How to Build a Battery Pack: The Complete Spot Welding Guide for Beginners

What You Will Need Before You Start

Getting your materials right before starting prevents the most common beginner problems — poor welds from wrong strip gauge, imbalanced packs from unmatched cells, and damaged packs from using the wrong BMS.

Cells

18650 lithium-ion cells are the standard for DIY pack building. Quality matters enormously — buy from reputable distributors who source directly from manufacturers (Samsung, Sony/Murata, Panasonic/Sanyo, LG Chem, Molicel). Avoid unbranded or rewrapped cells.

Key cell specs you need to know before building:

• Nominal voltage: 3.6V or 3.7V per cell
• Capacity: Typically 2,000–5,000mAh per cell for quality 18650s
• Continuous discharge current: The maximum current the cell can sustain (typically 10–30A depending on chemistry and model)
• Internal resistance: Lower is better; well-matched cells within a parallel group should be within 5–10mΩ of each other

Match your cells. All cells in the same parallel group should be from the same batch, same model, and within 0.05V of each other before assembly.

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Nickel Strip

Pure nickel strip (not nickel-plated steel) in an appropriate gauge for your current requirements. Common options:

• 0.1mm × 8mm: For low-drain packs (up to ~5–8A continuous per strip)
• 0.15mm × 8mm: Standard for most 18650 e-bike packs (up to ~10–15A per strip)
• 0.2mm × 8mm: For higher-drain applications or double-layer connections

Most builds use either 0.15mm or 0.2mm pure nickel. For the detailed comparison of nickel vs copper strip and when to use each, see our nickel strip vs copper strip guide.

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Spot Welder

A battery spot welder capable of handling your chosen nickel strip gauge reliably. Minimum for a serious pack: a mid-range bench welder (Sunkko 737G+ range, $250–$350). For professional-quality results, a capacitive discharge system. The technology behind CD welding is covered in our what is a CD spot welder guide, and the full welder comparison is in our best battery spot welders guide.

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BMS

A Battery Management System (BMS) protects your pack from overcharge, over-discharge, over-current, and short circuit. Match your BMS to:

• Cell count (string): A 10S BMS for a 10-cell series pack, 13S for 13-cell series, etc.
• Continuous current rating: Should match or exceed your expected discharge current
• Balance function: Most BMS units include passive balancing; active balancing is available at higher cost

Common BMS form factors: flat strip for inline packs, rectangular for box configurations. Many e-bike builds use a separate charge/discharge port BMS for better protection.

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Tools and Safety Equipment

• Digital multimeter: Voltage checking is mandatory at multiple steps
• Kapton tape (or fishpaper): Insulating exposed terminals during build
• Cell holders/spacers: Plastic spacers that keep cells aligned (18650 spacers are widely available)
• Heat shrink tubing: Final pack insulation; sized for your pack dimensions
• Non-conductive tools: Plastic or ceramic tweezers for positioning during welding
• Safety glasses: Required during welding
• Fire-resistant work surface: Li-ion cells can vent if short-circuited; work on a concrete or metal bench, not on a wood surface

Watch this complete DIY battery pack build with spot welding:

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Understanding Battery Pack Configurations

Series vs Parallel: What the Numbers Mean

Series (S): Connecting cells positive-to-negative increases voltage. Each cell in series adds its nominal voltage to the pack total. Two 3.7V cells in series = 7.4V pack.

Parallel (P): Connecting cells positive-to-positive and negative-to-negative increases capacity (amp-hours) and current capability. Two 3,000mAh cells in parallel = one 6,000mAh group, still at 3.7V.

Combined (e.g., 10S4P): A 10S4P pack has 10 groups in series, each group containing 4 cells in parallel. Total voltage = 10 × 3.7V = 37V nominal. Total capacity = 4 × cell capacity.

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Common Configurations: 3S, 4S, 10S, 13S

Voltage limits:

• Fully charged (max): 4.2V per cell × number of series groups
• Fully discharged (min): 3.0V per cell × number of series groups (your BMS should cut off at 3.0V)
• Nominal: 3.6–3.7V per cell × number of series groups

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Calculating Your Pack Voltage and Capacity

Voltage: Number of series groups × nominal cell voltage

Capacity (Ah): Number of parallel cells per group × individual cell capacity in Ah

Energy (Wh): Voltage × Capacity in Ah

Example — 10S4P with 3,000mAh cells:

• Voltage: 10 × 3.7V = 37V nominal
• Capacity: 4 × 3.0Ah = 12Ah
• Energy: 37V × 12Ah = 444Wh

This is a typical e-bike pack that would deliver approximately 20–30 miles of range depending on the bike and riding conditions.

