Get one of those "USB powerbank"s.
They're dirt cheap. If you don't know what they are, they are one or two 16850 LI-ion cells, a mini-USB port, and a USB-A port. The mini-USB port is used to charge the cells in the powerbank, and then you can plug your gadgets into the USB-A port, to charge them later.
Use the powerbank to suck the power from a public port first, then plug in your devices. The downside is that the whole process takes longer. The upside is that all you're risking is blowing up your powerbank. That sucks, but as I said, they're dirt cheap, and you just get another one.