Use a Ni-Cad charge on Li-ion Battery?
Maybe Fixer-builder edition
Repurposing a Ni-Cd Charger for Li-Ion Batteries
(Maker-Fixer Edition)
Your old Ni-Cd charger can live again. It’s got the guts — transformer, rectifier, and housing — it just needs brains. We’ll keep the power section and add modern control so it can safely charge Li-ion tool packs or custom builds.
What You’ll Need
- Your Ni-Cd charger (rated 9.6 – 18 V output)
- One DC-DC buck converter (example: LM2596 module)
- One BMS board (Battery Management System) for your cell count
- 1S = 4.2 V cutoff
- 2S = 8.4 V cutoff
- 3S = 12.6 V cutoff
- 4S = 16.8 V cutoff
- Connectors (XT60 or barrel plug)
- A bit of 14–18 AWG silicone wire
- Optional: heatsink or small printed enclosure
How It Works
- The Ni-Cd charger becomes your DC power supply. It no longer controls charging — it just provides raw DC voltage.
- The buck converter trims that voltage to the exact target for your Li-ion pack (4.2 V per cell × number of cells).
- The BMS board connects directly to the battery pack and handles all the smart stuff — balancing, over-current, and full-charge cutoff.
Example:
If you’re charging a 3-cell (3S) Li-ion pack, set the buck converter to 12.6 V output.
For a 4-cell (4S) pack, set it to 16.8 V.
Wiring Sequence
Ni-Cd charger (+) → input of buck converter
Ni-Cd charger (–) → input of buck converter
Buck converter output (+) → BMS “B+”
Buck converter output (–) → BMS “B–”
Battery pack wires → BMS board according to your cell count (B1, B2, B3, etc.)
Your tool or device connects to the BMS “P+” and “P–” pads.
Optional Add-Ons
- Use a small voltmeter display on the output so you can quickly confirm charging voltage.
- Keep the original LED on the Ni-Cd charger housing and rewire it to the BMS “charge complete” or power LED if you like indicators.
- You can even add a small 12 V fan to the case if you plan on charging large packs.
Typical Part Sources
- LM2596 buck converter module – a couple bucks online
- 3S/4S BMS board – about five bucks
- XT60 connectors – a few dollars for a pack
- Heat shrink and wire – grab a cheap silicone kit
Summary
Your Ni-Cd charger’s job now is simple: be the power brick.
The buck converter and BMS do the thinking.
No more junk drawer electronics — just a solid, functional Li-ion charger that fits in your maker space.