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Step 1: Plan Your Pack

Choose Your Cell Configuration

Start with your application requirements:

• What voltage does your motor controller/device require?
• What range or runtime do you need?
• What continuous current will you draw?

Match the configuration to these needs using the voltage table above. For e-bike builds, 10S (37V) is the most common for 36V motors; 13S (48V) for 48V motors. Verify your motor controller's accepted voltage range before building.

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Calculate How Much Nickel Strip You Need

For each parallel group, you need strip for both the positive and negative ends. Approximate strip length per connection: width of your cell group + 20mm overlap on each side.

Example calculation — 10S4P pack (4 cells wide):

• 4 × 18650 cell diameter (18mm each) = 72mm wide group
• Strip length per end: 72mm + 20mm each side = ~110mm per strip
• Two strips per group end (some builders use double-layer for higher current): 20 strip pieces total for 10S
• Plus inter-series connection strips: 9 series connections × strip length
• Buy 30–40% more strip than your calculation suggests — you will make mistakes and need retests

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Step 2: Prepare Your Cells

Testing Cells Before You Build

Every cell gets measured before going into the pack. You need:

1. Resting voltage: Should be within 0.05V of all cells in the same parallel group. Measure with your multimeter after at least 30 minutes at rest (not immediately after charging).
2. Internal resistance (IR): Use a dedicated cell internal resistance meter if available. Well-matched cells within a parallel group should be within 5–10mΩ. Cells with significantly higher IR than their group average will underperform under load.
3. Physical inspection: Check the wrapper for tears (a torn wrapper exposing the cell can cause shorts), dents in the can, or damaged positive cap. Discard any physically compromised cells.

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Sorting and Grouping by Voltage

After measuring, sort your cells into groups with matched voltages. All cells going into the same parallel group should be within 0.05V of each other. If you can't get within 0.05V, charge or discharge cells to bring them closer before building.

Never put cells with significantly different states of charge into the same parallel group — the higher-voltage cell will immediately discharge into the lower-voltage cell at whatever current the nickel strip allows. For a 0.2V difference in two parallel cells connected through 0.15mm nickel, this can be hundreds of amps briefly — dangerous.

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Arranging Cells in Your Holder

Place cells into your cell holders/spacers. Most 18650 holders are designed for specific configurations (2×5 for 10S1P, 4×5 for 10S4P, etc.). Check which end faces which direction for your intended strip routing — a consistent orientation (all cells positive-up, or alternating for compact series connections) is important before you start welding anything.

Apply Kapton tape or fish paper to any exposed areas where accidental short circuits could occur during the build process — particularly over the positive terminal tops of cells adjacent to where you'll be working. This is critical safety practice.

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Step 3: Set Up Your Spot Welder

Choosing Your Power Setting

Before touching your real cells, calibrate your welder on scrap material (old discarded cells and scrap nickel strip of the same gauge you'll use in your pack).

Start at a lower power setting and work up until your test welds:

• Create visible, circular weld spots on the nickel where the electrodes contacted
• Don't burn through the nickel strip
• Hold firmly on a pull test (see below)

Typical starting settings for a Sunkko 737G+ on 0.15mm pure nickel strip: Start at approximately 50–60% power and test. Adjust up if the weld peels off in the pull test; adjust down if the strip burns through.

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Electrode Placement and Pressure

For the standard two-probe handpiece: electrodes should be placed 2–5mm apart on the nickel strip directly over the cell terminal. The current flows through the strip between the two electrodes, then down through the nickel-to-terminal interface.

Apply firm, consistent downward pressure before triggering. Inconsistent pressure is the most common cause of uneven welds. On a bench welder with a fixed head, the spring or pneumatic actuation handles this; on a handheld stylus, you must develop consistent hand pressure.

Don't fire the welder while lifting, pivoting, or repositioning. Position fully, apply pressure, then trigger.

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Test Welds Before You Start

On your scrap material: weld a piece of strip to an old cell. Then perform a pull test — grip the strip with pliers and pull perpendicular to the cell terminal. A good weld: the strip tears (metal failure) before the weld spots separate (weld failure). A bad weld: the strip peels off the terminal cleanly, leaving the terminal unscratched.

