I mostly keep my devices charged up using chargers that have been tested and I trust, but there are times when I'm out and about where I'm forced to make the choice between using a public charge point, or running seriously low of juice.
Having a USB "data blocking" condom is a cheap way to have peace of mind.
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The SyncStop USB Condom comes in two types -- cased and uncased. The cased version looks like a small innocuous dongle, and the "Apple" white color means that it is unlikely to draw attention.
You pop it on the end of a charging cable, and then charge as normal.
There's also an uncased version, both designed for USB-A. This is essentially a bare circuit board with a USB connector on one end, and a USB port on the other. There are a few resistors on the board, but apart from that, it allows power to flow while blocking data.
The fact that it's uncased not only makes it smaller, but also allows the ultra-paranoid to make sure it's not been tampered with.
SyncStop USB Condom (in pictures)
The cased version is priced at $12.99, with the uncased version coming in at $6.99.
I've tested this extensively and found no problem with charging when using SyncStop, with the dongle allowing up to 2.1A of power to flow across it, with the device allowing absolutely no data connection between charger and smartphone or tablet. There's also no overheating or noise.
I had to find out what was inside the cased StopSync dongle (people have asked me), and after liberating the interior from the very solid plastic casing (without liberating any blood), I discovered -- as expected -- that it was very much like the uncased version.
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It's worth bearing in mind that, while a data blocker can prevent data transfer, the port might still be able to damage a device, either through damage or misconfiguration, or malicious tampering.
It's a shame that SyncStop don't make a USB-C-to-USB-C version of the USB Condom. There is a company that does -- PortaPow -- and you can find that on Amazon. That said, public USB-C charging ports are not that common yet (which is why it's a good reason to have USB-A-to-Lightning or USB-A-to-USB-C cables when traveling).
Highly recommended bit of kit for travelers.