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I am thinking to replace the old Ni MH Battery 12V pack of a B&D PD1200 portable vacuum cleaner with an x3 Li-ion batteries with an overcharge protection circuit. The new Li-ion pack is about 11.1V (3.7x3) instead of a nominal 12V.

So my question is, are 11.1V enough to powerup the vacuum cleaner or it will slow down its functionality? What are my options there?

Thank you.

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  • \$\begingroup\$ do bear in mind the voltage will change from 12.6 at full charge to 10(ish) at minimum, a much larger relative voltage swing than with NiMH \$\endgroup\$
    – Neil_UK
    Nov 7 '20 at 19:02
  • \$\begingroup\$ @Neil_UK which means? \$\endgroup\$
    – Maverick
    Nov 7 '20 at 20:00
  • \$\begingroup\$ I think 4×LiFePO₄ cells would suit you better. Slightly more than 12V at 50% charge (so the vacuum will perform well), and nearly constant voltage, much like the NiMHs. \$\endgroup\$
    – anrieff
    Nov 7 '20 at 21:16
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using steady supply for a battery powered device is much more reliable. first, about the voltage range, the wikipedia page on NiMH :

A fully charged cell supplies an average 1.25 V/cell during discharge, declining to about 1.0–1.1 V/cell (further discharge may cause permanent damage in the case of multi-cell packs, due to polarity reversal). Under a light load (0.5 ampere), the starting voltage of a freshly charged AA NiMH cell in good condition is about 1.4 volts.

  • [probably] 10x NiMH batteries : from 13v (fully charged) down to 10v, 12.5v most of the time
  • 3 Li-Ion batteries : from 12.6v down to 9v. 11.1v most of the time.

so it doesn't seem much of a problem as the difference is marginal.

I have a B&D cordless drill which I changed the obsolete NiCad battery pack to Li-Ion. I used 2x2 batteries to produce 7.4v, and used a step up converter module and a big CAP and it works perfectly.

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Given that NiMH have a very stable voltage as they discharge, it’s quite likely that the motor runs directly from this supply, assuming that it will be more or less constant. Running from a lower voltage will give less power. This is more or less proportional to the square of the voltage so you’d expect 85% or normal power. On the other hand, if the appliance uses a switch-mode supply to run the motor then you may see no degradation in power, although that would seem less likely because switch mode supplies cost money and 12v is a sensible voltage to use directly for a small motor.

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