Can a USB cable burn out your smartphone?

​Short answer: Yes it can! If it's a 2-in-1/Y-cables. Let's look at why they are so dangerous and whether exceptions exist.

Factor1. What voltage is transmitted through the USB cable?​

• ​Regular USB cables with any type of connectors (UsbC, micro Usb, UsbA etc.) if it is not broken or damaged cannot harm your device because safety is built into the USB standard and all USB devices are either able to accept 5V or simply won't turn on because the voltage is too low for operation. 

• The higher voltages of the fast charging standards are set between the charging adapter and the device being charged by “negotiation”. “Negotiating” is done via D+/D- pins for UsbA-MicroUSB, UsbA-UsbC, UsbA-Lightning cables, for the more modern UsbC-UsbC the devices communicate via CC pins.

• By default 5V is transmitted via USB cable and only after successful negotiation between the charger and the device to be charged, where the device to be charged informs what charging protocol and voltage it wants to receive and the charger confirms its ability to provide it, a higher voltage (e.g. 9V or 20V) is set and charging is switched to a faster one.​

Factor2. Why USB splitters, cables 2in1, 3in1 or so-called Y-cables are dangerous?

• The danger lies in the fact that at least GND (-) and POWER VDC (+) conductors in such a cable are connected to each other and if one of the devices connected through such a cable, in the course of “negotiations” with the charger, sets the voltage above 5V, it will lead to the fact that all connected cables (heads) are set to a common higher voltage.

• If you connect at this point another device to such a cable with a current above 5V, it will damage it despite the fact that the device itself could also work with the increased voltage and supports fast charging. But first of all, different devices may work with different charging protocols and the most important thing is that until the device itself asks for higher voltage, it does not expect to receive more than 5V, which can lead to power controller failure and even device fire.

• At best such a cable will only support basic 5V charging, at worst it will damage your devices.

• These hydra cables do not meet USB standards and are extremely dangerous to use.

Factor3. Are all Y-cables dangerous?

• No, not all cables, but most of those you can meet on the market.

• There are exceptions and they cost much more because safe cables are technically not a cables anymore, they have a special module that is responsible for separate power supply to each of the “heads”. In fact, such a cable contains a separate power controller that is responsible not only for power redistribution between connected devices, but can even provide charging via power protocols that could not be supported by the main power adapter.

• Different manufacturers implement such technology in different ways. I know only 4 of such cables: Anker 2in1 140W, CUCKTECH 2in1 100W, Club3D CAC-1527 100W и Cable Matters 2in1 140W - all of them were tested, they don't work the same, study the test results carefully.​

​Conclusions: Avoid using such cables, don't take any risks, a charged device is usually much more expensive than an additional cable or charger. In case you need such a device, look only for reliable alternatives that are safe to use as recommended above.