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Im currently planning out my (first) conversion of an old 18V drill with dead NiCad cells to 18650 Li-Ion. I have enough cells that should support the high discharge rates and I'm putting them into a 5S configuration.

My question: should I use a buck/boost coverter to keep the voltage steady at 18V or will the motor (most likely) be fine with the voltage range of fully charged and discharged Li-Ion cells (roughly 15 - 20V)?

If possible, would a 5S2P configuration be preferrable, to "spread out" the current over 2 cells? Would a different BMS be required for that?

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  • \$\begingroup\$ It would be better to match Vbat to motor as the current load is 10x on acceleration which increases converter switch losses 100x \$\endgroup\$ May 26 at 11:44
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    \$\begingroup\$ The motor will be fine over that range of voltages. The speed controller, not so much. (You might consider 6S LFP cells which have a very flat discharge curve except for a few % of capacity at either end) \$\endgroup\$ May 26 at 15:11
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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.

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  • \$\begingroup\$ This souds like I should be usig a buck converter. Stuff going bang is scary. \$\endgroup\$
    – Evgeni
    May 26 at 11:53

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