Batteries in Series va Parallel - Detailed Comparison

In the world of solar power systems, the connection of batteries is a critical factor influencing overall performance. The decision to wire batteries in series or parallel, or a combination of both, significantly impacts the efficiency and longevity of the system. This comprehensive guide explores the intricacies of these options.

Batteries in series vs. parallel - What's the difference?

Choosing the right battery configuration depends on your device's requirements and your energy needs. Here's a quick breakdown of how each connection type works:

• Series Connection = Voltage Adds Up, Capacity Stays the Same.
• Parallel Connection = Capacity Adds Up, Voltage Stays the Same.

Feature	Series Connection	Parallel Connection
Voltage (V)	Sum of all battery voltages	Stays the same as a single battery
Capacity (Ah)	Stays the same as a single battery	Sum of all battery capacities
Max Current (A)	Stays the Same (Limited by the weakest battery)	Adds Up (Sum of all batteries' max currents)
Load Distribution	Full System Current on Each Battery	System Current is Divided Among Batteries

Wiring battery in series

Connect the negative terminal of the first battery to the positive terminal of the second battery. Continue this "daisy chain" pattern for all batteries. The final output of your bank will be the positive terminal of the very first battery and the negative terminal of the very last battery.

Number of batteries in series

Connecting batteries in series increases the system voltage, so the number of series connections depends on your system's required voltage, such as 12V, 24V, or 48V.

• A 12V system is the most basic solar configuration, requiring only a single 12V battery. It suits motorhomes, boats, camping gear, and low-power lighting, delivering modest output for mobile use.
• A 24V system requires connecting two 12V batteries in series to achieve a total voltage of 24V. Common in medium-sized motorhomes, small off-grid homes, and telecom stations, it halves current compared to 12V, reducing losses and improving efficiency.
• 48V system is the most common configuration for residential solar energy storage, requiring four 12V batteries in series. It is most widely used in residential storage and larger installations.

Furthermore, safety certifications and insurance requirements may impose restrictions on series connections. For instance, the PowMr 12V 100Ah battery allows series connections of up to 4 units.

Always adhere to the manufacturer's specified maximum values while taking into account battery management system limitations, safety standards, certification requirements, and warranty obligations.

Advantages of wiring batteries in series

Increased Voltage: By connecting four 12V 100Ah LiFePO4 batteries in series, a robust 48V system is achieved without sacrificing overall capacity. This connection maintains a 100Ah capacity, showcasing the versatility of series wiring.

Lower Current & Thinner Cables: For the same amount of power (Power = Voltage × Current), a higher voltage results in a lower current. This allows you to use smaller, less expensive wiring and reduces energy loss over long cable runs.

Enhanced Efficiency in Power Transmission: The boost in system voltage resulting from series wiring translates to a reduction in system current. This, in turn, enables the utilization of thinner wiring, leading to improved efficiency in power transmission and minimizing voltage drop within the system.

Disadvantages of wiring batteries in series

Limited by the Weakest Battery: In a series connection, overall performance is constrained by the weakest battery. Differences in State of Charge among batteries can compromise the entire bank. Therefore, effective balancing with a high-quality BMS is crucial to protect the pack and ensure long-term reliability.

Reduced Redundancy: If one battery in a series configuration fails or drops in capacity significantly, it can impact the entire battery bank, affecting the overall performance of the connected devices or the entire system.

Elevated Voltage Application Limitations: Performing this action requires all your applications to operate at an elevated voltage. For instance, when you link two 12V batteries in series, the resulting voltage is 24V. Without employing a converter, you won't be able to supply power to any 12V appliances in this setup.

Wiring batteries in parallel

When connecting batteries in parallel, link the positive terminals together and connect the negative terminals together. While wiring is relatively straightforward for small numbers of batteries, large-scale parallel connections require sound circuit design to ensure balanced current distribution.

It is advisable to employ diagonal wiring or battery busbars to prevent overheating or reduced lifespan caused by uneven current distribution. For more detailed wiring schemes and best practices, please refer to our professional guide.

Number of batteries in parallel

The number of batteries that can be connected in parallel depends on several factors, including the capacity of the Battery Management System (BMS), wiring specifications, and the manufacturer's specific recommendations.

Within safe limits, the number of batteries can be determined based on the duration of power supply required by the system. Most battery systems support 2-4 batteries connected in parallel for residential applications.

Consider an off-grid motorhome with a daily consumption of 10kWh:

• A 48V, 100Ah (5kWh) battery requires two parallel banks for two days' backup.
• Four parallel banks are needed for four days' backup.

The scalability of parallel battery connections simplifies capacity planning, proving particularly advantageous for applications requiring extended autonomous operation. Examples include off-grid systems, backup power during prolonged outages, or seasonal energy storage.

Advantages of wiring batteries in parallel

Increased Capacity & Runtime: Connecting batteries in parallel increases the total capacity while keeping the voltage of the battery system constant, extending the system's power supply duration.

For example, if you have two 24V 100Ah batteries connected in parallel, the total capacity becomes 200Ah. The voltage remains the same, but the working duration of the batteries is doubled.

