instead of trying to minimize the impact of voltage difference errors, this coulometer calculates the charge required for each battery to reach a fully charged state (qneed), as shown in Figure 5. This balancing algorithm turns on the battery balancing FET during charging to provide the required qneed. This kind of battery power monitoring meter can easily implement the battery balance scheme based on qneed, because the total power and SOC are stable and available in the monitoring function. Because the battery balance does not distort the battery impedance difference, it can work independently of the battery charging, discharging or even idle state. More importantly, it achieves the best balance accuracy

Figure 5: battery balance based on qneed

due to the limited balance ability of the passive battery balance technology using the integrated FET solution, the battery difference or imbalance rate may exceed the battery balance. In addition, due to the low bypass current, it may take several cycles to correct the general imbalance. Using existing components to design some external bypass circuits can enhance the battery balance (see figures 6 and 7). In Figure 6, when it is decided to balance a certain battery, the internally balanced MOSFET is turned on first. This forms a low current path through the external filter resistance connecting the battery terminals (battery 1 and battery 2) and the IC pins. When the internal FET gate source voltage is formed in the resistor, the external MOSFET is turned on. The disadvantage is that adjacent batteries cannot be balanced quickly and simultaneously. For example, if the adjacent internal FET is turned on, Q2 cannot be turned on because there is no current through R2

figure 6

Figure 7 shows the latest example of passive battery balancing. It is a low-cost, single-chip battery power monitoring meter solution. Unlike the battery level monitor solution described above, this IC does not have internal battery balance, but requires a similar external bypass circuit to complete the balance. However, since the balancing circuit is an open drain inside an IC, it can balance several batteries including adjacent batteries at the same time. This balancing circuit uses an improved voltage algorithm, as shown in Figure 6. However, the external FET driver in Figure 7 describes a more efficient battery balancing method

Figure 7

active battery balance

since 100% of the excess energy in high-energy batteries is dissipated in the form of heat, passive balance is not the preferred method during discharge. Active battery balancing uses capacitive or inductive charge shuttle to transfer charge between batteries, which is an extremely efficient method. This is because the label should be located at the eye-catching place of the smallest sales package of the product, so that the energy is transferred to the place where it is needed, rather than being released. The cost of doing so is to add more parts and costs

the patented bq78pl114 powerpump battery balancing technology is the latest example of active battery balancing using inductive charge transfer. It uses a pair of MOSFETs (n-channel and p-channel) and a power inductor to establish a charge transfer circuit between two adjacent batteries

the battery pack designer sets the unbalance threshold between series batteries. If the IC measures an imbalance that exceeds this threshold, it will enable powerpump. Figure 8 shows the circuit diagram of various physical and mechanical properties of buck boost wire material using two MOSFETs (Q1 and Q2) and a power inductor. The top battery (V3) needs to transfer energy to the low battery (V2), and the P3s signal (working at about 200kHz and 30% duty cycle) triggers this energy transfer, and then the energy flows to the inductor through Q1. When the P3s signal is reset, Q1 is closed and the inductance energy level is at the highest level. Because the inductive current must flow continuously, the body diode of Q2 is positively biased to complete the charge transfer to the battery at v2. It should be noted that due to its low series resistance, the energy stored in the inductor has only a slight loss

figure 8: battery balance using powerpump technology

assuming that the length and capacity of the series battery are uncertain, there are some restrictions on charge transfer. One consideration is how far we can move energy before we no longer get the optimization of energy supply? In other words, how far can we move the charge before the inefficiency of the converter outweighs the many benefits of a balanced battery? Using an estimated efficiency of 85% in our test, powerpump only transfers energy to less than 6 batteries away. However, it is important to achieve "regional balance" before the whole battery pack can reach complete balance, regardless of efficiency

in addition to these obvious advantages, the advantage of powerpump battery balancing technology is that balancing may ignore the voltage of a single battery. This means that if you decide to transfer charge between two batteries, it can be done during any sequence of battery operating modes (charge, discharge and reset). The transfer can be completed even if the battery voltage providing the charge is lower than the battery voltage receiving the charge (for example, the low voltage caused by the low battery resistance during charging or discharging). Compared with the "resistance leakage" balance, the heat loss of energy is smaller

the following are three optional balancing algorithms:

* terminal voltage (TV) extraction

* open circuit voltage (OCV) extraction

* state of charge (SOC) extraction (pre balancing)

TV extraction is like the voltage passive battery balance introduced above. As shown in Figure 4, TV balance during charging does not always produce a trend

Copyright © 2011 JIN SHI