MacBook Pro 87W Adapter and S9

You're also advised against using unofficial chargers to charge your smartphone, as it can lead to battery damage and even thermal runaway.

The smartphone ans battery was made to accommodate the charger that you got with your phone, so it fits perfectly with that. It isn't made to work with the MacBook charger, so there could be unforeseen consequences. Lithium-ion batteries are actually quite fragile batteries, which is why there is so much software/hardware to monitor their charging, unlike other battery types.

If the MacBook charger is putting 9v and 3A into your smartphone battery, then you're just ruining your smartphone battery rapidly, and it is at risk of becoming way too hot. Your smartphone typically has a nominal cell voltage of around 3.7 V, which means that if you have 9V and 3A going to it, then you're putting way way above 3A into your battery, which is way too much.

The second mode of 5V and 2.4A is a lot closer to the official charger, and could potentially be alright, but it is going to increase the deterioration of your battery a little bit, as a result of the higher charging rate, and the increased temperatures that come with it. Li-ion does not like deviating from room temperature, the further it deviates, the worse it is for it.

You should just make sure that the MacBook charger is using the second mode, as the first mode is seriously not safe.

/r/GalaxyS9 Thread