Why our connection method Matters for Pack Reliability

A battery pack is only as reliable as its weakest connection. Because every cell in the pack is connected in series or parallel through nickel tabs, a single failed connection can disable the entire pack — or worse, create a hot spot that leads to thermal runaway under high-current conditions. Our engineering team has investigated enough field failures to know that connection quality is the number one predictor of pack reliability in the field.

The choice between spot welding and solder tabs is not just a manufacturing preference — it is a design decision that affects the pack’s electrical resistance, mechanical strength, thermal management, and production cost. Because Ni-MH and Ni-CD cells have different thermal sensitivities and seal designs than lithium-ion cells, our connection method must be chosen with these chemistries’ specific characteristics in mind.

Spot Welding: Process, Parameters, and Quality

Resistance spot welding uses a brief, high-current pulse to fuse a nickel tab to our cell terminal. The weld energy is concentrated at the tab-terminal interface — because our weld duration is 1-10 milliseconds, the heat does not propagate into our cell body. This is the critical advantage of spot welding: it creates a strong metallurgical bond without thermally damaging the cell’s internal components.

How Spot Welding Works

our spot welder applies two electrodes to the nickel tab, one on each side of the intended weld point. A controlled current pulse (typically 1,000-5,000 amps for 1-10 milliseconds) flows through the electrodes, heating the tab-terminal interface to the melting point of nickel (1,455°C). The molten metal fuses the tab to the terminal, and the electrode pressure forges the joint as it cools. The entire weld cycle takes less than 100 milliseconds.

Our spot welding equipment uses pulse-shaping technology that controls the current waveform — ramp-up, hold, and ramp-down — to minimize weld spatter and ensure consistent fusion depth. Because our weld parameters (current, time, pressure) are programmable, we can optimize the settings for each cell size and tab thickness combination.

Weld Quality Indicators

• Weld nugget diameter: 2-3mm per weld, visible as a circular indentation on the tab surface
• Peel strength: Tab must withstand 5N peel force without separating from the terminal
• Electrical resistance: Weld joint resistance must be less than 1 milliohm to minimize power loss
• Visual inspection: No spatter, no burn-through, no discoloration on our cell body

Spot Welding for Ni-MH and Ni-CD Cells

Ni-MH and Ni-CD cells are well-suited for spot welding because their positive terminal (the button top) is a flat nickel-plated steel surface that welds readily to nickel tabs. The negative terminal (the can bottom) is also nickel-plated steel. Because both terminals accept spot welds directly, there is no need for pre-applied solder or special surface preparation.

Our production line spot-welds Ni-MH and Ni-CD packs at a rate of 200-500 welds per hour per welding station, depending on our pack configuration. A typical 4-cell AA Ni-MH pack requires 6-8 inter-cell welds plus 2 lead wire welds — total production time per pack is approximately 30-60 seconds.

Solder Tabs: Process, Advantages, and Limitations

Solder tab attachment uses a soldering iron to melt pre-tinned solder onto nickel tabs that are pre-attached to our cell terminals during cell manufacturing. Because the tabs are already welded to the terminal at our cell factory, our pack assembly operator only needs to solder our inter-cell connections and lead wires.

How Solder Tabs Work

During cell manufacturing, a nickel tab is spot-welded to the cell terminal and pre-tinned with solder. our pack assembly operator bends the tabs to connect adjacent cells, applies flux, and uses a soldering iron (typically at 300-350°C) to melt the solder and create the inter-cell connection. The solder solidifies in 2-3 seconds, forming a mechanical and electrical bond.

