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I built a 22V battery pack with 6 18650 cells connected them to a 6s BMS. I will use this to power a portable audio amplifier.

I also want to use the same battery pack to power a Raspberry Pi Zero W embedded in the device.

According to the specs, the Zero W has a typical / max current draw of 150mA / 1.2A.

My idea is to connect the second tap (7.4V) with a resistor and the B- on the BMS to the RPi 5V USB input.

I have two questions:

  1. Will this work? Is the BMS designed to be used this way, drawing the full 22.2V from the whole array and 5V from only 2 batteries?

  2. In calculating the voltage drop resistor for the 7.4 → 5V conversion, which current draw shall I use? The max. rated 1.2A?

Thanks in advance.

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    \$\begingroup\$ voltage across a resistor is proportional to the current flow through the resistor ... if the Pi Zero W curent draw increases, then the voltage at the Pi power supply pins will decrease .... a dropping resistor is not a viable solution, use a buck converter instead \$\endgroup\$
    – jsotola
    Nov 28, 2021 at 0:46

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No and no. Neither are good ideas.

Unparallel draining of your batteries will lead to issues. 2 will die sooner than the others in best case.

A resistor or a resistor divider is not a good way to drop the voltage for a non-constant load like a SBC. As the current varies the voltage drop will vary.

You should just use a switching step down power regulator. A common off the shelf 24V car usb charger would do well here. Many 12V car usb chargers accept 24V as well. Or any number of modules for sale online or make your own from the numerous ICs.

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  • \$\begingroup\$ Moreover using resistors to drop voltage means to waste battery power. You'll loose R*I2 Watts across resistor. In the specific case, supposing your Zero will consume use constant current, @1.2A roughly 5Watts are waste for a drop of 2.4V. As suggested in the answer a switching regulator has high efficiency so to limit waste to few milliWatts. \$\endgroup\$
    – Frankie_C
    Nov 28, 2021 at 10:22
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    \$\begingroup\$ Just to be clearer about "the voltage drop will vary". 7.4V on a 5V input will be almost instantly fatal for the RPi. And at switch-on, everything starts with zero current and hence the full 7.4V on it. Zap. Oops. :) \$\endgroup\$
    – Graham
    Nov 28, 2021 at 16:48
  • \$\begingroup\$ @Graham The 5V bus is connected only to the connectors (that includes HDMI) and a PAM2306 regulator. Now the PAM2306 has a rated absolute maximum input voltage of 6.5 V, but being a regulator I would actually expect it to work with 7.4 V, at least for some time and at low temperatures. \$\endgroup\$
    – AndreKR
    Nov 28, 2021 at 21:15
  • \$\begingroup\$ I use these with good success. Note that it says "12 V" in the title but the input voltage specs are actually 8-35 V. \$\endgroup\$
    – AndreKR
    Nov 28, 2021 at 21:17

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