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You could try a power only USB cable such as this: https://www.amazon.co.uk/kenable-Power-Charging-Cable-Extension/dp/B005HRSI3C lots of others about on the net.
This stops the phone starting any data connections to its host
You could just break the connection of the data with something like this:
If you're plugging it into a computer you can use something like this power-only USB passthrough to avoid any of the data pins from even being used.
So use wireless or buy a power only USB for a couple of quid. Handy things to have...
It's possible that you just can't.
Evident by the fact that 6th and "7th" gen iPods didn't come with a wall adapter, it may not have been intended to charge that way. In order to trigger charging, it may require input on the data line of the USB, and not just the power line, in order to charge. That's something most if not all wall adapters don't do as... there's no data coming in through the wall.
A way to test this would be to get a "power only" cable extension like this one and plug your iPod into the computer with it, and see if it charges.
Another way to test would be putting 3 volts into either pin 2 or 3 of the USB while it's plugged into a wall adapter or power bank. (the two pins in the middle. If looking at it with the plastic bar on the bottom, from left to right the pins are: 4, 3, 2, 1; 1 is 5 volt power in, 4 is ground. 2 is data- and 3 is data+)
It's definitely not about not having enough 'output' or voltage; USB ports on computers output 5 volts at 1.5 amps, and it charges an iPod just fine.