We purchased the iPhone 13 today from Verizon. The salesperson stated that we should only use the 20w fast charge block with the 13 and it will damage the battery if it’s left on the charger overnight. Is this true? Will it hurt anything to use the charging block that we used with our iPhone 11 to charge our iPhone 13 overnight?
iPhone 13
No. Everything is forwards and backwards compatible. The phone itself controls the charging and will adjust to the amount of power that it has available. It will stop charging when it needs to; it won't have any sort of "trickle charge" when it's considered full. It will actually stop at a certain charge level and will be allowed to drop (from supplemental current and/or self-discharge) to a certainly level before it starts a short "top off" charge where it will stop again.. Internally, it charges a battery at under 4.3V and any compatible power source will be converted to the proper DC voltage inside the iPhone. It's really just a DC power supply and there's nothing specific about one that make it damaging or safe based only on the power.
This shows how it's done, although a lower available current would mean the first stage is considerably longer before it reaches the "saturation phase". You'll see stage 3, where it's not charging yet dropping, then stage 4 where it continues. If it's left connected to power, it will just repeat stages 3 and 4 indefinitely. This is perfectly safe.
If anything, slower charging is beneficial for slightly better battery longevity as it creates less heat when charging. But it's only a tiny benefit, and one that many would gladly trade for faster charging.
No. Everything is forwards and backwards compatible. The phone itself controls the charging and will adjust to the amount of power that it has available. It will stop charging when it needs to; it won't have any sort of "trickle charge" when it's considered full. It will actually stop at a certain charge level and will be allowed to drop (from supplemental current and/or self-discharge) to a certainly level before it starts a short "top off" charge where it will stop again.. Internally, it charges a battery at under 4.3V and any compatible power source will be converted to the proper DC voltage inside the iPhone. It's really just a DC power supply and there's nothing specific about one that make it damaging or safe based only on the power.
This shows how it's done, although a lower available current would mean the first stage is considerably longer before it reaches the "saturation phase". You'll see stage 3, where it's not charging yet dropping, then stage 4 where it continues. If it's left connected to power, it will just repeat stages 3 and 4 indefinitely. This is perfectly safe.
If anything, slower charging is beneficial for slightly better battery longevity as it creates less heat when charging. But it's only a tiny benefit, and one that many would gladly trade for faster charging.
Verizon likes to sell things.
If you have an older 5 watt power adapter and cable and want to use that with the iPhone 13, that is fine. Apple encourages it. See the following from Buy iPhone 13 and iPhone 13 mini - Apple
We encourage you to re‑use your current power adapters and headphones that are compatible with these iPhone models. But if you need any new Apple power adapters or headphones, they are available for purchase.
Also see the Compatibility section of Apple 5W USB Power Adapter - Apple where it states that it is compatible with the iPhone 13.
Finally, it will not harm the battery to leave it connected overnight.
Short version: No it will not hurt your phone.
iPhone 13 charging
