How to Rebuild a Snap-on 18V Battery Pack (CTB8185 / CTB8187 / CTB8189, 5S2P, 18650 Cells) | Step-by-Step Guide
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⚠ WARNING: Rebuilding lithium-ion battery packs is dangerous. Risk of short circuit, sparks, fire, and serious injury. Do not attempt unless you are experienced and using proper tools. If the cells are rusted, the BMS is damaged, or you are unsure — DO NOT rebuild. Dispose of the pack safely instead.
This complete guide shows you how to rebuild the Snap-on CTB8185 18V battery pack. The CTB8185 uses a 5S2P configuration of 18650 lithium-ion cells. ⚡ Note: The rebuild process is the same for the Snap-on CTB8185, CTB8187, and CTB8189, since all three packs follow the same basic design and configuration. We’ll cover specifications, tools, step-by-step rebuilding, safety notes, and professional tips. Parts and tools can be sourced directly from RenewSpark
Battery Pack Specifications
- Configuration: 5S2P (5 series × 2 parallel = 10 cells)
- Nominal Voltage: ~18V (3.6V × 5)
- Full Charge Voltage: ~21V
- Capacity: ~5Ah (2 × 2500mAh)
- Cells Used: Samsung 18650-25R, 3.6V 2500mAh
- Energy: ~90Wh
- Nickel Strips: 15mm × 0.2mm nickel plate
- BMS: 5S, ≥30A with temperature sensor
Required Tools & Materials
- Spot Welder – set to gear 40, tips 2–4mm apart, pulse 2ms
- Soldering Station (e.g. WEP 927 IV) with solder wire
- Desoldering Tool – for thick solder removal
- Pliers & precision cutters – for nickel strips
- Cut-resistant Gloves – nickel plates are sharp
- Kapton Tape – high-temp insulation
- Fish-paper Insulation Rings – for positive cell ends
- Clear Electronic Grade Silicone – to secure cells & reattach sensor
- Internal Resistance Tester (e.g. YR1035+)
- Desoldering Wick – for cleaning BMS pads
- Snap-on CTB8185 Charger – must be the correct charger
Step-by-Step Rebuild Process
- Open the battery case: Carefully disassemble the Snap-on CTB8185 housing to expose the pack.
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Remove the BMS:
- Cut the nickel strips carefully and detach the BMS.
- Use the desoldering tool or solder wick to remove old solder.
- Clean the pads for fresh nickel connections.
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Check the old cells:
- Measure voltage with a multimeter.
- Check internal resistance using YR1035+ tester.
- If cells are rusted, swollen, or out of spec — stop and recycle.
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Prepare the new cells:
- Use Samsung 25R 18650 cells.
- Install cells into the plastic cell holder.
- Place insulation rings on positives.
- Secure with a little caulking.
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Spot-weld the nickel strips:
- Weld 15×0.2mm nickel strips according to the 5S2P layout.
- Spot welder settings: gear 40, 2–4mm spacing, 2ms pulse.
- ⚠ Avoid short circuits — nickel across two cells can spark/fire.
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Reconnect the BMS:
- Solder new nickel to the BMS pads.
- Reattach the temperature sensor with caulking.
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Final checks:
- Check voltages between BN–V1, V1–V2, V2–V3, V3–V4, V4–V5 (~3.6V each).
- Total BN–V5 should read ~18V.
- Confirm balance wires are correct.
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Charge and test:
- Only use the Snap-on CTB8185 charger.
- If it charges normally, your rebuild is successful.
- Recycle old cells: Take dead lithium-ion cells to a recycling center.
Safety & Cautions
- Never short cells — even momentary contact can spark and ignite.
- When removing nickel strips with pliers, avoid touching adjacent cells.
- Always wear cut-resistant gloves.
- Do not reuse rusted, swollen, or damaged cells.
- If the BMS is corroded or broken, replace it — do not reuse.
- Work in a safe space with a fire extinguisher nearby.
Pro Tips
- Apply caulking with a cotton swab applicator for precision.
- Label the pack with rebuild date, cell type, and capacity.
- Take photos of wiring before teardown to use during reassembly.
Disclaimer: This guide is for educational purposes only. Battery rebuilding is dangerous. Proceed at your own risk. RenewSpark is not responsible for injury, damage, or misuse.
