Ok, let’s go through it. I see, you removed the “overcharge” part of your answer, which was the most dangerously wrong part, but let’s go through the rest:
Hi and hello
The voltage range and current are common for PD chargers and the Fairphone uses QC
[Power Dilivery or Quick Charge]
They are different protocols which communicate the charging rate required by the device, depending upon it’s state of charge, but most modern chargers handle both protocols.
Nope, devices don’t communicate the required charging rate, and that communication doesn’t depend on the state of charge.
They communicate power requirements, and the value is usually negotiated at the beginning of the communication and not changed afterwards. This is important, because USB PD can also power devices that don’t actually have a battery. (E.g. a laptop with disconnected battery.)
I the Fairphone gets no response from the charger it will just use the lowest voltage and but as, according to your ‘specs’ still provides 3A it should charge fairly rapidly. (rapidly may not happen)
Nope, it’s not 3A, but 0.5A on USB 2 and 0.9A on USB 3. That’s the maximum a device is allowed to draw, if no negotiation determined another voltage/current.
The only concern is that if there no control the phone may take a bit too much when it is already nearly fully charged, you can only tell by looking at the rate of charge which isn’t easy. (This refers to the fact that a fully charged battery left plugged in will be topped up keeping the battery at an increased stressful level)
Not true. USB PD communication will change nothing at that fact. Any USB power supply (PD or not) will keep delivering power, because it knows nothing of the charge state. And no matter what kind of power supply you use, the phone’s battery charging circuit will keep the battery topped up. And that circuit also reduces the current needed, if necessary.
Ensure you keep an eye on the phone as it reaches 90% 95% and see if it says it is charging slowly, that means it is communicating and there should be no concern that it overcharges.
Not true. Again, the charging speed during topup charge is controlled completely and only by the battery charging circuit built into the phone. There is no communication happening with the power supply that would limit anything here.
Saying that, there are many recommendations, including by Fairphone that battery ‘life’ can be extended by not fully charging the battery. Some people use 80% as a max so they either root the phone to install a control app or, as I do, keep my eye on the phone.
This is true, but again, has nothing to do with the USB power supply. In these control apps, the phone shuts down it’s charging circuit. It does not communicate with the USB power supply to turn of the power.
On the note of battery ‘longevity’ it is also touted not to let the battery go too low, especially in cold climes. Although I have no routine I rarely let the battery drop below 10% nor charge it above 90%, with commonly an even smaller range.
This is actually correct.
But the main point where you went wrong is that you think, a USB power supply can actually reduce it’s power output below 5V.
To get back to the basics: The PSU can only determine the voltage. And it has a certain “power” to hold that voltage, depending on how much current is drawn.
When a PSU says, it does 5V@3A, that doesn’t mean it can somehow limit how much current is flowing. That means, it promises to be able to safely deliver a voltage close to 5V if no more current is drawn than 3A. If you draw more, the PSU will likely get hot and the voltage will drop below the rated 5V.
A PSU usually has no way of limiting the current.
So when PD negotiation failed, the phone will only draw 5V@0.5A/5V@0.9A, not because the PSU did anything, but because that’s the highest current, that the phone knows is safe to draw. There is nothing stopping the phone from drawing more, but the phone doesn’t know if that might damage the PSU. So it sticks to the safe limit it knows.