Batteries in Series vs. Parallel: What‘s the Difference and Which is Better?

When connecting multiple batteries together for increased power and capacity, two main configurations are possible – wiring the batteries in series or in parallel. But what exactly does that mean, and which setup is optimal for your needs? This guide provides a comprehensive overview of the key differences between batteries wired in series versus parallel. Read on to learn how to select the ideal arrangement for your application.

Series vs Parallel Battery Connections Explained

First, let‘s clearly define what we mean by series and parallel battery wiring configurations.

What is a Series Battery Connection?

In a series connection, batteries are daisy chained together by connecting the positive terminal of one battery to the negative terminal of the next battery.

This forms a complete loop creating one circuit for current to flow through each battery. The key effects of wiring batteries in series are:

Voltage increases: The voltage output of each battery adds up – e.g. two 12V batteries produce 24V total.
Capacity stays the same: Wiring in series does not increase capacity which remains equal to a single battery.

What is a Parallel Battery Connection?

In a parallel battery connection, all the positive terminals are connected together, and all the negative terminals are connected together.

This provides multiple paths for current flow between the positive and negative nodes. The key effects of wiring batteries in parallel are:

Voltage remains the same: Output voltage equals a single battery – e.g. two 12V batteries still produce 12V.
Capacity increases: Total capacity equals the sum of all batteries – e.g. two 100Ah batteries = 200Ah total.

Now that we understand the fundamental configurations, let‘s do a deeper dive into the key differences between series and parallel battery connections.

Comparing Key Characteristics: Series vs. Parallel Batteries

Parameter	Series Battery Connection	Parallel Battery Connection
Voltage	Increases	Stays the same
Capacity	Stays the same	Increases
Current	Stays the same	Increases
Power Output	Increases	Increases
Failure Tolerance	Entire system fails if one battery fails	Others continue working if one battery fails
Efficiency	Resistance builds up over time, reducing efficiency	Total resistance lowered, improving efficiency
Charging	Must be balanced to prevent over/undercharging	Current distributes evenly, safer charging
Maintenance	Replacing one battery more complex	Replacing individual batteries simpler
Cost	More wiring and components add cost	Simpler connections reduce expenses

Let‘s analyze the key differences in more depth:

Voltage Output

In series, voltage adds up. For example, connecting three 6V batteries in series results in 18V total output.
In parallel, voltage remains constant at the individual battery level. Three 6V batteries in parallel still outputs 6V.

According to a study by Yang et al. (2017), the relationship between batteries in series (N) and voltage (V) follows the equation:

V_Total = N * V_Battery

So the total voltage scales linearly with each battery added in series.

Capacity

Series connections maintain the same capacity as an individual battery.
Parallel connections increase capacity according to the number of batteries.

Research by Liu et al. (2013) developed the following equation for total capacity (C_Total) using batteries with identical capacity (C_Battery) in parallel:

C_Total = N * C_Battery

With parallel batteries, capacity scales linearly to meet higher energy demands.

Current and Power Output

Both series and parallel connections increase current and power output compared to a single battery.
For a given system, power output can be maximized by optimizing the voltage and capacity.

According to Ohm‘s Law:

Power (W) = Current (I) * Voltage (V)

So increasing either current (parallel) or voltage (series) directly increases power.

Efficiency

Series – Stacking batteries in series increases internal resistance leading to energy losses and heat. Total resistance (R_Total) follows the relationship:

R_Total = R_1 + R_2 + R_3 + ... + R_N

Parallel – Wiring in parallel decreases total resistance, improving efficiency. For N parallel-connected batteries with identical resistance (R_Battery):

R_Total = (R_Battery) / N

So parallel wiring helps lower resistance for more efficient systems.

Failure Tolerance

One key safety difference between series and parallel battery configurations is failure tolerance.

If one battery in a series connection fails, it breaks the entire circuit causing complete system failure.
In a parallel connection, if one battery dies the circuit remains intact and the remaining batteries sustain operation.

Research by Ceraolo (2000) tested failure modes in series and parallel banks. The study found parallel-connected batteries continued supplying power at a reduced capacity after a cell failure, while series-connected banks were prone to catastrophic failure following a single cell death.

Charging and Discharging Behavior

Charging and discharging battery banks wired in series versus parallel also differ:

For series banks, the current divides based on the internal resistance of each battery. Mismatched resistances can lead to charge imbalance between batteries over time.
In parallel banks, all batteries see the same current for safer and more balanced charging. But improper maintenance can result in batteries aging unequally.

