# Raspberry Pi lightning bolt or under-voltage warning: power supply

> Fix the Pi under-voltage warning: use the official or a good supply for your model, a short thick cable and keep USB loads low.

Canonical: https://roboticsengineering.in/fix/raspberry-pi-undervoltage-lightning-bolt-power-supply/ | Published: 2026-11-03 | Updated: 2026-11-04

**The lightning bolt means the supply voltage fell below the safe level.** Use a proper supply and a short, thick cable.

## What the message means

A Pi needs steady 5 V and enough current. Thin cables and weak chargers drop the voltage, and the Pi may slow down, lose Wi-Fi or corrupt the card.

## Fix it in this order

1. Use a supply made for your Pi model and its rated current.
2. Use a short cable with thick wires.
3. Measure the voltage on the test points if your board has them.
4. Unplug USB devices that draw much current, or use a powered hub.
5. When on a battery, use a good regulator rather than a plain charger lead.
6. Check that servos and motors are not on the same supply.

## Why it happens

The lightning bolt icon appears when the Pi measures its supply below about 4.63 V. The usual cause is a weak charger, a thin or long cable, or a load such as a camera, a USB device or motors on the same supply.

Use the official power supply for your model, a short good cable, and power motors and servos from a separate supply with a shared ground.

## If you see this, do this

| You see | Do this |
| --- | --- |
| Warning appears under load | Supply or cable too weak. |
| Random freezes | Check power first. |

## Related

[ESP32 brownout](https://roboticsengineering.in/fix/esp32-brownout-detector-was-triggered/), [power bank cut-off](https://roboticsengineering.in/fix/power-bank-switches-off-low-current-robot/).

Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.

## Why this happens

The Pi checks its 5 V input and shows the lightning bolt when it falls below about 4.63 V. The usual cause is not the charger label but the voltage lost on the way. A cable has resistance, and the voltage lost is current times resistance. A cable with 0.25 ohm round trip carrying 3 A loses 0.75 V, so 5.1 V at the charger becomes 4.35 V at the board.

The load can also jump suddenly. Starting a camera, plugging in a USB drive or switching a motor on the same supply pulls the voltage down for a moment, which is enough to trigger the warning.

## How to confirm the cause

- Run vcgencmd get_throttled. A value of 0x0 means no problem has been recorded since boot.
- Read the bits. Bit 0 is under-voltage now and bit 16 means it has occurred since boot. Bit 2 is currently throttled and bit 18 is throttling has occurred.
- Run dmesg and search for the under-voltage message with the time it appeared.
- If your board has test points, measure the voltage between them while the Pi works hard. Below about 4.75 V is a problem.
- Unplug USB devices one at a time and watch whether the warning stops.

## Worked example

Decoding a result from vcgencmd get_throttled, and the cable loss calculation.

```
vcgencmd get_throttled
throttled=0x50000
0x50000 = bit 16 + bit 18
-> under-voltage has occurred, throttling has occurred, neither is happening now

drop = I * R = 3 A * 0.25 ohm = 0.75 V
5.1 V - 0.75 V = 4.35 V at the board (too low)
```

## Common mistakes

- Using a phone charger without checking that it can supply the current your model needs.
- Powering servos or motors from the Pi’s 5 V pin.
- Using a long, thin or old cable.
- Feeding 5 V into the GPIO pins with thin wire, which bypasses the protection and the drop is larger.
- Ignoring the warning because the Pi still boots.

## If this did not fix it

- A power bank that cuts out or does not deliver full current on its port.
- A failing SD card, which causes freezes that look like power faults.
- A hot Pi. Temperature throttling is a different warning from under-voltage.