Spot Welding vs Compression Fittings for Battery Packs

The connection challenge

Building a battery pack from individual cells requires reliable electrical connections between cells. These connections must:

• Carry high current (potentially 100A+ for a home storage system) without overheating
• Have low resistance — every milliohm of connection resistance wastes energy as heat
• Be mechanically robust — vibration, thermal cycling, and handling shouldn't loosen them
• Be safe — a loose or high-resistance connection is a fire risk

The two main approaches are spot welding (for cylindrical cells) and mechanical compression/bolting (for prismatic cells).

Spot welding

How it works

A spot welder delivers a brief, high-current pulse (thousands of amps for a few milliseconds) through two electrodes pressed against a nickel strip sitting on the cell terminal. The current creates enough heat at the contact point to fuse the nickel to the cell surface, forming a solid metallurgical bond.

Equipment

The kWeld is widely regarded as the best DIY spot welder for battery building. It uses a car battery (or similar high-current source) for its energy supply and delivers precise, adjustable pulses.

Nickel strip

You need nickel strip (pure nickel or nickel-plated steel) to connect cells:

• Pure nickel — lower resistance, more expensive (~£15–£30/metre for 0.15mm x 10mm)
• Nickel-plated steel — cheaper, slightly higher resistance, adequate for most applications
• Thickness: 0.15mm is standard for single-layer; 0.2mm for higher current applications
• Width: 8–10mm for standard connections; wider for high-current bus connections

Pros of spot welding

• Very low contact resistance (under 0.1 milliohm per weld)
• Permanent and mechanically strong
• Industry standard for cylindrical cell packs
• Compact — minimal space between cells
• Proven in billions of commercial battery packs (laptops, power tools, EVs)

Cons of spot welding

• Requires equipment investment (£80–£300)
• Learning curve — consistent weld quality takes practice
• Not easily reversible — replacing a cell requires cutting and re-welding
• Risk of cell damage if weld energy is too high (overheating the cell)
• Nickel strip can be fragile if too thin for the current

Compression fittings

How it works

Compression fittings use mechanical pressure to maintain electrical contact between cells and bus bars. No fusion or melting occurs — the contact depends on the pressure being maintained over time.

Methods

Bolted bus bars (for prismatic/blade cells): The most common method for large prismatic cells (the type used in most EV battery modules and home storage). Copper or nickel-plated copper bus bars are bolted to threaded terminals on the cell tops.

• Torque specification matters — too loose creates high resistance; too tight can damage the terminal
• Use calibrated torque wrenches (typically 3–6 Nm for M5/M6 terminals)
• Apply thread-locking compound (Loctite 243) to prevent loosening from thermal cycling
• Cost: £2–£10 per bus bar, plus bolts and washers

Spring contacts (for cylindrical cells): Metal springs or spring-loaded pins press against cell terminals. Used in some commercial battery holders but generally not suitable for high-current applications due to higher contact resistance.

Cell holders with compression plates: Plastic cell holders that position cylindrical cells, with copper plates pressing against terminals from above and below. The plates are held by bolts running through the holder assembly.

Pros of compression fittings

• No special equipment needed (just a torque wrench for bolted connections)
• Easily reversible — replace individual cells without cutting or re-welding
• Widely used for prismatic cells in commercial products
• Lower skill barrier — less practice needed than spot welding

Cons of compression fittings

• Higher contact resistance than welded connections (can be mitigated with proper torque and contact area)
• Risk of loosening over time (thermal cycling, vibration)
• Requires periodic checking of bolt torque
• Takes more space than welded connections for cylindrical cells
• Spring contacts are unsuitable for high-current applications

Which method for which cell type?

For home solar storage specifically

Most DIY home solar storage projects in the UK use one of two approaches:

Approach 1: Large prismatic LFP cells (EVE 280Ah, CALB 100Ah)

• 16 cells in series (16S) for a ~48V pack
• Bolted copper bus bars between cells
• Torque to manufacturer specification
• Simple, reliable, low-skill requirement
• Typical pack: 16S 280Ah = ~14.3kWh

Approach 2: Repurposed EV modules

• Modules are already assembled with internal connections
• Connect modules together with bolted bus bars
• Focus on module-to-module connections rather than cell-level
• BMS connects to module-level voltage taps

In both cases, the actual cell-to-cell connections within modules are pre-made. Your job is connecting modules or cells at the pack level, which is almost always done with bolted bus bars.

Spot welding is mainly relevant if you're building packs from loose cylindrical cells (18650/21700) — less common for home solar storage due to the sheer number of cells needed (a 10kWh pack requires ~900 21700 cells vs 16 prismatic cells).

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