How to Choose the Right Lithium Battery for a Solar System？

How to Choose the Right Lithium Battery for a Solar System？

   Amid the global energy transition and sustainable development, lithium batteries are a key energy storage solution and a core component of solar power systems, serving as a crucial carrier for achieving solar-to-electricity conversion.

As a new and developing product in the battery industry, how should we choose the right lithium battery when installing solar system?

Six Key Points for Lithium Battery Selection

1. Determine Power Requirements
2. Determine Battery Voltage
3. Calculate Battery Capacity (Ah)
4. Key Influencing Factors
5. Introduction to Lithium Battery BMS and Current Selection
6. Lithium Battery Communication Protocol Selection

1. Determine Power Requirements

List Load Devices: Record the power (W) and operating hours (hours) of all appliances requiring power, and calculate the total daily power consumption (Wh). 
Formula: Daily power consumption = Device power × Usage time × Number of units
*For example: A 100W lamp is used for 5 hours per day → 100W × 5 hours = 500Wh*

Considering consecutive cloudy days (self-sufficient days): Depending on the local climate (such as the rainy season or days with low sunlight in winter), it is common to reserve 2-3 days of backup power.

Formula: Total power demand = Daily power consumption × Self-sufficient days
*For example: 500Wh/day × 3 days = 1500Wh*

2. Determine the battery voltage

Common voltages for solar systems are 12V, 24V, or 48V, and high voltage, such as 96V, 192V, 240V, and 380V, etc. 
The lithium battery voltage needs to match the system's DC voltage. Lithium battery voltages available on the market include 48V/51.2V and 24V/25.6V, etc. 
 48V/51.2V corresponds to a system DC voltage of 48V. This can usually be determined by the DC voltage of the inverter in the system. For example, a 5kW/48V inverter typically uses a 51.2V lithium battery; a 3kW/24V inverter typically uses a 25.6V lithium battery.

*Small systems (<1-3kWh) can choose 12V or 24V; medium systems (>5-10kWh) can choose 48V to reduce current loss. *Large systems (>12kWh) can consider higher voltages such as 96V, 192V, 240V, 380V, etc. Choosing the appropriate battery voltage depends on the specific configuration of the solar system.

3.Calculate Battery Capacity (Ah)

Battery capacity should be selected based on power consumption and the voltage of the solar system.

Formula: Battery Capacity (Ah) = Total Power Required (Wh) ÷ Battery Voltage (V)
*For example: 1500Wh ÷ 12V = 125Ah*

Consider Depth of Discharge (DoD): For lead-acid batteries, the recommended DoD is ≤50%, while for lithium batteries, it can reach 80-90%.

Also, consider the solar panel configuration. If you only want solar panels to charge the battery, consider the estimated daily charge capacity of the system's solar panels and choose the appropriate battery. If the battery capacity is too large, insufficient solar panels will not fully charge the battery. If the battery capacity is too small, using too many solar panels will result in a rapid charge and a shorter power supply time. Therefore, the battery selection should be based on power consumption, system configuration, and other factors.

4. Key Influencing Factors

Temperature Compensation: Low temperatures reduce battery capacity, while high temperatures can damage the battery. Choose a model suitable for the environment or install temperature control. 

System efficiency: The inverter and line losses are about 10-20%, which can be compensated by increasing the capacity appropriately. 

Future expansion: Reserve 10-20% margin for new loads. 

5. Introduction to lithium battery BMS and current selection

BMS (Battery Management System) is the core control unit of the lithium battery pack, responsible for monitoring, protecting and managing the battery's charge and discharge process to ensure the battery's safe, efficient and long life operation. 

Core functions of BMS

(1)Battery protection (core responsibility)

 Overcharge protection: Prevents the voltage of a single cell from exceeding the upper limit (such as lithium batteries are usually 4.2V/cell) to avoid fire or explosion. 
Over-discharge protection: Prevents the voltage from being too low (such as 2.5V/cell) to avoid permanent damage to the battery. 
Overcurrent protection: Limits the charge and discharge current to prevent short circuit or overload damage to the battery. Temperature protection: Monitors the battery temperature and stops working at high temperature (>60℃) or low temperature (<0℃). 
Balance protection: Eliminate voltage differences between cells through passive balancing (resistance discharge) or active balancing (energy transfer) to extend overall life.