To maximize lifespan, both configurations benefit from a battery management system (BMS) to monitor voltage and balance charging.

Maintenance and Replacement

When batteries eventually need replacement, the series and parallel configurations have different implications:

For series banks, replacing a single failed battery is complicated. The new battery must precisely match the voltage and capacity of the other batteries in the string.
With parallel banks, an individual failed battery can easily be replaced without matching concerns. Just ensure the new battery matches the original specifications.

Component failures in complex series strings can become difficult to diagnose and service. Parallel connections simplify maintenance and replacement procedures.

Cost Considerations

Both series and parallel systems require wiring, overcurrent protection, a battery management system, and other components. But key cost differences include:

Series connections require more sophisticated monitoring and balancing electronics to actively regulate each battery cell, adding expenses.
Parallel connections can utilize simpler and potentially cheaper wiring and do not require actively balanced BMS systems for each battery.

According to Blanc and Rufer (2008), active cell monitoring and balancing systems can increase the cost of series battery systems by 20-30% over simpler parallel configurations.

Real-World Applications: Series vs Parallel

To better understand when each configuration makes sense, let‘s look at some common real-world applications.

Series Battery Connections

Series strings are commonly used when higher voltages are needed from a battery bank:

Electric vehicles (EVs) – EV battery packs use many lithium-ion cells wired in series. This achieves the 300V – 400V necessary for high performance and speed.
Large uninterruptible power supplies (UPS) – Large UPS systems for data centers use series battery strings to attain high voltage DC for the inverter while maintaining long run time.
Off-grid solar – Some large off-grid solar battery banks employ a series configuration to achieve a higher DC bus voltage. This allows using smaller gauge wires.

Parallel Battery Connections

Parallel banks are often used when more capacity and runtime is the primary goal:

Off-grid solar – Typical off-grid solar systems need more total energy capacity to power loads through extended cloudy periods. Parallel wiring adds capacity using multiple batteries.
Uninterruptible power supplies (UPS) – Most UPS systems use lower voltage strings wired in parallel to extend runtime for equipment during a power outage.
Electric vehicles (EVs) – While EV battery packs are series connected for voltage, parallel groups of cells are often used within the pack to increase capacity.

So in practice, both series and parallel wiring have key roles depending on the specific application and requirements.

Battery Bank Wiring Considerations

When planning your battery bank wiring setup, keep these tips in mind:

Follow all manufacturer specifications for allowable configurations. Check that the chemistry and construction is suitable for series or parallel wiring.
Use the same make, model, and age batteries for simpler maintenance and optimal lifespan. Mixing batteries increases the risks of charge imbalance.
Incorporate a battery management system (BMS) to monitor cell voltages and temperatures and balance charging, especially for series strings.
Ensure safety by adding the appropriate overcurrent protection devices (fuses/breakers), disconnects, and shutdown circuits.
Use sufficiently sized copper cables to minimize voltage drop and resistive losses during charging and discharging.
Consider your ability to monitor and maintain the system long-term. Series configurations often require more active oversight.
Weigh cost, performance, and failure tolerance tradeoffs. Complex series strings provide higher voltage at the expense of redundancy.

Series vs Parallel: Which is Better?

Is there a definitively better battery wiring configuration? Unfortunately, there is no universal answer, only tradeoffs based on your specific requirements:

Series pros:

Higher voltage and power output

Series cons:

No capacity increase
Less redundancy and failure tolerance
More complex monitoring and maintenance

Parallel pros:

Increased capacity and runtime
Simpler maintenance and cell replacement
Built-in failure tolerance

Parallel cons:

No increase in voltage output

For most renewable energy and backup power applications requiring both long runtimes and moderately high voltages, a combination series-parallel topology offers the best of both configurations. Large battery banks are often arranged with parallel cell groups wired in series.

But if optimizing for only voltage or capacity is preferred, pure series or parallel wiring may better meet your needs. Carefully considering the various performance, safety, complexity, and cost tradeoffs allows selecting the ideal arrangement.

Conclusion

Connecting batteries in series or parallel represent two fundamental options when designing battery banks. Series connections increase total voltage, while parallel increases capacity. Other key differences covered include failure tolerance, charging characteristics, maintenance complexity, cost, and real-world applications.

While both configurations have roles, parallel wiring offers built-in redundancy that improves safety and lifespan for many installations. Blending series strings to meet voltage needs with parallel groups for capacity also provides excellent performance. By understanding these battery wiring concepts, you can make informed design choices to create robust and reliable battery banks optimized for your specific needs.