System Redundancy: If one battery in a parallel configuration experiences degraded performance or fails, the remaining batteries can continue to supply power to the system. This provides better fault tolerance compared to series configurations, though overall capacity will be reduced.

Voltage Compatibility: Parallel wiring maintains the same voltage as individual batteries, eliminating the need for voltage converters. A 12V parallel bank can directly power all 12V appliances and devices without additional conversion equipment.

Disadvantages of wiring batteries in parallel

Higher Current Requirements: Parallel connections increase the total current for the same power output because the system voltage is lower than in series arrangements. This requires thicker cables to safely handle the higher amperage, adding complexity to installation.

Increased Cable Costs and Losses: Using larger-diameter cables reduces voltage drop and internal resistance but increases material costs. Overall, parallel setups are generally more expensive and labor-intensive than equivalent series systems.

Imbalanced Load Distribution: Batteries in parallel may discharge unevenly due to differences in internal resistance or state of health. Without proper balancing, some batteries work harder than others, leading to uneven wear and potentially shortening the lifespan of the entire battery bank.

Batteries in series vs parallel: Which is right for you?

The decision between series and parallel wiring is not about which is "better," but which is the right tool for your specific job. Let this simple guide make the choice clear:

Choose a SERIES Connection if:

• You need higher voltage. Your primary goal is to run a 24V, 48V, or higher-voltage inverter or appliance.
• Efficiency over long distances is key. You are building a larger stationary system (like an off-grid cabin) where minimizing wire thickness and power loss is important.
• You have identical, brand-new batteries and a high-quality BMS to manage the critical task of cell balancing.

Choose a PARALLEL Connection if:

• You need longer runtime. Your main objective is to maximize the amp-hour capacity to keep your existing 12V (or 24V) system running for as long as possible.
• You need to power high-draw devices. You need to combine the maximum discharge current of multiple batteries to start a powerful inverter, motor, or other heavy load.
• System redundancy is a priority. You want your system to remain operational even if one battery fails.

Can you wire batteries in series and parallel?

You can connect batteries in series and parallel, simultaneously achieve higher voltages and greater capacity, whilst distributing current across multiple paths to reduce the load on individual cells.

Here is an example of building a 24V system using 12V 100Ah batteries, which allow maximum of four in series and four in parallel. The optimal solution is to connect two battery groups in series to achieve a 24V 100Ah specification. By creating four of these series pairs and then connecting the pairs in parallel, the system voltage remains 24V while the capacity increases to 400Ah.

This approach proves particularly practical when existing battery specifications cannot fully meet voltage and capacity requirements, enabling flexible customisation solutions using standard batteries.

Tips of solar batteries connection

Connecting solar batteries involves several considerations to ensure a safe and efficient setup. Here are some tips for connecting solar batteries:

Battery selection

It is important to ensure that all batteries used in a series/parallel connection are of the same type and the same age, have the same capacity and voltage, and are properly balanced to prevent overcharging or discharging of individual batteries. Lead-acid and lithium-ion batteries are common types of solar batteries for inverter. Consider factors like capacity, voltage, and cycle life.

Charge controller and inverter compatibility

Adjust charge controller settings based on battery manufacturer recommendations, covering absorption voltage, float voltage, and temperature compensation. Ensure inverter compatibility with battery bank voltage and type, noting specific connection requirements.

Cable sizing

Use appropriately sized cables to connect batteries. Undersized cables can lead to voltage drops and reduced system efficiency. Refer to a battery cable sizing chart of the product manual based on the current and distance.

Temperature and ventilation

Install batteries in a well-ventilated area, and consider venting systems if necessary. Additionally, batteries perform optimally within a specific temperature range. Avoid exposing them to extreme temperatures, and consider temperature-compensated charging if your charge controller supports it.

Fusing and circuit protection

Install appropriate battery fuses and circuit breakers to protect the battery bank from overcurrent and short circuits. Place fuses on both the positive and negative sides of the battery bank.

Always refer to the manufacturer's guidelines and specifications for the specific batteries and components you are using in your solar power system. Additionally, local electrical codes and regulations should be followed during installation. If you are unsure or uncomfortable with the installation process, consider hiring a professional to install and connect the solar batteries. This can help avoid mistakes and ensure a safe and efficient system.

Frequently asked questions (FAQ)

Can I mix old and new batteries in my bank?

Absolutely not. An older battery will cripple the performance and lifespan of the entire bank, regardless of the connection type.

Which connection puts more stress on individual batteries?

It depends on the type of stress.

• For current stress, Series is higher. In a series circuit, every battery must handle 100% of the system's current. In a parallel circuit, the current is divided.
• For voltage balancing stress, Series is more complex. The BMS must actively manage the voltage of each battery individually. A parallel setup is largely self-balancing from a voltage perspective.

My inverter needs 150A, but my batteries are only rated for 100A max discharge. What should I do?

You must use a parallel connection. If you connect two 100A-max batteries in parallel, your new bank can deliver a combined 200A, safely meeting your inverter's needs. A series connection would not increase the m ax current.

Do my cables need to be the same length?

For parallel connections, yes, it is critical. Using cables of identical length and gauge ensures that current is distributed evenly. Uneven cable lengths will cause uneven load sharing.