Paralleling Lithium Batteries in Solar Systems: Principles, Operation, and Selection Guide

Paralleling Lithium Batteries in Solar Systems: Principles, Operation, and Selection Guide

Amid the accelerating global transition to clean energy, solar systems, with their zero-emission and sustainable advantages, have become a core choice for energy storage in homes, businesses, and industrial sectors. Lithium batteries, the "energy storage" of solar systems, directly determine their capacity and stability, directly determining the system's power supply capabilities. When the capacity of a single Battery Pack is insufficient to meet nighttime power supply or peak load demands, paralleling lithium batteries becomes a key solution for increasing storage capacity and ensuring system reliability. This article will thoroughly explain the core knowledge of paralleling lithium batteries in solar systems, from principles and practical applications to misunderstandings and selection, helping you efficiently build a safe and stable energy storage system.

First, why do solar systems require paralleling lithium batteries?

Before delving into the technical details, let's first clarify a core question: Why is paralleling lithium batteries often used to expand the capacity of solar systems? This requires addressing the conflict between the intermittent nature of Solar Power generation and the continuous nature of loads:

Compensating for the Insufficient Capacity of a Single Battery Pack
Solar power generation relies on sunlight, with peak power generation during the day and zero power generation at night. This requires lithium batteries to store sufficient energy to support nighttime loads (such as home lighting and appliances). If a single lithium battery pack can only provide three hours of nighttime power, connecting two similar battery packs in parallel can double the storage capacity, extending the power supply to six hours and adapting to longer periods of off-grid operation.

Improving System Redundancy and Reliability
Commercial and industrial solar systems (such as small factories and agricultural greenhouses) require extremely high power continuity. A single battery failure can cause the entire system to shut down. A parallel architecture allows multiple battery packs to work together. Even if one fails, the remaining batteries can still maintain power, reducing the risk of system downtime. For example, a solar irrigation system in an agricultural greenhouse uses three lithium battery packs in parallel. If one battery pack fails due to low-temperature protection, the remaining two packs can still ensure normal operation of the water pump, preventing crop water loss.

Flexibly Adapting to Load Expansion Needs
As user loads increase (such as adding electric vehicle charging stations in homes or adding new production equipment in commercial and industrial settings), the existing energy storage capacity may not be able to meet demand. Compared to directly replacing a single battery pack with a larger capacity (which is costly and complex to disassemble and assemble), parallel expansion eliminates the need to replace existing batteries; instead, new battery packs of the same specifications are added, significantly reducing expansion costs and construction complexity.

Second, the core principle of paralleling solar lithium batteries: Voltage consistency is key.

The essence of paralleling lithium batteries is "capacity stacking," but it's not simply "positive to positive, negative to negative." The core premise is that the voltage, capacity, and cycle status of the multiple battery packs must be highly consistent. Otherwise, "internal circulation" will occur, leading to battery overheating, lifespan degradation, and even safety risks.

1. Basic Electrical Logic of Parallel Connection
When multiple lithium battery packs are connected in parallel, the system automatically adheres to the "voltage balancing" principle:

If the voltage of battery pack A (3.2V/cell, 12V pack voltage) is higher than that of battery pack B (3.15V/cell, 11.85V pack voltage), a "circulation current" will be generated from battery pack A to battery pack B upon parallel connection, effectively forcing battery pack A to "force charge" battery pack B. Under normal circumstances, when the voltages of two battery groups converge (error ≤ 0.05V), the circulating current gradually disappears. At this point, the total capacity equals the sum of the capacities of each battery group (for example, two 100Ah battery groups connected in parallel have a total capacity of 200Ah), while the total voltage remains the same as a single group (for example, 12V).

This is also the core difference between parallel and series connection of lithium batteries in solar systems: series connection increases voltage (for example, two 12V battery groups in series increase to 24V), while parallel connection increases capacity (for example, two 100Ah battery groups in parallel increase to 200Ah). However, solar inverters and controllers have fixed voltage requirements (commonly 12V/24V/48V), making parallel connection the optimal option for capacity expansion without changing the system voltage.

2. Risks Caused by Inconsistency

Ignoring battery consistency and forcing parallel connection can lead to two core issues:

Local overheating and lifespan reduction: The low-voltage battery acts as a "load" and is continuously charged by the high-voltage battery, causing the battery temperature to rise (possibly exceeding 60°C), accelerating electrolyte decomposition, and causing the cycle life to drop sharply from 1500 cycles to less than 500 cycles.

Safety Hazard: If a battery group has an internal short circuit (such as a damaged separator), the other batteries in parallel will continuously discharge into it. The instantaneous current may exceed 100A, causing battery bulging, leakage, or even fire and explosion.

