Introduction

The connection of battery cells to one another makes up the reliability of the pack as a whole.

From an EV to an ESS and from residential inverters through to large-scale lithium batteries, current must be passed thoroughly from cell to cell. It is possible for you to use the best battery cells, but if the interconnection has been poorly designed, excessive heat, increased resistance, a voltage imbalance, and possibly even a premature failure of the battery will occur.

Busbars have a great role to play here.

A correctly specified busbar will do more than simply connect the battery cells together, it will assist in the current flow through the battery system, provide thermal management, assist in providing mechanical support, and contribute towards the overall safety of the battery pack. As the requirement for increased power and extended life continues to grow for battery systems, the selection of the most suitable cell-to-cell interconnection solution has become as critical as the selection of the actual cell itself.

What Is Cell-to-Cell Interconnection?

Cell-to-cell interconnection is the method used to electrically interconnect the individual battery cells located inside a battery module or battery pack.

In the case of a prismatic lithium battery, the electrical interconnection between cells uses customized aluminum busbars or copper busbars, which rely on the electrical and mechanical requirements of the application.

The objective is clear:

• Transfer current between cells with minimum electrical resistance

• Provide constant voltage across the battery pack

• Handle continuous charging and discharging cycles

• Support safe operation under vibration and temperature changes

• Provide long-term reliability over the life of the battery

Although this sounds relatively simple, designing these connections takes careful engineering. A slight variation in the design of a busbar can directly impact the performance of a complete battery pack.

Why Cell Interconnection Matters More Than Many People Realize

Cell interconnection plays an important part in determining the efficiency of the battery over time but is typically ignored during the early design stage of battery pack assembly. Well-designed busbars (structural support for cells) help to

• Minimize electrical losses

• Reduce heat produced during operation

• Improve cell-to-cell current distribution

• Improve overall battery efficiency

• Enable higher charge/discharge rates

• Lengthen the service life of the overall battery by reducing stress on individual cells

Increased cell capacity continues to grow and therefore increases contact resistance, which causes increased energy loss from the battery and greater heat generation.

These small design considerations can have a very significant effect on product quality and long-term reliability for manufacturers of EV batteries, solar battery storage systems, telecom backup batteries, and industrial lithium-ion batteries.

Investigating the Common Design Obstacles of Prismatic Cell Interconnection

Building a reliable prismatic cell copper busbar is not simply about connecting the terminals together. Engineers need to be concerned with several important considerations: electrical performance, mechanical performance, manufacturability, and cost.

1. Amperage Rating

The first thing to consider is the ability to carry the proper load and not overheat when carrying the load.

When a busbar is too small, causing the electrical resistance to be high, the following detrimental effects will occur:

• Overheating

• Voltage drops

• Decreased efficiency

• Decreased performance of the prismatic cell(s)

Selecting the proper material, thickness, and cross-sectional area to provide a reliable busbar is very critical for the operation of the cells. 

2. Contact Resistance

Every contact interface has some amount of electrical resistance at that interface.

Improper surface finish, the absence of equal pressure, improper fastening methods, and inadequate quality control significantly increase the contact resistance.

The higher the contact resistance, the more likely the following conditions will occur:

• Localized heating

• Loss of charging efficiency

• Loss of power

• More rapid aging or degradation of the components within the battery

This is why high-precision manufacturing processes are essential, as well as proper surface finish.

3. Thermal Expansion

Whenever a battery pack is charged or discharged or the ambient temperature changes, the battery cells will naturally expand and contract.

If the design of the interconnection does not account for this movement, the bus bar will experience the following:

• Mechanical stress

• Material fatigue

• Cracking damage over time

• Loose electrical connections

Most modernly fabricated aluminum busbars are designed with an appropriate level of flexibility to account for these thermal cycles and not damage the bus bar or any interconnects.

4. Limited Space Within Battery Packs

With the ongoing efforts by battery manufacturers to increase energy density, it is crucial that every millimeter inside a battery module be considered in regard to the busbars used in the battery pack.

When creating a compact battery assembly, busbars must meet all of the following requirements:

• Safe electrical clearances

• Adequate electrical insulation

• Reliable current carrying capability

• Ease of assembly during the production process

To meet these requirements, busbars will typically require a custom design; therefore, they will no longer be standard flat connectors.

Why Aluminum Busbars Are a Preferred Choice

For the majority of new lithium-based battery applications, the use of aluminum busbars has become the #1 choice due to the performance and cost advantages that they offer. Some of the major benefits of aluminum busbars are the following:

• The weight of aluminum busbars is significantly less than copper.

• Aluminum has a high level of conductivity for most battery applications.

• Much more cost-effective when producing large quantities.

• More efficient for handling in high-quantity manufacturing.

• The ideal choice of busbar for EV battery packs and energy storage systems where weight reduction is a critical factor.

If properly designed, laser-welding type aluminum busbars will yield high-quality electrical connections with excellent mechanical integrity for the life of the battery pack.

Copper vs. Aluminum Busbars for Prismatic Batteries.

One common question asked by battery manufacturers is, should we use copper busbars or aluminum busbars?

Your choice of busbars really depends on how you intend to use them.

Feature	Copper Busbar	Aluminum Busbar
Conductivity	Higher	Slightly lower
Weight	Heavier	Much lighter
Cost	Higher	Lower
Corrosion Resistance	Good	Good with proper treatment
EV Applications	Common	Increasingly popular
Large ESS Systems	Used	Widely used

Many contemporary battery packs, particularly with regard to electric vehicles and energy storage systems, derive great value from the use of custom-engineered aluminum busbars due to their superior conductivity, low weight, and reasonable price point.

