20 Watt charger harmful for the battery of a device?

I'm considering to buy a 20 watt charger for my iOS-devices. Obviously it charges a device two to three times faster then the 5 watt chargers, which I currently use.

I'm not sure about that, the BMS on the iOS device should alter its power draw to match what it's programmed to accept, given the current state of the battery.

If 5 W is the most it can charge at, then plugging it into a 20 W charger isn't going to force more than 5 W in.

If your device is capable of drawing more than your current charger provides then you'll see some improvements in charge time with a higher capacity charger but not necessarily by the ratio of how many watts the new charger provides vs the old one, ie. 20/5 = 4x faster. The BMS has to accept 20 W and even then things are a bit more nuanced as 20 W would be the maximum, not continual power draw.

Might it reduce the life-time of the battery? Or is it even dangerous (strong heating up, explosions)?

It really shouldn't, unless there's an existing flaw in the battery or charger.

Should I definitely get an original Apple charger? Or are others vendors safe too?

I think that's going to be tough to get a definite answer for unless you have a teardown on the specific device, from someone qualified to comment on its quality. I've seen some teardowns of USB chargers with terrible isolation and there have been cases of people being shocked while using their phone as it's charging.

I'd put money on there being plenty of properly made, safe chargers out there. The question I ask myself is: do I feel safe plugging a $1,000+ phone into this to save a few dollars, or is my peace of mind worth the ~$30 for an OEM part that I've seen a teardown confirm that at least the design is sound?

A higher wattage charger will not harm the device, but it will also not charge a device any faster than a matched rating charger.

The power or current rating of a charger (or any power source) is its delivery capacity. The power or current rating of a device (or any load) is what it draws. No matter how much power or current a source can deliver, a load will only draw up to its own rating. Conversely an under-rated source will be insufficient for the load. In general, using an over-rated charger is a waste of money, but it won't cause harm.

USB is USB is USB, the brand doesn't matter, & any exception is equipment that differs is breaking USB spec. USB-C delivers 5 to 20 Volts (negotiated between source & load) at up to 5A. USB-A tops out at 3A & is 5V only. That's why USB-C can charge devices that support it faster.

If you have multiple iOS devices I suggest a better choice would be a multi port USB-C charger with ports of at least 5W capacity.

P = V x I, where P is power in Watts, V is Voltage, & I is current in Amps (all 3 units are capitalised because they're people's names). So a 5W USB-C charger could be delivering 5V at 1A or 20V at 0.25A, as negotiated between the devices. A 20W USB-C charger could deliver 5V at 4A or 20V at 1A, as negotiated. A top rated USB-A charger will deliver 5V only at 3A = 15W, but many supply lower current for lower resultant power.

20W USB chargers are not dangerous to use with an iOS device. A non-defective USB charger coupled with a non-defective iOS-device will not overheat, explode, or reduce the life-time of the device.

It is perfectly valid and normal to use a 20W USB-charger with an iOS device.

Any standards-compliant charger is fine. Apple chargers follow the same standards as chargers from other vendors.

You might want to stay away from the cheapest, no-brand chargers, as the quality is usually not that good.

Note: A device won't necessarily charge faster with a higher capacity charger. Each type of device has a maximum wattage that they can draw from a charger - having a charger with a higher rating doesn't speed up anything.

For example an iPhone 12 can take advantage of the full 20W rating, where as an iPhone 7 only draws 10W.

Buying a non apple charger is fine, as long as you take a reputable brand. Anker is known to be good for exemple.

Avoid cheap Amazon or Aliexpress no name chargers at all cost!

As far as battery life is concerned, yes, it will have a small negative effect on battery life: Faster charge means more heat, which is known to have a small cumulative effect on battery life.

I don't think it's particularly worth worrying about since the phones are designed to be charged this way.You could also mitigate this buy using 5W charging for night time, and 20W for quick day time top ups.

Don't forget that your phone battery WILL degrade anyway, and that the battery can be exchanged (for a fee, or free under warranty)

The iPhone 12 has a 2815mAh battery. At a nominal voltage of 3.7V, this is 10.4Wh. Charging at 20W is equivalent to a C rate of 1.92 (20 / 10.4). Typically for maximum lifespan, Li-ion manufacturers generally recommend charging at < 0.8C for optimum battery lifespan. (The other answers are correct in that the BMS in the phone will not accept charge current in excess of the design spec, but 20W is now an allowed option, at least with an official 20W Apple charger).

For comparison, a Tesla Model Y has a 75kWh battery pack. The peak supercharger power is 150kW which is a C rate of 2 (150 / 75), so scaled for battery size, the 20W charger is roughly like supercharging your phone. It's pretty well accepted that while supercharging your Tesla is safe and not catastrophically destructive to your battery pack, it also isn't ideal for long-term battery health.

Will your phone, or your Tesla explode? No, not unless something else is wrong. But is it ideal for your long-term battery capacity? No.

My recommendation is that if you need to charge fast, go for it. But if you're going to have your phone on the charger overnight anyway, why not use a slower charger in that location and save your battery a little stress?