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Replacing lithium ion cells in an old laptop battery pack, I want to connect the new cells in parallel before removing the old ones to avoid the BMS locking (if it even would do that). The pack uses three parallel pairs of cells in series to make 10.xx volts with low and medium taps going to the BMS. I bought six new 18650 from a reputable national battery company that are the same ampere-hour spec as the old ones. All my new cells arrived with 3.55V. After charging the laptop battery pack, the old cells in it are within -50 mV of the new cells, but the old cells are not able to run the computer for more than a few second before it shuts down, which makes me think they are simply high internal resistance. I think putting the new cells in parallel temporarily shouldn't cause significant current flow as long as the old ones are fully charged. My new cells are spot welded in the correct configuration and I merely need to connect the jumper wires appropriately to the open battery pack. I've read that after replacing all the cells, the battery pack should be run through a charge cycle or two, to calibrate the BMS.

So I'm wondering, would it be safer to use resistors as shown in my diagram when connecting the new batteries before removing the old ones? I've worked with electronics all my life and understand current and voltage very well, but feel I should be prudent from all the warnings I've heard about lithium batteries failing.enter image description here

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  • \$\begingroup\$ Saying thanks here is by "upvote" ans "accepting" answers ;) \$\endgroup\$
    – datenheim
    Jan 21, 2023 at 6:21
  • \$\begingroup\$ @datenheim, I chose your answer as most helpful, but Davide Andrea's also influenced me. I performed the cell replacement and the laptop is running a charge cycle right now. Everything looks good! \$\endgroup\$ Jan 21, 2023 at 15:18

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You could do it that way, but I'd recommend to do it pair after pair.

Use a resistor to level the first new pair to the first old pair. If the Voltage of both pairs is the same (within 5 mV or so).

@gbarry (thanks) mentioned it is simplest to measure the voltage drop over the resistor going to near zero. A meter with a resolution of 1 mV would be great, but one with 10 mV will do too.

Now it is save to connect the new pair directly and cut out the old pair.

Repeat for the other pairs.

With 100 Ω it may take very long to level!

Let's assume you have 0.2 V mismatch, you can allow 0.2 A of leveling current. So yo could savely reduce the resistor to a few Ω.

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  • \$\begingroup\$ This does sound like a more logical sequence. \$\endgroup\$ Jan 20, 2023 at 20:33
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    \$\begingroup\$ With the resistor in place, simply watch the voltage across it as it goes to zero. \$\endgroup\$
    – gbarry
    Jan 21, 2023 at 3:13
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If the old cells have such high resistance, connecting the new cells in parallel to them (even without the mentioned resistor) won't do much of anything. You're fine.

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    \$\begingroup\$ I assumed the old cells have high resistance based on their fast voltage drop, and thought that paralleling probably would be safe, so I appreciate your verification. I am wary from scary stories of lithium ion cells, so trying to be careful (maybe too much so). \$\endgroup\$ Jan 20, 2023 at 20:44
  • \$\begingroup\$ That is a good assumption. \$\endgroup\$ Jan 20, 2023 at 21:38
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    \$\begingroup\$ The only thing you lose by being extra cautious is a little time. \$\endgroup\$
    – gbarry
    Jan 21, 2023 at 3:14
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    \$\begingroup\$ This answer was meaningful information for me. I measured no more than 5 mA when paralleling old and new pairs no matter what the voltage difference was, so that gave me the confidence not to worry. \$\endgroup\$ Jan 21, 2023 at 15:21
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    \$\begingroup\$ Well done. Thank you for sharing your test results with us. \$\endgroup\$ Jan 21, 2023 at 15:32

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