If power is all you are looking for you can always play it safe with a USB Condom.
The normal PortaPow tells the device that it's a charger.
The extra paranoid version with no components on the board will often limit charging, it depends on how the device treats a complete lack of data pins.
But if you're worried then get one that has a resistor and only a resistor.
If you want to block attacks that use the external data pins, then a USB condom will keep you safe. Regardless of whether it has a chip in it.
If you want to block standalone attacks from a malicious cable, then a USB condom wasn't going to help in the first place. For standalone attacks in particular, the risk from a chip-having condom is similar to the risk from a cable, but a cable can use bigger and scarier chips than the one in the PortaPow.
Transparent casing, no-chip design and custom made USB connector with data pins visibly removed means you can be sure the blocker is secure.