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I'm trying to power my Raspberry 3 model B by means a Catlyn Power Bank 10000mah just purchased and seems to work fine with an iPhone 11.

The problem is that when I connect it to Raspberry, even if the Raspberry starts without any issue and the Catlyn is full charged, I see on the right corner a "Low voltage warning".

Any idea?

Thanks in advance

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  • 3
    I'm not sure what you want us to say. The powerbank is not providing enough voltage when loaded.
    – joan
    Nov 30 '21 at 16:16
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    Try a different cable. Nov 30 '21 at 16:53
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I've had good luck powering my pi4b with an Anker PowerCore Essential 20000 and a high-quality USB-A to USB-C cord (Brexlink 6ft, 3A) plugged into one of the ports. However, the pi is headless with no keyboard, no mouse, and WiFi turned off. I am using extensively the RJ45 jack for ethernet and some pretty demanding programs in C++. The Anker Power bank reads 5.14 v with no load attached, and ~4.9 v at the pi GPIO 5v pin with the pi attached and running. That power pack can run the pi for at least 8 hours and is then only down to 3 of 4 bars. As has been mentioned in several posts you can check if the pi has dipped (is dipping) below 4.63 volts with the vcgencmd get_throttled command.

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  • It's worth mentioning: Voltage readings that are not made with an oscilloscope are not particularly informative - an o'scope can capture short transients that will trigger the LVW, while most meters will display only dc/average values. IOW, one can experience many LVWs while a meter shows > 4.63V
    – Seamus
    Dec 1 '21 at 14:44
  • Good point. Any idea how good the pi itself is on recording transients as indicated by the vcgencmd get_throttled command. If I understand it correctly a 0x5000 response indicates a low voltage was detected earlier but is not currently present.
    – gorlux
    Dec 1 '21 at 20:07
  • I believe I have a reasonably good idea how the RPi's Power Management IC (PMIC) determines the existence of an undervoltage condition](raspberrypi.stackexchange.com/a/133677/83790). I'm not sure about the 0x5000 value you mentioned... I didn't bother to check the "official docs" as they're now difficult to navigate, but this article is fairly current & may have other useful info.
    – Seamus
    Dec 2 '21 at 3:59
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Low voltage warning

When the supply cannot deliver enough current (amperage), the output voltage will drop:

https://electronics.stackexchange.com/q/218344/52138

tested with my iphone 11, seems works fine

Yes, but your iphone has an internal battery, and that is what it is running off of. Notice that the amount of time it takes to charge a phone is (usually) substantially less than the amount of time that charge will last last, point being that as long as the phone is plugged in, the charge on the battery is still increasing even if you are actively using the phone. It isn't running off the powerbank, it is running on it's own internally regulated power, which is provided by the battery which is charged by the powerbank.

While this implies the amperage from the powerbank is at least as much as the amperage used by the phone, we need to qualify this with over time. At any given instant, the amperage used by the phone may be greater than that coming from the power bank, even though averaged over time it is not.

The phone's battery thus serves as a sort of a giant capacitor. But a Raspberry Pi does not have a battery. When in use, like the phone the amperage consumed will fluctuate widely and rapidly.

Powerbanks are not designed (or sold) to power electronic devices and respond properly to that kind of instantaneous current draw. They are designed and intended to recharge batteries. These are very different scenarios. Delivering fast instantaneous current is not something you want happening with a lipo battery -- they can explode. But that is the opposite of what you need in a Pi power supply.

The take-away here is don't bother shopping around for a "better" powerbank. If you want to power the pi from a battery, you need some hardware intended for that purpose (there are hats, and suppliers like Adafruit carry various things).

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Yes... I feel @goldilocks' answer is correct in all important respects, but fundamentally the issue you're seeing is the result of the RPi's input voltage sensor: It is designed to generate a "low voltage warning" when the input voltage falls to 4.63 volts.

If you consider your question for a moment, it might be re-phrased:

If my power bank outputs 5V, why does the input voltage at my RPi's power circuitry drop to 4.63 volts?

The answer is inevitably that the load (a.k.a current) imposed by your RPi acts to reduce the voltage seen at the RPi's input terminals. There are numerous potential causes for this; e.g. :

  • R x I losses in your cable & connectors iaw Ohm's Law. You can see the effect that wire diameter (AWG) has on resistance in tables such as this one. Higher resistance results in higher voltage drops.

  • Internal battery resistance for LiPo batteries is always present, and will increase as the current sourced by the battery increases

  • The performance of the power bank's voltage regulator may have a significant effect. Some power banks use linear (LDO) regulators, some use switch-mode regulators, but regardless, all voltage regulators must comply with the laws of physics which tell us that there will be losses.

As a practical matter, given that you have invested in this power bank, the only variable you have any control over at this point is in your cable selection. In other words, try to find a cable that has the least resistance; i.e. :

  1. Your cable should be as long as it needs to be - but no longer.
  2. Your cable should have the largest power wiring you can find; in my experience this means about 22 AWG for cables you buy, or 18 AWG for cables you build.
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These devices are of variable quality and are designed to charge batteries, but should deliver 5V.

I have used many reputable devices quite successfully, however there are 2 issues which may cause problems.

  1. Most USB cables use fine wire which causes excessive voltage drop. Unfortunately there is no supplier with adequate cable - I make my own. Use the shortest cable possible.
  2. Powerbanks are designed to supply 5.0V; Pi power supplies have decently sized wire but still suffer voltage drop, so are designed to supply 5.1V.

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