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I have an old ThinkPad X220T notebook of which the internal battery is broken (and removed). To get my notebook working on the go again I want to build an external portable power supply with 2 LiPo batteries and 2 buck converters I already have.

Battery A: 5000 mAh, 3S/11.1V, 55.5Wh, 20C Battery B: 4500 mAh, 5S/18.5V, 83.2Wh, 20C

buck converters: 7-32V to 0.8-28V 12A 300W

I want to connect to the 20V connector of the ThinkPad to which normaly the AC Adapter of the standard power supply connects.

The plan is to connect each battery to a seperate buck converter and regulate the voltage down. Output of A+Buck: 8V Output of B+Buck: 12V

Those two outputs I want to connect in serries to get to the required 20V and feed them into the ThinkPad.

  1. Can the buck converters influence each other?
  2. Do I need protection from current spikes when connecting the batteries?
  3. Do you see any other problems?

sketch of the circuit

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    \$\begingroup\$ As the buck converters go up to 32V why not set both to 20V and in parallel? Oh and add some fuses. \$\endgroup\$
    – Solar Mike
    Commented Apr 20, 2023 at 18:08
  • \$\begingroup\$ connecting two converters in parallel is likely to just use one and the other one will be idle. Connecting them in series is likely to drain one battery before the other. Neither is a great solution \$\endgroup\$
    – Stack Exchange Supports Israel
    Commented Apr 20, 2023 at 18:14
  • \$\begingroup\$ I calculated the voltages A+Buck: 8V and B+Buck: 12V so that the batteries are drained equally. But as long as I don't got below the minimum voltage of any cell draining one first shouldn't be a problem. \$\endgroup\$
    – BigBangBattery
    Commented Apr 20, 2023 at 18:55
  • \$\begingroup\$ My Buck Converters have a current limiting circuit inside them. So current spikes (when connecting the battery) shouldn't be a problem right? I'm a bit worried since my batteries can deliver currents up to 100A because they have a high C-rating. \$\endgroup\$
    – BigBangBattery
    Commented Apr 29, 2023 at 14:50

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