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I'm interested in building lithium ion battery packs, and I was wondering if there is a way to change the pack on the fly from series to parallel - basically to allow charging in parallel and discharging in series. Ideally something that can be done with solid state switches or MOSFETs, not with relays. That would seem to avoid the need for a charger specific to the pack voltage, as well as the need for balancing.

P.S. I've looked at BMS chips and there are series battery pack monitoring/balancing chips like BQ76940 (works for 9S to 15S) and BQ79616 (6S to 16S) but they seem pretty hard to use: at a minimum would need a microcontroller and a fair bit of supporting circuitry.

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  • \$\begingroup\$ What is the cell arrangement you plan on using in your battery packs? \$\endgroup\$
    – MOSFET
    Commented Apr 5 at 0:53
  • \$\begingroup\$ @MOSFET Hmm, it would be all series, maybe as high as 30S \$\endgroup\$
    – Alex I
    Commented Apr 5 at 1:00
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    \$\begingroup\$ So you want to build a battery pack with 30 cells, charge all 30 in parallel, and switch them in series for usage? Not impossible to do. But extremely unorthodox. And yes, this would require a fair bit of circuitry. What application would you need a battery pack like that? That's going to be over 100VDC with 30 cells in series. \$\endgroup\$
    – MOSFET
    Commented Apr 5 at 1:08
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    \$\begingroup\$ Something like this is done in some power tools batteries - e.g when charged or used with some less demanding load, the battery is 5s3p, on request it switches to 15s1p \$\endgroup\$
    – fraxinus
    Commented Apr 5 at 5:12

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Yes, it's possible, but fraught with a few problems:

  • Balancing is trickier
  • While in series, one of the two batteries will be the first to be emptied; when its BMS shuts it down, the load will be unpowered even if there is still energy in the other battery.
  • The BMS's protector switch is likely to be rated only for its battery voltage. When it opens under load, the negative of the full battery voltage (twice as much, and reverse) appears across its terminals, which may blow MOSFETs.
  • There will be some voltage difference in the two batteries when reconnecting in parallel, which will cause an inrush of current which may degrade the battery with the lowest SoC.

So, doable, but with a lot of care and risks.

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  • \$\begingroup\$ On this "Balancing is trickier" isn't balancing simply charging the batteries to the same voltage? If charged in parallel, they would always be perfectly balanced at the beginning of discharge \$\endgroup\$
    – Alex I
    Commented Apr 5 at 2:44
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    \$\begingroup\$ "which will cause an inrush of current which may degrade the battery with the lowest SoC" - true, I thought about that. may have to connect some decent sized resistors in between for a while, and then start charging \$\endgroup\$
    – Alex I
    Commented Apr 5 at 2:46
  • \$\begingroup\$ "in the two batteries" - oh, I'm thinking of a decently big stack, around 20-30. I figured that can definitely be done with relays (just use relays rated at 200V 5-10A or so) but I was curious if there is any solid state method to do the same \$\endgroup\$
    – Alex I
    Commented Apr 5 at 2:48
  • \$\begingroup\$ "When it opens under load..." - that is a big problem. How do series packs usually handle this? I guess I could only limit to a very moderate discharge (let's say from 4.2V down to 3.5V per cell or so) but it really seems any series pack would need some way to disconnect the load well before any individual battery's BMS tries to do so \$\endgroup\$
    – Alex I
    Commented Apr 5 at 2:50
  • \$\begingroup\$ "How do series packs usually handle this? " The problem is avoided by not doing series batteries in the first place. Instead of two batteries in series, we make single higher-voltage battery. \$\endgroup\$
    – Davide Andrea
    Commented Apr 5 at 12:10

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