Do 10–15 test welds, varying pressure and power, until you can consistently produce welds that tear the strip rather than peel off. Document your working power setting. Only then start your actual pack.

For detailed technique including electrode conditioning, dual pulse settings, and troubleshooting, our how to spot weld battery tabs guide covers the welding process in depth.

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Step 4: Weld the Parallel Groups

Welding the Negative End

The negative end (the flat end on standard 18650 cells) is welded first. It's typically easier to start here because the flat surface provides better electrode contact.

Cut your strip to the correct length for your parallel group width. Position the strip flat across all negative terminals in one parallel group. Make sure it doesn't overhang onto adjacent cells' positive terminals — this is a short circuit risk.

Apply your two weld spots per terminal, positioning your electrodes on the strip directly over each cell. Move methodically: weld all cells in the group at the negative end before moving to the positive.

For a 4P group: 4 cells × 2 weld spots = 8 trigger presses per strip piece, per end.

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Welding the Positive End

The positive end has the raised button terminal — a smaller target than the flat negative. The technique is the same but electrode placement requires more care to stay on the button.

Critical: Before welding the positive end, verify your strip is clear of the negative ends of any adjacent cells. In a typical 18650 holder, the cells alternate direction — positive of one group faces negative of the next. A strip that overlaps onto the negative of the next group would short the series connection.

Apply your weld spots to the positive terminal strip in the same sequence as the negative.

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Testing Each Weld as You Go

After welding each parallel group, do a quick check: grip each strip end and apply light sideways pressure with your finger. Any strip that moves has a failed weld underneath it. Find and re-weld any suspect spots immediately, before moving on.

Also check with your multimeter: the voltage across each parallel group should match your individual cell voltage (within a few millivolts). If a group reads zero or anomalously low, investigate before continuing.

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Step 5: Make the Series Connections

Once all parallel groups are welded internally (positive and negative ends), you connect the groups in series. This is the step where the high-voltage pack assembly begins — be careful.

The series connection strip runs from the positive terminal of one parallel group to the negative terminal of the next group. In a side-by-side cell layout where cells alternate direction, this strip bridges across the gap between groups — connecting the positive caps of Group 1 to the negative flat ends of Group 2.

Before starting series connections: Insulate any exposed nickel that won't be part of the series connection path with Kapton tape. As series groups get connected, you'll have increasing pack voltage present on the strips — accidental shorts at this stage can discharge significant energy.

Work one series connection at a time. Connect Group 1 positive to Group 2 negative. Measure the voltage across Groups 1 and 2 in series — should be approximately 2× cell voltage. Continue through the pack.

After all series connections: measure the full pack voltage from negative terminal of Group 1 to positive terminal of Group 10 (for a 10S pack). Should be approximately 37V (for cells at 3.7V nominal).

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Step 6: Connect the BMS

The BMS connects to:

1. Main power leads: The pack's full voltage runs through the BMS for protection. Connect the BMS B+ to the pack's positive and B- to the pack's negative.
2. Balance leads: A wire from each series group connection point goes to the BMS balance connector. This allows the BMS to monitor each group's voltage individually for balancing.
3. Output leads: The BMS C- (charge output) and P- (discharge output) connect to your output connector (or a single output for common port BMS).

Follow your specific BMS wiring diagram exactly — these vary by manufacturer and model. Before connecting, verify your balance lead voltages with a multimeter: each successive lead should read approximately one cell voltage higher than the previous.

Solder the wiring leads to the nickel strip tabs on the pack (not to the cell terminals — see our spot welding vs soldering guide for the complete explanation of which connections use which joining method). Use quality stranded wire with an appropriate gauge for the current (12–14 AWG for moderate current; 10 AWG for higher current e-bike applications).

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Step 7: Test the Finished Pack

Voltage Check

Measure the full pack voltage at the output leads after BMS connection. Confirm it reads your expected nominal voltage (e.g., 37V for a 10S pack). If it reads zero or significantly low, the BMS protection circuit has tripped — check for wiring errors.

Measure each balance tap wire voltage: group 1 should read approximately 3.7V, group 2 approximately 7.4V, group 3 approximately 11.1V, and so on through all groups. Any group reading out of expected sequence indicates a wiring error.

