Yep, the dmesg log is the way to see those undervolt events. I know that (at least some of the Pi versions) the power LED is supposed to flash or change color, but I swear there have been times I haven't seen that, and then there were lots of u-v events in the log.
I have had cables be the culprit, with an otherwise OK power supply, on several occasions. You want one with heavier wires, there are some amazingly thin ones out there, though this isn't often documented. Try ones that say they're for charging rather than just data. If they document that... A cheap, super thin wire cable will have more drop, as TOP has said, and with enough drop your voltage goes too low. A "5.1V" vs a "5V" would give you another tenth of a volt to lose over a cable, so that would be leeway. If the supply is doing that.
For testing, try the following. It might be worth trying to create as much load as you can, run high bitrate radio, turn on resampling to high bit and data rates, maybe other CPU using features, for a worst case current draw. Then try swapping different cables in and out, and reading the log for undervolt events. That's the quickest, easiest thing to do.
Then try swapping the supplies, with a known "better" cable. Standard process of elimination process. Hopefully you end up with at least 2 good cables, throw away a few bad ones, and find both supplies worthy.
Interesting note on the "5.1V" battery. There is no such thing as a 5.1V battery. All "batteries" that provide a USB compatible voltage are batteries
(usually single cell Lithium at 3.6V or therabouts) with a switching power supply to bring it up to 5 or 5.1V. So, you're actually at the mercy of the switcher supply in the battery pack, as to stiffness under load. If well designed, that would be better than a raw battery. I'd normally ask if you had a decent meter to measure the voltage, but it's difficult on USB systems, since it's hard to measure at the connectors unless you make an adapter tap cable or something, and probing on the Pi happens to be very hazardous, in that the convenient spots near the connector happen to be very near places to blow out stuff if you slip. (Don't ask me how I know!!) Be careful if you try that.
An aside about these USB "Batteries". This makes me giggle a bit, as a hardware electronics guy, about people hoping for better "audiophile" effects of running off a battery, but using one of these USB supply packs. An actual battery is a pretty low noise power supply, if that's going to be important. USB battery paks invariably have a dreaded switcher, probably built cheap by some mega producer? Hmmm... you might be better off with that phone charger. Or, maybe your modern Hat designer did enough with a decent design, and it rejects normal noisy USB power, and the "audiophile" needs are satisfied with either?
You just need a well regulated supply that won't sag under your load. And a solid, low resistance cable that won't drop too much getting it there.
I have had cables be the culprit, with an otherwise OK power supply, on several occasions. You want one with heavier wires, there are some amazingly thin ones out there, though this isn't often documented. Try ones that say they're for charging rather than just data. If they document that... A cheap, super thin wire cable will have more drop, as TOP has said, and with enough drop your voltage goes too low. A "5.1V" vs a "5V" would give you another tenth of a volt to lose over a cable, so that would be leeway. If the supply is doing that.
For testing, try the following. It might be worth trying to create as much load as you can, run high bitrate radio, turn on resampling to high bit and data rates, maybe other CPU using features, for a worst case current draw. Then try swapping different cables in and out, and reading the log for undervolt events. That's the quickest, easiest thing to do.
Then try swapping the supplies, with a known "better" cable. Standard process of elimination process. Hopefully you end up with at least 2 good cables, throw away a few bad ones, and find both supplies worthy.
Interesting note on the "5.1V" battery. There is no such thing as a 5.1V battery. All "batteries" that provide a USB compatible voltage are batteries
(usually single cell Lithium at 3.6V or therabouts) with a switching power supply to bring it up to 5 or 5.1V. So, you're actually at the mercy of the switcher supply in the battery pack, as to stiffness under load. If well designed, that would be better than a raw battery. I'd normally ask if you had a decent meter to measure the voltage, but it's difficult on USB systems, since it's hard to measure at the connectors unless you make an adapter tap cable or something, and probing on the Pi happens to be very hazardous, in that the convenient spots near the connector happen to be very near places to blow out stuff if you slip. (Don't ask me how I know!!) Be careful if you try that.
An aside about these USB "Batteries". This makes me giggle a bit, as a hardware electronics guy, about people hoping for better "audiophile" effects of running off a battery, but using one of these USB supply packs. An actual battery is a pretty low noise power supply, if that's going to be important. USB battery paks invariably have a dreaded switcher, probably built cheap by some mega producer? Hmmm... you might be better off with that phone charger. Or, maybe your modern Hat designer did enough with a decent design, and it rejects normal noisy USB power, and the "audiophile" needs are satisfied with either?
You just need a well regulated supply that won't sag under your load. And a solid, low resistance cable that won't drop too much getting it there.