Third, Practical Steps for Parallel Connection of Solar Lithium Batteries: From Preparation to Testing

After understanding the principles, correct operating procedures are key to ensuring parallel connection safety. The following is a standardized procedure for home and small- to medium-sized commercial and industrial solar systems (using a 48V lithium battery pack as an example):

1. Preliminary Preparation: Select Battery Packs with "Homogeneous Source and Homogeneous State"

Before parallel connection, carefully screen the batteries to ensure they meet the "three similarities" principle:

Same Model and Specifications: Select lithium batteries of the same brand and model (e.g., both 48V 100Ah lithium iron phosphate). Avoid mixing ternary lithium and lithium iron phosphate batteries (the former has a voltage range of 3.6-4.2V, while the latter has a voltage range of 3.2-3.65V, resulting in a significant voltage difference).

Same Cycle Count: Prefer battery packs with a cycle count difference of ≤50 (this can be checked using the battery management system (BMS)). For example, a new battery (0 cycles) should not be connected in parallel with an old battery that has been used 100 times. The capacity degradation of the old battery (approximately 5%-8%) will result in a low voltage and induce circulating current.​
Same voltage condition: Before connecting in parallel, use a multimeter to check the open-circuit voltage (OCV) of each battery group, ensuring the error is ≤0.05V. If the voltage difference is large (e.g., one group is 49.2V, the other is 48.8V), first recharge the lower-voltage battery using a matching charger until it matches the higher-voltage group. Then, let it sit for 2 hours and re-check the voltage to confirm stability before connecting in parallel.

Special tools are also required: an insulating wrench (to prevent short circuits), a torque wrench (to ensure connections are tight; recommended torque is 5-8 N·m), a multimeter (accuracy ≥0.01V), and insulating tape (to cover the terminals).

2. Power-off Operation: Ensure Construction Safety

The solar system contains high-voltage components such as inverters and controllers. Before parallel connection, the power must be completely disconnected:

Turn off the DC switch of the solar photovoltaic array (if any) to prevent the panels from continuously supplying power to the system;

Disconnect the AC output switch of the inverter to prevent backflow of current from the grid;

Disconnect the main switch of the lithium battery pack to ensure there is no voltage output from the battery terminals;

Use a multimeter to check the voltage at the battery terminals. Verify that the display reads 0V before proceeding.

3. Wiring Procedure: Follow the principle of "positive first, negative second, series first, parallel later."

If connecting multiple battery packs in parallel (e.g., three 48V 100Ah packs), first complete the series connection within each pack (this step is not required for modular batteries), then proceed with parallel connection between packs:

Securing the battery packs: Secure each pack to the battery rack, maintaining a spacing of 10-15cm to allow for heat dissipation (lithium batteries operate optimally at 15-35°C; close spacing can lead to localized overheating).

Connecting the positive terminal: Use a dedicated copper busbar (cross-sectional area ≥ 50mm², selected based on maximum current, e.g., 70mm² for a 200Ah battery). Connect the positive terminals of each battery pack with a copper busbar. Apply conductive paste to both ends of the busbar (to reduce contact resistance and prevent overheating) and tighten the screws with a torque wrench.

Connect the negative terminals: Repeat the steps for connecting the positive terminals to the negative terminals of each battery pack. (Note: The positive and negative busbars must not cross to prevent short circuits.)

Insulation: Cover all exposed terminals with insulating tape to prevent dust and moisture from causing leakage.

4. Power-On Test: Verify Parallel Connection

After wiring is complete, test the system status in three steps:

No-load Test: Close the main battery switch and use a multimeter to measure the total voltage after parallel connection (it should be consistent with the voltage of a single battery group, e.g., 49.2V). Observe the BMS display (if available) to confirm that the current of each battery group is 0A (no circulating current).

Light-load Test: Turn on a low-power load (e.g., a 100W light) and run it for 30 minutes. Measure the total current (it should be ≤ the sum of the rated discharge currents of each battery group; e.g., for two 100Ah battery groups, the total current should be ≤20A). Use an infrared thermometer to measure the terminal temperature (it should be ≤40°C).

Full-load Test: Turn on the system's full load (e.g., all household appliances or commercial equipment) and run it for 1 hour. Observe the BMS data: the voltage difference between each battery group should be ≤0.1V, the temperature should be ≤50°C, and there should be no alarms (e.g., overcurrent or overtemperature).

Fourth, Common Parallel Connection Mistakes: Avoid These "Invisible Killers"

Even if you master the operating procedures, overlooking details can still lead to system failure. The following are five common mistakes in paralleling solar lithium batteries and how to avoid them:

1. Mistake: Mixing Batteries of Different Brands/Types

Case: A user connected Brand A's 48V/100Ah ternary lithium battery and Brand B's 48V/100Ah lithium iron phosphate battery in parallel. Three days later, the battery pack temperature rose to 75°C, triggering the BMS overtemperature protection. Disassembly revealed lithium deposition in the positive electrode of the ternary lithium battery, resulting in a 30% capacity degradation.

Cause: The charge/discharge curves and voltage plateaus of ternary lithium and lithium iron phosphate differ significantly (the full charge voltage of ternary lithium is 4.2V/cell, while that of lithium iron phosphate is 3.65V/cell). Parallel connection generates continuous high circulating current.

Avoidance: Use batteries of the same brand and chemistry (e.g., both lithium iron phosphate), with identical models and capacities.