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Tug Test on Welds

Do a final tug test on all visible nickel strip sections — any strip that moves indicates a potentially failed weld that should be addressed before the pack is enclosed and put into service.

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Load Test

Before final assembly and heat shrink, connect a known load (a light bulb, a resistor, or your actual device at low load) and verify the pack outputs power and maintains stable voltage. Check the BMS protection by simulating a short (carefully, momentarily) — the BMS should cut output current immediately.

Apply heat shrink tubing and seal the pack when all tests pass.

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Common Beginner Mistakes and How to Avoid Them

Using nickel-plated steel instead of pure nickel. Nickel-plated steel has higher resistance and welds differently. Buy from a battery supplies specialist who explicitly identifies their strip as pure nickel.

Skipping the test weld phase. Every welder behaves differently with every nickel gauge. Skipping calibration on scrap means your first real welds are experiments on your actual cells.

Not matching cell voltages before parallel connection. Putting cells at different states of charge into the same parallel group can cause high current flow between cells during the parallel connection itself. Match within 0.05V first.

Inconsistent electrode pressure on the handheld stylus. The #1 cause of variable weld quality. Develop a consistent pressure habit before building, or upgrade to a bench welder with mechanical actuation.

Welding nickel strip that overhangs onto adjacent terminals. A strip that connects positive of one group to negative of an adjacent group is a short circuit. Measure and cut precisely.

Not insulating exposed strips during series connection. Once series connections are being made, the pack voltage builds up across exposed nickel. A metal tool contacting the wrong strips at this stage can cause a serious short. Kapton tape all exposed nickel that won't be part of the next connection before proceeding.

Skipping load testing before final assembly. A pack that passes static voltage checks but fails under load has a connection problem somewhere. Find it before you heat-shrink the pack closed.

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Frequently Asked Questions

How many cells do I need to build a battery pack?

This depends entirely on your voltage and capacity requirements. For a 36V (10S) e-bike pack: 10 cells minimum for 1P configuration; 20 cells for 2P, 40 cells for 4P, etc. The series count determines voltage; the parallel count determines capacity. A typical e-bike pack using quality 3,000mAh cells in a 10S4P configuration uses 40 cells and delivers approximately 37V nominal at 12Ah (444Wh). Calculate your required voltage from your controller's spec sheet, then calculate the parallel cell count from your desired range.

What nickel strip thickness should I use for 18650 batteries?

For most 18650 e-bike and power tool pack builds: 0.15mm pure nickel strip is the standard starting point. It handles 10–15A continuous per strip run and welds reliably with mid-range bench welders. For higher-current applications or wherever you want lower strip resistance, use 0.2mm or run a double layer of 0.15mm (weld the first layer, then weld the second layer on top). Use 0.1mm only for low-drain applications or prototyping. Always use pure nickel rather than nickel-plated steel for lower resistance and better weld quality.

What is the minimum spot welder I need to build an e-bike battery pack?

A mid-range bench welder with sufficient power for your nickel strip gauge. The Sunkko 737G+ ($250–$350) is the most community-recommended minimum for e-bike pack builds — it handles 0.15–0.2mm nickel in fixed-head mode with adequate consistency for the application. Budget rechargeable handhelds ($30–$150) can weld 0.1mm nickel for learning, but produce inconsistent results on the heavier strip gauges appropriate for e-bike packs. If you're building packs that will cycle under real load, don't compromise on the welder.

How do I test whether my spot welds are strong enough?

The pull test is the standard field check: after welding, grip the nickel strip with pliers and pull it perpendicular to the cell terminal. A good weld tears the nickel strip metal rather than separating at the weld interface — the strip fails, not the bond. A weak weld peels cleanly off the terminal. Perform pull tests on scrap welds during calibration before starting your real pack, and do spot-check pulls on the actual pack after welding each group before proceeding. If any weld peels, re-weld and test again.

How long does it take to build a battery pack?

For a beginner's first pack (10S2P, 20 cells): expect 4–8 hours including planning, cell preparation, welding, BMS wiring, testing, and packaging. The welding phase itself for 20 cells is 30–60 minutes; the cell preparation, voltage matching, and BMS wiring take most of the total time. A second or third pack of the same configuration takes 2–4 hours as your efficiency improves. Production builders with good cell holders, a bench welder, and established processes can complete a 10S4P pack in 2–3 hours.