Sometimes when I open VNC viewer on my RPI I get low voltage report. I have measured supply voltage several times, but always measure bit more then 5 volts. I don't know what to think. My question is:
Is there a way to read the supply voltage, so I can put it in a trend chart?
There is no way to measure supply voltage to a Raspberry Pi without extra components, all you have is the vcgencmd get_throttled command which can tell you if the Pi has had an undervoltage incident since booting. There is a script to help interpret vcgencmd get_throttled at https://gist.github.com/aallan/0b03f5dcc65756dde6045c6e96c26459
I suppose you could use Node-Red to call vcgencmd and display the time undervoltage occurred.
An undervoltage can be a very transitory voltage dip; you might need an oscilloscope to detect it.
Use a good power supply, and if the USB cable is removable, make sure it's a good quality one.
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Have a look at node-red-contrib-vcgencmd (node) - Node-RED
That's a very easy to use solution to monitor for low voltage throttling, plus lots of other useful information.
Another tip is don't use a USB power cable longer than what you need! The longer the lead - the greater the voltage drop.
If the power lead resistance measures just 0.5 Ohm, that would give a voltage drop of up to 1 Volt, before it even gets to the pi.
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@jbudd @Paul-Reed , thanks very much for your advise guys, I will make use of this node. I think it might be helpfull in monitoring what;s going on.
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My Pi3 ran for years with periodic warnings. Never seemed to actually be an issue. Even so, Paul's advice is spot-on.
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