Is a buck converter necessary for a 18V drill conversion? (NiCad to Li-Ion)
Both my father and I had/have (respectively) 18 V NiCd-powered drills, which took identically connectored power packs.
I converted my dead NiCd pack to a 5S lithium setup, and at the just-charged voltage of 21 V, sometimes the motor brushes would smell rather, but it's worked for a year or two now, and is excellent.
I plugged the pack into my father's drill (same polarity battery, but different make drill), and it went bang. Just that extra volt or so, unless it was a very long co-incidence.
You'll have to draw your own conclusions about whether you need that regulator.