Advantages of Solder Tabs

• Lower tooling cost: A soldering iron costs $20-$50, while a spot welder costs $2,000-$20,000
• No special equipment: Solder tab assembly can be done with basic hand tools in any workshop
• Flexible configuration: Tab bending allows complex 3D pack geometries that are difficult with rigid spot-welded connections
• Prototype-friendly: Ideal for 1-50 unit prototype runs where spot welder setup time is not justified

Limitations of Solder Tabs

• Thermal damage risk: our soldering iron tip operates at 300-350°C, which can heat the cell terminal and potentially damage the seal or vent mechanism if applied for too long
• Weaker mechanical bond: Solder joints are more brittle than spot welds and can crack under vibration or impact
• Slower production: Each solder joint takes 3-5 seconds, versus 100 milliseconds for a spot weld
• Operator skill dependent: Solder quality varies with operator technique — over-heating, cold joints, and solder bridges are common defects

Our spot welding equipment uses programmable pulse-shaping technology that our maintenance team calibrates weekly. Because we control our weld parameters — current, time, pressure — through our equipment rather than relying on operator skill, our weld quality is consistent across shifts and across different operators. This machine-controlled consistency is what allows us to guarantee the same joint quality on the first pack and the thousandth pack.

Our Ni-MH rechargeable battery pack assembly service includes design-for-manufacturability review at no extra charge. Before we start production, our engineering team reviews the buyer’s pack design and recommends optimizations for weldability, tab routing, and insulation placement. This upfront review prevents production issues that would otherwise require costly mid-production design changes.

Side-by-Side Comparison

Our production data from Ni-MH pack assembly shows that spot welding produces a first-pass yield of 99.2% — meaning only 0.8% of welds require rework. Solder tab assembly, by contrast, typically shows a first-pass yield of 94-96% depending on operator skill level. This yield difference directly translates to lower production cost and fewer quality escapes in the finished packs.

Tab Materials and Specifications

Our Ni-MH and Ni-CD battery pack assembly service covers the complete workflow from cell sorting through final testing. We sort cells by voltage and internal resistance before assembly to ensure balanced packs, and our spot welding parameters are optimized for each cell size.

Our engineering team specifies our standard tab material for spot-welded Ni-MH and Ni-CD packs is pure nickel (Ni 200/201) at 0.1-0.2mm thickness. Pure nickel provides excellent corrosion resistance, consistent weld quality, and good electrical conductivity. For cost-sensitive applications, we also offer nickel-plated steel tabs, which reduce material cost by 30-40% while maintaining acceptable weld quality.

Tab Thickness Selection

• 0.1mm: Standard for AA and AAA cell packs — sufficient for currents up to 2-3A
• 0.15mm: Standard for C and D cell packs — handles currents up to 5-8A
• 0.2mm: For high-current applications (power tools, medical devices) — handles 10A+ continuous

Tab Width and Configuration

Tab width is determined by the cell spacing and the current requirement. Standard widths are 5mm, 8mm, and 10mm. For parallel cell connections, wider tabs distribute current more evenly and reduce the voltage drop across the connection. Our engineering team calculates the optimal tab width based on the pack’s maximum continuous current and the acceptable voltage drop (typically less than 10mV per connection).

Quality Verification for Assembled Packs

Because our connection method directly affects pack reliability, our quality verification protocol tests every assembled pack — not just a sample from the batch.

Electrical Tests

• Pack voltage: Measured open-circuit after 1-hour rest — must be within ±2% of nominal
• Pack capacity: Discharge at 0.2C to cutoff — must meet rated capacity within ±5%
• Internal resistance: Measured at 1kHz — must be within spec sheet range, no outliers
• Connection resistance: Each tab-to-terminal joint measured individually — must be less than 1 milliohm

Mechanical Tests

• Peel test: 5N force applied to each tab — no separation allowed
• Vibration test: Pack subjected to 10-55Hz sweep for 2 hours — no connection failures
• Drop test: Pack dropped from 1 meter onto concrete — no electrical discontinuity

Visual Inspection

• Weld quality: No spatter, no burn-through, no discoloration on cell body
• Tab alignment: Tabs centered on terminals, no short circuits between adjacent cells
• Insulation: All exposed metal covered with fish paper or insulation sleeve where required

Choosing between spot welding and solder tabs for Ni-MH and Ni-CD battery packs depends on your production volume, quality requirements, and pack configuration. For production volumes above 100 units, spot welding delivers superior joint quality, faster production, and consistent results. For prototype runs below 50 units, solder tabs offer a low-cost alternative with acceptable quality. Our engineering team at Johnson Eletek can help you choose the right method and optimize your pack design for manufacturability.