Laser-Welded vs Bolted Interconnections

Laser-welded and bolted interconnections between batteries have differing associated advantages:

Laser-welded busbars:

• Lower contact resistance

• More compact

• Greatly resistant to vibration

• Can be manufactured automatically

• Consistent connection quality

Best suited for:

• Electric vehicles

• High-volume battery production

• Premium battery packs

• Space-constrained designs

 Bolted busbars:

• More easily maintainable

• More easily replaceable

• Lower cost to tool

• Good for prototype builds

 Best suited for:

• Industrial battery systems

• Low-volume production

• Serviceable battery packs

• R&D projects

 Many manufacturers begin with bolted connections while developing their product and then switch to using laser-welded aluminum busbars for production runs.

Useful Busbar Design Suggestions

In the course of working on numerous projects with battery packs, there are three or four common-sense lessons that become evident.

Keep Current Paths as Short as Possible

As current paths have a longer length, they exhibit an increase in electrical resistance; thus, having short current paths lowers the amount of resistance, creating greater efficiency.

Avoid Sharp Corners.

Sharp corners will create greater concentrations of stress, whereas rounded edges create a more even distribution of current and, therefore, reduce stress concentrations.

Consider Heat from the Start

Thermal performance should be an integral factor to the overall design process and not simply an afterthought.

Use Consistent Fastening Torque

Connections should not experience inconsistent contact resistance due to uneven torque in assembly.

Plan for Manufacturing Tolerances

A design that works perfectly on paper may pose difficulties in assembly.

Typical Errors in Designing Battery Interconnections

• Selecting busbars that are not sized correctly

• Not accounting for thermal expansion of the busbar materials

• Using incompatible materials that have not been plated properly

• Poor welds between the busbars

• Using insufficient insulation clearance

• Overlooking vibration requirements for the interconnections

• Using the lowest-cost supplier rather than the most capable supplier/manufacturer

These issues can be found months after deployment and can be costly to resolve.

Selecting the Right Busbar Manufacturer

If you are sourcing busbars for a battery project, ask these questions before you place your order:

• Can they manufacture custom busbars?

• Are they experienced with prismatic cell battery packs?

• Will they be able to support laser welding applications?

• What materials and plating options do they offer?

• Do they provide prototype quantities prior to volume production?

• Can they consistently hold dimensions throughout large production runs?

• Do they have an understanding of battery-specific safety requirements?

An experienced supplier in this area can often identify design problems before production begins.

What Is Cell-to-Cell Interconnection in a Prismatic Battery?

A prismatic battery cell interconnects to other cells in the same prismatic cell type using busbars or other types of connectors, providing power to the loads from all the cells. Current can flow through the entire battery pack with little electrical resistance, therefore producing a very efficient use of power through long-term reliability.

Benefits of Using Custom Busbars for Prismatic Batteries

• Lower electrical resistance

• Increased thermal performance

• Improved current distribution

• More compactly designed battery packs

• Added reliability in vibration

• Reduced production time for assembly

• Extended battery service life.

Frequently Asked Questions

1. What is the function of busbars on prismatic battery packs?

Busbars connect the separate battery cells together and allow for easier current flow, therefore increasing overall efficiency and minimizing heating on the busbars.

2. Which material should I use for my battery busbars (copper or aluminum)?

A copper busbar will provide the highest conductivity however, a significant amount of weight and cost can be saved by using an aluminum busbar. The best option will depend on the power requirements of the application and whether weight and budget are factors.

3. What causes heat in battery busbars?

High current draw, the insufficient cross-section of busbars, lack of contact pressure, oxidization, and inadequate connections can all lead to heat generation in battery busbars.

4. Is a laser-welded busbar going to be stronger than a bolted busbar?

Generally, busbars that are laser welded will experience less resistance and fatigue during vibration than bolted connections. On the other hand, bolted connections are more easily maintained and replaced.

5. What is the recommended thickness for busbars?

When determining the required thickness of the busbar for the application, the current carrying capacity, allowable temperature rise, conductivity of the busbar material, and available installation space are the primary factors.

6. Are aluminum busbars suitable for use in battery packs for EVs?

Yes, aluminum busbars are widely used in today’s EV battery packs since they provide lightweight designs while offering high electrical performance.

Conclusion

Interconnecting cells through interconnection is one of the most important components of a prismatic-type battery pack. Even if the battery has high-quality cells, state-of-the-art electronics for the BMS, and advanced cooling systems, the entire battery system will ultimately rely on how well the electrical connections between the cells were made.

Properly designed custom aluminum busbars and copper busbars reduce electrical resistance between the cells, help manage thermal energy, improve the reliability of the battery, and help the battery to operate at peak performance for an extended period of time. As battery systems become smaller and more powerful, busbar design will continue to be an important factor in providing competitive advantages to manufacturers.

Do You Need Custom Busbar Solutions?

If you are in the process of designing a lithium battery pack, energy storage system (ESS), electric vehicle (EV) battery module, or inverter battery assembly, working with a manufacturer of custom aluminum busbars will save you significant time on prototype production.

Custom-designed busbars that are designed based on your current requirements, cell layout, and final assembly processes will typically have superior performance characteristics when compared to off-the-shelf alternatives.