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Can I replace 6 NiMH batteries in series (7.2 V discharged, 8.4 V charged) with a 2S Li-ion battery pack (7.4 V discharged, 8.4 V charged), using the same charger, with close monitoring and multiple safety measures in place?

What I'm trying to do is to revive an old VHF walkie talkie.

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    \$\begingroup\$ "also close monitoring and multiple safety measures in place" - exactly what are these measures? \$\endgroup\$ May 4, 2023 at 2:20
  • \$\begingroup\$ kinda long story but will try to resume, I put that phrase in part because I've seen lots of people frowning to the idea of mixing charger types which is correct, in my case in the testing period I plan to not leave the battery being charged alone, also I have some experience handling batteries and work with batteries on daily basis, also fuses and a FLIR one thermal camera, oh, and my entire workbench is fireproof, short of a foam based extinghisher with CO2, pretty sure I can be safe, even if nobody is ever really safe, as you probably know. feel free to advise anything except not doing it. \$\endgroup\$ May 4, 2023 at 4:51
  • \$\begingroup\$ Well I'm gonna go with the crowd and suggest (strongly) you not do this. Besides the obvious safety issues, there's really NO REASON to monkey with it. You can obtain a replacement 6S NiMH battery pack that will be reasonably close to the energy storage possible with the LiIon. (with no risk of fire...) \$\endgroup\$
    – Kyle B
    May 4, 2023 at 7:08
  • \$\begingroup\$ Thanks for the advice, Kyle, still, if you check the voltage levels, they are pretty similar, that's the main reason for this endeavour, only issue I foresee is full charge cutoff, which I plan to instrument via time and temperature, not trusting the original charger. regarding the low cost you mention, I'm currently living in Argentina, south america, which multiplies any cost you might have in mind by 750, so this is the only way (think Afganistan without the flying bullets ), as I said, this is just to try and resuscitate some old vhf Motorola portable ham radio. \$\endgroup\$ May 4, 2023 at 10:38

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Your question is about a charger designed for NiMH/NiCD 6s, and whether it can be used with lithium.

Absolutely not! Every battery chemistry needs the charger to be tuned to its requirements so the battery isn't damaged. A lithium battery needs this a great deal more, because lithium batteries tend to catch fire!

You would be wisest to identify a lithium battery charger appropriate for 2S, and modify the device or charge the battery externally.

"even if nobody is ever really safe, as you probably know" You might as well say "When it's your time, God will take you". People who pay attention to safety codes and best practices are perfectly safe.

Honestly. Look no farther than FAR part 121 (commercial) aviation in the United States. When people care about safety, safety works.

"Feel free to advise anything except not doing it" Then consider this a Frame Challenge.

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  • \$\begingroup\$ I like your way of thinking, will hookup my oscilloscope and some other electronics paraphernalia to perform a close current/tems monitoring for the first 10 charge/discharge cycles and see how it goes, if anything looks weird or out of the ordinary, I promess I'll hardwire a proper charger, this is for a hobby project, not something of scale. \$\endgroup\$ May 4, 2023 at 10:48
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The problem here is that NiMH batteries are charged at constant current with no voltage limit, while Li-ion batteries need a voltage limit to prevent them from blowing up. Any voltage above 4.2 V per cell (even only 0.05 V higher) will be fatal.

Therefore you cannot use the same charger without modifying it to stop the voltage going over 8.4 V. That may not be easy. It might be better to disable the charging circuit and use a separate Li-ion charger.

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No, you cannot use the same charger.

NiMH batteries are charged with constant current, and a maximum in dV/dt and/or dT/dt is used to detect if the battery is full, often with an added timer for when the detection fails. They sometimes are charged with a low current and a timer.

Li-ion batteries are charged using a constant current/constant voltage algorithm with a very strict and precise end voltage.

The algorithms for NiMH don't have that voltage limit, which makes charging Li-ion batteries with NiMH chargers dangerous. Monitoring time and temperature alone won't do.

You could get six new NiMH batteries and keep things original, or use two Li-ion cells in series and replace the charging bit with a proper 2S Li-ion charger. There are plenty of ICs about that integrate such a charger and need just a few external components. The 2S battery pack should have a 2S BMS module for safety; these are also available off-the-shelf.

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