Technical Information｜Three major design challenges that battery energy storage systems need to overcome

Solar and wind energy bring renewable energy to the grid, but the imbalance between supply and demand has become a major limiting factor in the utilization of this energy . Although solar energy is plentiful at noon, electricity demand is not high enough at this time, so consumers' electricity costs remain high.

Grid energy storage, home energy storage, and commercial and industrial energy storage systems (ESS) can collect energy from renewable sources such as solar and wind during the day and release the stored energy during peak demand periods or when grid electricity prices are high. By storing energy for use during peak times, energy storage systems can stabilize the grid and reduce energy costs.

Design challenges associated with battery energy storage systems (BESS for short, which is the more common type of energy storage system) include: 1) safe use; 2) accurate monitoring of battery voltage, temperature, and current; and 3) battery to battery and battery Powerful balancing capabilities between packages. These challenges are detailed below.

The first challenge is maintaining battery safety throughout the entire life cycle of the battery energy storage system , which typically exceeds 10 years. Battery energy storage system applications typically use lithium-ion (Li-ion) batteries, specifically lithium iron phosphate ( LiFePO ₄ ) batteries.

When the voltage, temperature and current exceed the maximum limit, lithium-ion batteries are prone to smoke, fire or explosion, so the monitoring and protection of battery voltage, temperature and current data are crucial . Therefore, the possibility of battery and battery management system failure should be considered and analyzed.

Figure 1 shows the architecture of a battery energy storage system. Texas Instruments Stackable Battery Management Unit for Energy Storage Systems reference design describes a stackable battery management unit (BMU) that monitors system issues by using the BQ79616 integrated redundant battery information detection, while the battery for energy storage systems The control unit reference design demonstrates a battery control unit (BCU) that ensures system safety through a reliable switch driver design.

Accurate battery data ensures safety and improves battery energy utilization. Considering that the charge and discharge curve of lithium iron phosphate ( LiFePO ₄ ) has a wide plateau area, even a small battery voltage measurement error will lead to a huge remaining power error. Therefore, accurate battery voltage and battery pack current measurement are very important for accurately estimating the power. important. Accurate power information is the key to avoiding incorrect battery balancing. Over-balanced charging and over-balanced discharging will destroy the maximum available capacity of the battery.

Another important measurement is temperature. Most battery fires and explosions are caused by battery thermal runaway.

Figure 2 shows Texas Instruments’ stackable battery management unit reference design. The design uses the BQ79616 battery monitor to achieve ±3mV battery voltage error from –20°C to 65°C. For home energy storage systems, the battery monitor BQ76972 can also be used, which can achieve ±5mV battery voltage error in the range of –40°C to 85°C. Multiplexer switches can expand the temperature measurement channels to enable temperature monitoring of each battery and power bus connector. The reference design also reserves additional temperature sampling channels for diagnostic checking of the multiplexer switches.

Figure 2: Stackable battery management unit reference design

Energy storage system power monitoring also requires accurate and reliable current measurement solutions. The BQ79731-Q1 voltage and current sensor integrates a dual-channel 24-bit current sensing analog-to-digital converter with redundant sampling channels to help ensure system safety and current data accuracy.

Battery packs may draw current at different rates due to load inconsistency. These changes can lead to imbalances in remaining power between battery packs and reduce the maximum available power of the entire energy storage system. Inconsistencies in new batteries and different cooling conditions can also cause imbalances between cells, even within the same battery pack. Passive cell balancing consumes battery energy across resistors and is not recommended for pack-level balancing because it consumes too much power and causes the battery pack to heat up.

Battery pack imbalances will gradually worsen over the life of the product, and the life of an energy storage system may exceed 10 years. Over a 10-year cycle, some battery packs may age faster than others, causing users to have to replace aging battery packs earlier. Without a powerful battery pack-level balancing circuit, the new battery pack must be charged or discharged manually so that the energy of the new battery pack is almost equal to the energy of the remaining battery packs in the energy storage system. However, this approach is not only risky, but also difficult, costly and labor-intensive.

Battery imbalance is also affected by battery capacity. In order to optimize the unit energy cost of the entire energy storage system, battery manufacturers are developing larger-capacity batteries, expanding the capacity from 280Ah to 314Ah or even 560Ah. In order to maintain the same energy for all batteries in the battery pack, the larger the battery capacity in the battery pack, the greater the effective balancing current required.

There are several methods of equalizing battery packs. Figure 3 shows one method of charging and discharging a battery pack via a bidirectional isolated DC/DC converter on the high-voltage bus. By controlling charge and discharge current, an isolated DC/DC converter can equalize the remaining capacity or voltage of the battery pack. Since both the normal charging current and the discharging current flow through the bidirectional DC/DC converter, the overall efficiency is low and the bidirectional DC/DC converter is required to have a larger power rating.

Figure 3: Bidirectional isolated DC/DC converter between battery pack and high voltage bus

Figure 4 illustrates another option for balancing energy between different battery packs: using a low-voltage bus instead of a high-voltage bus to relay energy to ensure high system efficiency. The isolated DC/DC converter is located between the battery pack and the low-voltage bus and only operates when the battery pack requires equalization. Since the balanced energy only flows between different battery packs, the rated power of the isolated DC/DC converter is small. In order to keep the low-voltage bus voltage stable, the energy delivered to the low-voltage bus and the energy extracted from the low-voltage bus must be ensured. Maintain dynamic equilibrium.

Figure 4: Bidirectional isolated DC/DC converter between battery pack and low voltage bus

A safe and reliable battery management system can eliminate the safety issues of lithium-ion and lithium iron phosphate ( LiFePO ₄ ) batteries and help extend the life of the energy storage system through carefully designed protection functions, even in the event of a single failure. Precise data detection and powerful balancing capabilities at the pack and cell levels can achieve equal battery capacity during charging and discharging and maximize energy utilization from solar and other renewable energy sources, ultimately enabling end users to achieve safe, Stable and low-cost renewable energy.

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