Those old DeWalt and Makita drills are built like tanks, but their NiCad batteries are ancient history. In this comprehensive project guide, we walk through the engineering of a 5S lithium conversion, explaining why high-drain cells are mandatory, how to integrate a tool-grade BMS, and the critical reason why you must throw away your original charger.
The Resurrection of the Yellow and Red Classics
If you have a collection of old "18V" or "14.4V" power tools from the early 2000s, you know the frustration. The drills themselves are indestructible—heavy-duty motors, all-metal gearboxes, and ergonomic shells that put modern cheap plastics to shame. But the Nickel-Cadmium (NiCad) batteries are their Achilles' heel. They suffer from memory effect, they self-discharge in a week, and they weigh as much as a brick while delivering a fraction of the power.
Converting these tools to Lithium-Ion (18650 or 21700) doesn't just fix them; it upgrades them. You end up with a tool that is 30% lighter, holds its charge for a year on the shelf, and has significantly more torque under load. However, this is not a "drop-in" project. It requires understanding high-current electronics, BMS integration, and the mechanical challenges of fitting cylinders into a tower-style case. This guide is the definitive blueprint for a professional-grade tool conversion.
1. The Voltage Math: Why 5S is the Magic Number
The first step in any conversion is matching the voltage of the old NiCad pack to a modern Lithium configuration.
NiCad cells are 1.2V nominal. Lithium-Ion cells are 3.6V-3.7V nominal.
- For 18V Tools: A NiCad 18V pack has 15 cells in series ($15 imes 1.2V = 18V$). A 5S Lithium pack ($5 imes 3.6V = 18V$) is a perfect match. The peak voltage of 21V (5 x 4.2V) is easily handled by any 18V brushed motor.
- For 14.4V Tools: Use a 4S Lithium pack ($4 imes 3.6V = 14.4V$). Peak voltage is 16.8V.
- For 12V Tools: Use a 3S Lithium pack ($3 imes 3.6V = 10.8V$). Peak voltage is 12.6V.
The Torque Secret: Because Lithium batteries have significantly lower internal resistance than NiCad, the "Voltage Sag" under load is much lower. Your drill will feel like it has "new life" because the motor is actually seeing higher voltage during a heavy cut than it ever did with NiCad.
2. Cell Selection: The High-Drain Requirement
This is where 90% of DIYers fail. They harvest cells from an old laptop battery and put them in a drill.
Do NOT use laptop cells for power tools.
A laptop cell (like the Panasonic NCR18650B) is designed for a 3 Amp discharge. A cordless circular saw or a stuck drill bit can pull 30 to 50 Amps. Pushing a laptop cell that hard will cause it to overheat, trip its internal CID, or vent. (See our guide on C-Rating and Tool Demands).
Recommended Cells:
- Samsung 25S: The gold standard for 18650 tool packs. 2500mAh, 25A continuous.
- Molicel P28A: 2800mAh, 35A continuous. The most powerful 18650 on the market.
- Samsung 30T / 40T (21700): If you have the physical space, these cells provide incredible torque and runtime.
3. The Brain: Selecting a Tool-Grade BMS
You cannot simply wire the cells to the drill terminals. You need a Battery Management System (BMS) to prevent over-discharge. If you drain a Lithium cell below 2.5V just once, its lifespan is permanently damaged. (Refer to our BMS Selection Guide for more on this).
BMS Requirements for Tools:
- Current Rating: Minimum 40A Continuous / 100A Peak. Tools have massive "inrush" current when you pull the trigger. A weak 20A BMS will simply trip and cut power every time you try to use the drill.
- Common Port: Makes wiring easier inside the cramped shell.
- Size: Look for "Tool BMS" boards which are long and narrow to fit alongside the cell tower.
4. Mechanical Assembly and Housing
Most old tool batteries are "Tower" style. You must gut the old NiCad cells and the paper insulators.
The Layout: Use plastic cell holders if they fit. If not, use high-temp Kapton tape and Fishpaper between the cells. Never glue cells together without insulation; vibration will wear through the shrink wrap.
The Contacts: You must reuse the original metal terminal block from the top of the NiCad pack. Desolder the old wires and solder new 12 AWG silicone wires from the BMS "P+" and "P-" pads to these terminals. Use a heavy-duty soldering iron to ensure the joints don't vibrate loose.
5. The Charging Problem: Abandoning the OEM Charger
CRITICAL WARNING: Never use your old NiCad charger with your new Lithium pack.
NiCad chargers use a "Delta-V" or "Delta-T" (temperature rise) detection method to stop charging. Lithium requires a strict CC/CV (Constant Current / Constant Voltage) protocol. A NiCad charger will keep pushing current into a full Lithium battery until it enters thermal runaway. It is a guaranteed fire hazard.
The Solution:
- Drill a small hole in the side of the battery case.
- Install a 5.5mm x 2.1mm DC Barrel Jack.
- Wire this jack to the BMS "C-" (or P-) and "P+" pads.
- Buy a dedicated 21V (for 5S) Lithium-Ion "Brick" charger.
6. Safety Protocol and Testing
Before closing the case, perform a load test. Clamp the drill in a vise and give it a few short bursts. Check for heat at the BMS MOSFETs. If everything stays cool, seal the case. Many builders use 3D Printed Adapters to allow their old tools to accept modern "Slide-on" batteries (like Milwaukee M18 or DeWalt 20V Max), which is often an easier mechanical route than rebuilding the old shells.
The Result
You now have a tool that is ready for another decade of work. A converted 5S2P (10 cells) pack using Molicel P28A cells will outperform a brand-new "Store Bought" drill in terms of raw torque and sustained power. By recycling the high-quality mechanical components of the past and pairing them with the energy density of the future, you have built a superior piece of equipment for a fraction of the cost of a new brushless kit.
