When you still have some older power tools lying around, which have defective NiMH- or NiCd-batteries and you want to reactivate them with newer Li-ion batteries, you are here on the right project page.
Those old power tools were often powered with 14.4V (12 cells @ 1.2V), whereas nowadays the power tools are powered with 18V (5 Li-ion cells @ 3.6V).
The circuit below is a converter from 18V to 14.4V. It implements also a deep discharge protection for the Li-ion battery. In order to not cause a permanent damage to the 18V Li-ion battery, it should not be discharged below ~16V (3.2V per cell).
For sure this simple circuit does not supervise the voltage of each single cell because a power tools' Li-ion battery typically does not offer this feature.
The voltage thresholds for powering on and off can be calculated with the formulas in chapter Simulation. The circuit provides the potentiometer R1 to exactly trim these voltage thresholds.
Also I recommend a heatsink for the mosfet and the diodes to not overheat those parts (depending on the used power tool several amps can flow).
The circuit has been simulated with the freeware LTspice. Because some part models were missing in the provided standard library I added those models in folder .
The switching-on threshold voltage can be calculated with:
U_on = 2.5V * (R2 + (R1||R4)) / (R1||R4)
The switching-off threshold voltage can be calculated with:
U_off = 2.5V * (R1 + (R2||R4)) / R1
