How do chargers with different output values work?

The USB-PD standard allows for multiple output voltages on USB-C devices, but every USB-C power supply must initially output 5V and communicate with the device on the other end. If the other device does not specifically ask the power supply to output more than 5V it will just continue to put out 5V and no more.

Can I charge my phone with 5V 2A?

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Can I use a 5V 2A charger with a 5V 3a device?

Yes, it is absolutely safe to charge a device with a charger that has more current capacity than needed. Since the voltage is held constant (5V), the only factor that determines current draw is the load (another term for resistance) the device places on the charger.

Does the output of a charger matter?

Power output has no meaning if the voltage is wrong. If the voltage output is wrong, it will either damage the device or battery, or not charge at all. It’s best if you also use the specific charger recommended for your device.

Can I use 6V instead of 5V?

If you’re mildly comfortable with electronic circuits, you could convert the 6V adapter to 5V. This can be done by hooking up the output of the adapter to a simple 5V linear regulator (the 7805) — the dropout would probably be low enough at 200mA to supply 5V.

Can I charge a 6V battery with a 3v charger?

No. As tomnexus has pointed out you need a voltage higher than that of the battery before it will charge properly.

Can I use 5V adapter for 6V?

Most of the times it won’t. Cannot tell for sure without checking the internal hardware of the repeater. But if it works without generating heat it will be fine as far as I know. The 5V adapter might generate more heat than the 6V as sometimes the lower voltage will increase the current draw.