I would like to replace/convert some old (non working) 18v Nicad power tool batteries to use 18650s instead. I've got some brand new 18650s and some 5s battery management system boards, but as a non-engineer, there is just one piece that isn't in the wiring diagrams and seems pretty critical.

Based on my online reading, it appears that 5 cells will give me the best match for 18v tools, it sounds like 4 isn't enough

I understand that each 18650 need to be charged at 4.2v, and if I was just charging one, that makes sense. When I look at the diagrams for the BMS wiring, they don't indicate line input voltage for five 18650s wired in series along with the BMS. Does my input voltage need to be 5 x 4.2 (21v) or do I have input of 4.2 to charge the whole system, and still expect it to deliver ~21 on the way out? Sorry for such a basic question.

Here is some additional info; I have two chargers that are interchangable for the old batteries; they indicate output of 21.75v 85ma 24v 210ma

In an ideal world, I'd use the existing chargers against the new 18650 5-packs to charge, and then just plug into my 18v tools and be able to use them again.

Thank you for any clarification!

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1 Answer 1


The charging profile / procedure of a NiCd chemistry is not compatible with the charging profile / procedure of a Lithium chemistry... so I would say "bad idea" where it comes to re-using the old charger at least.

  • \$\begingroup\$ The old charger is just a wall wart PSU, rather than a 'managed' charger- the only advantage to using it is that it has the same connector shape as the tool, which would (if possible) save me from having to add a power jack on the side of the battery pack for a barrel plug \$\endgroup\$
    – KERCAP
    Mar 31, 2019 at 19:56
  • \$\begingroup\$ So let's assume a new wall wart, in this configuration do I need a 21v or 4.2v input? Thank you! \$\endgroup\$
    – KERCAP
    Apr 1, 2019 at 23:43
  • \$\begingroup\$ Since the batteries are in series, 4.2V isn't going to be very useful. The datasheet for the pictured board should tell you how it's intended to be used. \$\endgroup\$
    – vicatcu
    Apr 2, 2019 at 3:09

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