2. Mistake: Connecting in parallel without checking voltage

Risk: If the voltage difference between two battery groups exceeds 0.1V, the instantaneous circulating current during parallel connection may reach hundreds of amperes, burning the terminal blocks or the BMS motherboard.

Workaround: Before connecting in parallel, be sure to check the open-circuit voltage with a high-precision multimeter. If the error exceeds 0.05V, recharge the batteries until the voltages are consistent. Let them sit for 2 hours, then check again.

3. Mistake: Neglecting the tightening and insulation of terminal blocks

Problem: Loose terminal screws increase contact resistance. High current discharge can cause the terminals to heat up (over 100°C), melting the insulation and causing a short circuit. If exposed terminals come into contact with metal parts, they can directly short-circuit the batteries.

Workaround: Tighten the screws to the specified torque with a torque wrench. After connecting, wrap the terminals with insulating tape or heat shrink tubing. Check the tightness of the screws regularly (every three months).

4. Misconception: Not Configuring Protective Devices After Parallel Connection

Hazard: If a battery group shorts internally without protective devices, the other batteries will continue to discharge into the faulty battery, potentially causing a fire.

Workaround: Connect a DC circuit breaker (rated for the battery's 1C discharge current, e.g., a 100A breaker for a 100Ah battery) in series with the positive terminal of each battery group. If the current in a battery group is abnormal, the circuit breaker will automatically trip, isolating the faulty battery.

5. Misconception: Neglecting Post-Maintenance and Balancing

Consequence: After long-term use, the capacity of each battery group may decay at different rates (e.g., one battery group decays faster due to poor heat dissipation), resulting in voltage inconsistency and circulating current.

Workaround: Monitor the voltage, capacity, and temperature of each battery group using the BMS every six months. If the voltage difference exceeds 0.1V, use a dedicated equalizer for equalization charging (transferring charge from the higher-voltage battery to the lower-voltage battery) to maintain battery consistency.

Fifth, Lithium Battery Selection for Solar Systems: Choosing the Right Battery for Easier Parallel Connections

The safety and stability of parallel connection fundamentally depends on the quality of the batteries themselves. When selecting lithium batteries suitable for parallel connection in a solar system, focus on the following four indicators:

1. Consistency: Prioritize "factory-matched" battery packs.

High-quality lithium battery manufacturers perform "capacity sorting" on battery cells before shipment, ensuring that the capacity variation within a batch is ≤2% and the voltage variation is ≤0.02V. They also provide "parallel-specific packages" (such as 2/4 pre-matched battery packs) to avoid errors caused by user selection. For example, a certain brand of 48V lithium iron phosphate battery packs undergo three charge-discharge cycle tests before shipment. Only batteries that meet the consistency standards are selected and assembled into parallel sets. Users can connect and use them directly without further testing. 2. BMS Functionality: Must Support "Parallel Balancing"

A battery management system (BMS) is the "guardian" of parallel system safety. A high-quality BMS must have the following functions:

Inter-group balancing: Real-time monitoring of the voltage of each battery group, using active balancing techniques (such as capacitor and inductor charge transfer) to reduce voltage differences and prevent circulating currents;

Overcurrent/overtemperature protection: Immediately disconnects the circuit when the current exceeds the rated value or the temperature is too high;

Communication: Supports linkage with solar controllers and inverters, uploading data from each battery group for remote monitoring.

3. Cycle Life: Choose long-life batteries to reduce replacement costs

The design life of a solar system is typically 10-20 years, so the cycle life of lithium batteries must match system requirements. Lithium iron phosphate batteries (with a cycle life of 1500-3000 cycles) are superior to ternary lithium batteries (800-1500 cycles) and are more suitable for long-term parallel operation. Based on 1500 cycles and one charge and discharge per day, a lithium iron phosphate battery can last for over four years. Even after degradation, the battery capacity can be expanded by connecting new batteries in parallel, extending the overall system lifespan.

4. Environmental Adaptability: Coping with Complex Outdoor Conditions
Lithium batteries in solar systems are often installed outdoors (such as on rooftops or in equipment rooms) and must withstand high and low temperatures, as well as fluctuating humidity.

Temperature Range: Select batteries with an operating temperature range of -20°C to 60°C (ordinary lithium batteries experience significant capacity degradation at low temperatures, with capacity potentially dropping below 70% at -10°C, while low-temperature-optimized batteries can maintain over 85% capacity at temperatures above -10°C).

Protection Rating: The battery casing must meet IP65 protection (dust and water jet resistance) to prevent rain and dust from entering and causing malfunctions.

VI. Conclusion: Parallel connection is the optimal solution for expanding solar energy storage capacity, but safety always comes first.

For solar systems requiring increased energy storage capacity, paralleling lithium batteries offers a cost-effective, efficient, and flexible solution. With a sound understanding of the principles, standardized operating procedures, and rigorous selection criteria, you can double system capacity and improve reliability. However, it's important to remember that the core of parallel connection is consistency, and safety lies in details. From battery selection to ongoing maintenance, strict adherence to standards is crucial for a solar system to consistently and stably provide you with clean electricity.