(2) State monitoring and calculation

SOC (State of Charge): Real-time estimation of remaining power (such as displaying the percentage of power).
SOH (State of Health): Assess battery health and predict remaining life.
SOP (State of Power): Calculate the maximum power that can be output at present.

(3) Communication and data recording

Communicate with external devices (such as inverters, solar controllers) through interfaces such as CAN bus, RS485, and Bluetooth.
Record historical data (such as cycle counts, fault logs) for easy maintenance and diagnosis.

(4) BMS battery protection board current selection

The lithium battery protection board needs to be selected according to the parameters of the solar system, especially the parameters of the inverter. The protection board has different currents, such as 100A, 150A, 200A, etc.

So how should I choose? See the example below:

For a lithium battery protection board used with a 5kW/48V inverter, the current must be calculated based on the inverter's power and voltage, taking into account a safety margin. The following calculation steps and recommendations are provided:

a. Calculate the continuous discharge current
Inverter nominal power: 5kW (5000W)
System voltage: 48V
Theoretical current = power / voltage = 5000W / 48V ≈ 104A

b. Consider inverter efficiency and peak power
Efficiency: Assuming an inverter efficiency of 90%, the actual power required from the battery is higher:
5000W / 0.9 ≈ 5555W
5000W / 0.9 ≈ 5555W
5555W / 48V ≈ 116A

Peak power: The inverter may be temporarily overloaded (e.g., when starting a motor), so allowance must be made for peak power. Typically calculated as 1.5 times:
116A times 1.5 ≈ 174A

c. Protection Board Selection Recommendations
Continuous Current: Select a protection board with a continuous discharge rating of at least 120A-150A to meet efficient operation requirements.

Peak Current: The protection board's peak discharge capacity must be ≥ 200A to handle transient loads.

d. Other Factors
Cell Type: For high-rate power cells (such as LiFePO₄), the discharge capacity must match the cell's capacity.

Heat Dissipation Design: High-current protection boards require a heat sink or active cooling.

Brand Reliability: Prefer reputable brands (such as Daly and JK BMS) to ensure sensitive overcurrent and overtemperature protection.

Final Recommendation:
Minimum Configuration: 150A continuous discharge, 200A peak.
Ideal Configuration: 200A continuous discharge (with room for upgrades).

6. Lithium Battery Communication Protocol Selection

Currently, lithium batteries on the market come from different manufacturers and use different communication protocols. For example, brands commonly used in photovoltaic batteries include Growatt, Pylontech, Huawei, BYD, CATL, Sungrow, and GoodWe.

When selecting a battery, it's crucial that the communication protocols used in the inverter and lithium battery match. For example, if the inverter uses the Pylontech protocol, the lithium battery should also use the Pylontech protocol; otherwise, communication will not be possible.

Therefore, when purchasing, users should inquire with the manufacturer about matching the communication protocol to their system.

Selecting lithium batteries and pairing them with solar systems requires professional expertise. If you encounter questions about purchasing a solar system or lithium batteries, please contact Vevsun. We offer professional system solutions, providing the most suitable products and high-quality lithium batteries. We also provide complete system packages and shipping services. If you need a solar system for home or a solar system project, please feel free to contact us!

Vevsun service 

1.Provide technical support: Help Customer matched different solar system, help calculate request system request solar panel quantities, inverter power, lithium battery voltage & capacity, etc. 
2.Provide complete solar system products, inlcude solar panel, controller, inverter, battery, mounting bracker, cables, connectors, etc. 
3.Offer solar system installation guide 
4.Offer products OEM&ODM service, solar panels, inverter, battery, etc. can do customer LOGO or case design, etc.