Amp up USB port power (no pun, well maybe)

I wish I had a good answer.

A PC USB charger delivers 2.5 Watts of power (5 volts at 500 mA). An iPhone charger delivers 5 Watts (5 volts at 1000 mA). A Retina iPad mini charger delivers 10 watts(5.1 volts at 2100 mA).
- Choose the Right Charger and Power Your Gadgets Properly | WIRED

I don't have the USB output specs of the Prius USB port. I do know if you put a 12v to USB adapter in the power port, you likely are getting more voltage to charge your device as you should be getting the full 12v battery behind it.

KEEP IN MIND that any 12v to USB adapter you buy can restrict the maximum voltage/wattage you get to the USB port for charging.

For my motorcycle, I got this to recharge my iPhone when on the road:

As you can see, the specs are 12v input, 2.1a / 5v output ... Pretty much what an iPad charger puts out. It's a bit overkill, but I saw no point in going to a slightly cheaper cable adapter that converted to less power. If anything it lets me charge my phone faster so I can "top off" while stopped someplace to eat rather than leave it plugged up for several hours.

 

Same phone. Ditto problem. Set on low power mode while using google apps. Charges fine. No mod. Costs zero.

BTW. It's not under delivering a charge. DW has iPhone 6s Plus without issue. My 6 battery is dying. One of the symptoms especially when using high drain apps like GPS or 3D games, the battery when old is unable to tell the phone what the correct charge is and will rapidly ramp down. First time it happened on my phone, I thought same as you. But when I plugged in at home, instantly jumped to 40%.