Main challenges with residential energy storage system

The architecture of household energy storage system is complex, involving batteries, inverters and other equipment. At present, products in the industry are independent of each other, which is easy to cause a variety of problems in actual use, mainly including: complex system installation, operation and maintenance difficulties, energy storage batteries can not be efficiently used and low battery protection levels.

A. System integration: complex installation

Household energy storage is a complex system that combines a variety of energy sources and faces ordinary households. Most users hope to use it as "household appliances", which puts forward higher requirements for system installation.

At present, the complex and time-consuming on-site installation of residential energy storage has become the biggest problem for some users. At present, there are two kinds of energy storage system for household use in the market: low voltage energy storage and high voltage energy storage.

Low-voltage household energy storage system (inverter and battery dispersion).The low-voltage energy storage system for household use refers to the energy storage system with the battery voltage range of 40~60V, which is connected to the inverter after several batteries are connected in parallel. Through the isolation DC-DC inside the inverter, it is coupled with the direct current output of photovoltaic MPPT at the bus bar, and finally converted into alternating

current through the inverter output and connected to the grid. Some inverters have backup output function.

Main problems of household low-voltage energy storage system:

1. The inverter and battery are independently dispersed, the equipment is heavy and difficult to install;

2. Inverter and battery connection line can not be standardized, the need for on-site processing;

As a result, the installation of the whole system takes a long time and increases the cost.

 B. Household high voltage energy storage system:

The battery cluster of household high-voltage energy storage system adopts two-stage architecture, which is output by several battery modules in series through the high-voltage control box. The voltage range is generally 85~600V. The output of the battery cluster is connected to the inverter, and through the DC-DC unit of the inverter, the battery cluster output is coupled with the direct current output of photovoltaic MPPT at the bus bar. Some inverters have backup output function.

Main problems of household high voltage energy storage system:

1. In order to avoid the direct series use of different batches of battery modules, it is necessary to do a strict batch management in the production, shipment, warehouse, installation links, which requires a lot of manpower and material resources, and the process will be very complicated, but also to the customer stock troubles;

In addition, the power consumption and capacity attenuation of the battery will cause the difference between modules to be enlarged. The general system needs to be checked before installation. If the difference between modules is large, it also requires manual recharge, which consumes time and power.

battery capacity mismatch: module difference caused capacity loss

Generally, the voltage range of the traditional household low-voltage energy storage system is 40 to 60V, and the capacity is expanded by connecting multiple battery strings in parallel. Due to the differences in cell, module and wiring harness, the charging/discharging current of batteries with high internal resistance is low, while the charging/discharging current of batteries with low internal resistance is high. Some batteries cannot be fully charged/discharged for a long time, thus causing partial capacity loss of battery system.

2. Series mismatch of household high-voltage energy storage system

Generally, the voltage range of the household high-voltage energy storage system is 85 to 600V. Multiple battery modules are connected in series to achieve capacity expansion. According to the series circuit characteristics, the charging and discharging current of each module is the same. However, due to the difference in module capacity, the battery with small capacity is charged/emptied first, resulting in the long-term failure of some battery modules to be filled/emptied, and the capacity loss of part of the battery cluster.

C. product operation and maintenance cost threshold is too high

In order to ensure the reliable and safe operation of household energy storage system, good operation and maintenance is one of the effective measures.

However, due to the relatively complex architecture of household high-voltage energy storage system, which requires high professional level of operation and maintenance personnel, maintenance difficulties and time-consuming problems often occur in the actual use. The main reasons are as follows:

1. Regular maintenance, the battery pack needs to carry out SOC calibration, capacity calibration or main loop inspection.

2. When the battery module is abnormal, the conventional lithium battery does not have the automatic balancing function, and the maintenance personnel need to manually recharge the battery on the site, which cannot quickly respond to customer needs.

D. old and new battery mix using: accelerate the aging of the new battery and capacity mismatch

For household low-voltage energy storage system lithium battery mix old and new, the battery internal resistance difference is large, easy to cause circulation, battery temperature rise, will accelerate the aging of the new battery;

In the household high-voltage energy storage system, the old and new battery modules are mixed in series. Due to the barrel effect, the new battery modules can only be used with the capacity of the old battery modules, resulting in serious capacity mismatch of the battery cluster.

For example, the available capacity of the new module is 100Ah, and the available capacity of the old module is 90Ah. If they are used together, the battery cluster can only use 90Ah. To sum up, it is generally not recommended to use new and old lithium batteries directly in series or parallel.