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I have a LiPo battery with specs 7.4V, 5200mah (50C if it is relevant), that I am trying to use to power a Raspberry Pi 4 for a project. I am using a BLDC motor in this setup, so I am trying to power it specifically through the BEC of the motor's ESC.

The BEC outputs 6.1V, which is too high for the Pi, so I hooked it up to a 5V regulator (using an L7805cv in parallel with a 10 and 100 uF capacitor). Now I have a 5V output (more like 4.96V). I tried to hook this up (using jumper wires) to the 5V and ground of the Pi and this is where my problem begins. The voltage drops to a low of about 4.1V and oscillates between that and 4.4V.

I know that the reason is because of insufficient voltage and it constantly restarting, but I am not sure exactly how to avoid this. I heard that the jumper wires have too much resistance, but I am not sure about that (they measure 0.04 ohms on my multimeter).

How can I work around this? Should I just use the 6.1V and add a resistor to the circuit? I am scared to because I saw that the Pi operates with a max voltage of 5.25V.

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  • \$\begingroup\$ The L7805CV has a dropout voltage of around 2V. I'd try using the LiPo battery itself as the input for the regulator. At 7.4V it should be enough to maintain regulation -- assuming the RPi doesn't consume more than 1A, the limit of the 7805. \$\endgroup\$
    – ErikR
    Jun 18, 2021 at 1:56
  • \$\begingroup\$ It sounds like you are missing the bypass capacitors for the regulator, post a schematic of your circuit, not a frizzy thing. \$\endgroup\$
    – Gil
    Jun 18, 2021 at 22:53

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Linear voltage regulators have a quality called "dropout voltage". It's the amount of voltage that develops between their input and output terminals at useful current outputs.

The original 7805 has a dropout voltage of around 2 volts. It sounds like you're using one with a greater dropout voltage, or the BEC is dropping more voltage under load. There are newer regulators that have much lower dropout voltages. You need to get your hands on such a regulator, and use it instead of a 7805.

Alternately, get a separate BEC that has an output of 5V, and use that.

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