Three design challenges for battery energy storage systems to overcome

 Solar and wind power bring renewable energy to the grid, but the imbalance between supply and demand is a major limiting factor affecting the utilization of such energy sources. Although solar power is abundant in the middle of the day, demand is not high enough at this time, so the cost of electricity for consumers remains high.

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

Design challenges associated with battery energy storage systems (BESS, 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. Strong balancing ability between batteries and between battery packs. These challenges are detailed below.

Challenge 1: Security

The first challenge is to keep batteries safe for the entire life cycle of a battery storage system, which typically exceeds 10 years. Lithium-ion (Li-ion) batteries are commonly used in battery energy storage system applications, especially lithium iron phosphate (LiFePO4) batteries.

When the voltage, temperature, and current exceed the maximum limit, lithium-ion batteries are easy to smoke, fire, or explode, so the battery voltage, temperature, and current data monitoring and protection 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 The Reference design describes a stackable battery management unit (BMU) that monitors system issues by using the BQ79616’s integrated redundant battery information detection. The Battery Control Unit Reference Design for energy storage systems demonstrates a battery control unit (BCU) that ensures system safety through a reliable switch drive design.

Challenge 2: Accurate battery monitoring

Accurate battery data ensures safety and improves battery energy utilization. Considering that the lithium iron phosphate (LiFePO4) charge and discharge curve has a wide platform area, even a small battery voltage measurement error can lead to a large residual charge error, so accurate battery voltage and battery pack current measurement is very important to accurately estimate the charge. Power information is the key to avoiding the wrong balance of the battery, over-balanced charging and over-balanced discharge will destroy the maximum available capacity of the battery.

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

Figure 2 shows Texas Instruments’ stackable battery management unit reference design. The design uses the BQ79616 battery monitor, which can achieve a ±3mV battery voltage error in the range of -20 °C to 65°C. For home energy storage systems, you can also choose the battery monitor BQ76972, which can achieve a battery voltage error of ±5mV in the range of -40 °C to 85°C. The multiplexer switch can extend the temperature measurement channel to enable temperature monitoring for each battery and power bus connector. The reference design also reserves additional temperature sampling channels for diagnostic checks of the multiplexer switch.

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 detection analog-to-digital converter with redundant sampling channels to help ensure system safety and current data accuracy.

Challenge 3: Balance between battery and battery pack

Due to inconsistent load, the battery pack may consume current at different rates. These changes can lead to an imbalance of remaining power between battery packs and reduce the maximum available power for the entire energy storage system. The inconsistency of the new battery and the different heat dissipation conditions can also lead to an imbalance between different batteries, even within the same battery pack. Passive battery equalization consumes battery energy on the resistance and is not recommended for battery pack level equalization because it consumes too much power and can cause the battery pack to heat up.

The imbalance of the battery pack will gradually worsen over the service life of the product, and the service life of the 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 the aging battery pack earlier. If there is no powerful battery pack level equalization 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 rest of the battery pack in the energy storage system. However, this practice is not only risky but also difficult, costly, and labor-intensive.

Battery imbalance is also affected by battery capacity. 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, and even to 560Ah. For all batteries in the battery pack to maintain the same energy, the larger the battery capacity in the battery pack, the more effective equalizing current is required.

Battery packs are balanced in a variety of ways. Figure 3 shows a method of charging and discharging a battery pack on a high-voltage bus via a bidirectional isolated DC/DC converter. By controlling charge and discharge currents, isolated DC/DC converters can equalize the remaining battery pack capacity or voltage. Since the normal charging current and discharge current will flow through the two-way DC/DC converter, the overall efficiency is low, and the rated power of the two-way DC/DC converter is required to be larger.

Figure 4 illustrates another option for equalizing 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 works when the battery pack needs to be equalized. Because the balanced energy only flows between different battery packs, the isolated DC/DC converter has a small power rating. To keep the voltage of the low-voltage bus stable, it is necessary to ensure that the energy delivered to the low-voltage bus and the energy extracted from the low-voltage bus are dynamically balanced.

At Last:

A safe and reliable battery management system eliminates the safety concerns of lithium-ion and lithium iron phosphate (LiFePO4) batteries and helps extend the life of the energy storage system with well-designed protection features, even in the event of a single failure. Accurate data detection and powerful battery pack and battery-level equalization can equalize battery capacity when charging and discharging, and maximize energy utilization from solar and other renewable energy sources, ultimately giving end users access to safe, stable, and low-cost renewable energy.