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Power-Tool 18650 Rebuild Side Quest

A work log for evaluating full-cell replacement in failed 18 V-class power-tool battery packs using new matched high-drain 18650 cells, retained OEM electronics, and spot-welded nickel interconnects.

Initial Packs

• Ryobi ONE+ 18 V 4 Ah pack, expected 5S2P / 10-cell architecture
• DeWalt DCB207 20 V MAX 1.3 Ah pack, 5S1P / 5-cell architecture

Pack History

Ryobi ONE+ 18 V 4 Ah

The pack was opened after the OEM charger would not recover it. The cell array measured approximately 9 V total. Multiple short external charge attempts and repeated OEM charger attempts did not return it to service. The current direction is complete 10-cell replacement while preserving the OEM board, thermistor, terminal assembly, carrier, and usable insulation.

DeWalt DCB207

This pack was previously rebuilt with recycled cells from another project. It returned to service and survived roughly one year of actual tool duty. The exhausted salvage set has now been replaced with five new matched high-drain cells. Because the replacement array is 5S1P, the 2500 mAh cell rating carries through as a nominal pack capacity of 2.5 Ah; the series connection increases voltage but does not add amp-hours. Compared with the original 1.3 Ah rating, this is an increase of about 92 percent, nearly doubling the nominal capacity in the same pack format.

Ordered Materials

• 25 BAK N18650COP flat-top cells, 2500 mAh nominal, 30 A listing
• 100 flat-top positive-terminal insulators
• 20 ft of 8 mm x 0.15 mm pure nickel strip

Order total: USD 108.06 shipped and taxed.

Equipment

• Portable battery spot welder, Amazon ASIN B09V2R1TKD
• Keenstone four-bay charger, Amazon ASIN B07JD77LVD
• Digital multimeter
• IR thermometer
• Kapton tape
• USB power meter and selectable high-load resistor board

Completed Teardown and Preparation

• Documented the original cells, pack layouts, series/parallel geometry, and board connections before removal.
• Cut the original interconnects destructively with side snips to separate the exhausted arrays.
• Observed an arc when a cut connection contacted the conductive cell casing, confirming the casing as an active shorting path during teardown.
• Released the original cells from the plastic cages while preserving the reusable pack structures and electronics.
• Desoldered the remaining cut board leads and cleared the connection points with a solder sucker.
• Cut replacement main leads, formed the required angles, and spot-welded the angled interfaces.

Cell Charging

The new cells were charged individually in four-cell lots. Each lot required roughly four hours to reach the charger's full indication, with final displayed voltage near 4.22 V per cell. This established a common charged starting condition for assembly; it was not a measured discharge-capacity result.

DeWalt Rebuild and Validation

• Reassembled the five-cell 5S1P battery array in the original carrier and orientation.
• Installed the positive-terminal insulators and tacked the new nickel interconnects in place.
• Added extra Kapton tape around exposed interfaces and contact areas for additional insulation.
• Partially installed the shell and performed an initial functional test in a small shop vacuum.
• Connected a third-party DeWalt adapter with USB output to a USB meter and selectable high-load resistor board.
• Held the electrical load until the resistor bank approached its thermal limit, then checked the resistor bank and pack temperatures.
• The meter showed approximately 4.80 V at 3.16 A, about 15.18 W.
• The resistor bank measured approximately 486.7 F, the pack measured approximately 91.9 F, and ambient temperature was roughly 80 F.
• Placed the partially assembled pack on the OEM charger, observed normal operation with no smoke or visible abnormal behavior, and allowed it to reach full charge.
• Trimmed limited plastic from the shell with side snips to clear the wider replacement nickel and completed final assembly.

The DeWalt pack passed the initial tool, resistive-load, thermal, charger, and full-charge checks. These are practical functional checks rather than calibrated capacity, cycle-life, or maximum-discharge qualification.

Assembly Assumptions

• Install a positive-terminal insulator on each new cell before laying out nickel.
• Preserve and reuse OEM carriers and insulation where intact.
• Use additional Kapton where replacement geometry creates exposed or compressed contact risks.
• Use 8 mm x 0.15 mm pure nickel, with parallel paths where needed to reproduce effective OEM bus geometry.
• Establish welder settings by destructive peel testing on scrap or retired cells before welding service cells.
• Use soldering only where needed for board tabs, sense leads, thermistor leads, or terminal connections, while avoiding prolonged direct cell heating.

Next Step

1. Reassemble the charged Ryobi cells in the original 5S2P configuration.
2. Reinsert the rebuilt array into the original plastic pack structure.
3. Verify orientation, parallel-group voltages, node progression, and total pack voltage before reconnecting electronics.
4. Reconnect the retained board leads to the new battery interconnects.
5. Perform mechanical, continuity, charger, tool-load, and thermal checks before final closure.

Pre-Energization Checks

• Verify cell orientation against the original layout.
• Verify every series-group voltage independently.
• Verify monotonic node progression from B- through intermediate taps to B+.
• Verify total pack voltage.
• Check for unintended continuity between nickel and isolated cell-can areas.
• Inspect all welds mechanically and visually.
• Confirm thermistor placement and attachment.
• Confirm no nickel edge can contact a cell shoulder or case under compression.

Open Items

• Record the exact Ryobi pack model and original cell markings.
• Record final Ryobi parallel-group and total pack voltages before board reconnection.
• Record exact spot-welder markings and successful settings for 0.15 mm pure nickel.
• Add Ryobi charger acceptance, tool behavior, and thermal observations.
• Track the rebuilt DeWalt pack through normal tool use for any early imbalance, heating, or mechanical issues.

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