Charging iPhone using 5W and 20W at one charging cycle

Yes, it is perfectly fine to do so. Apple’s iPhones are designed to handle different charging speeds, and the internal circuitry adjusts to the current provided by different chargers. Therefore, you can charge your iPhone with a 5W charger up to 50% and then switch to a 20W charger to finish charging, or vice versa.

The main difference you’ll notice is that a 20W charger will charge your iPhone faster than a 5W charger due to its higher power output.

Remember, regardless of the charger you use, it’s recommended to avoid charging your iPhone in extremely hot conditions to prevent potential damage to the battery over time.

As always, make sure you are using genuine Apple chargers or certified third-party chargers to ensure the safety and longevity of your device’s battery.

Mysteriokiller wrote:

Doing so does it affect my iPhone battery health…

No, it doesn’t. It doesn’t hurt or help your battery health. And it is pointless. Fast charging only fast charges to 80%, so you are slow charging to 50%, then fast charging 30% more, then it slows down to the 5 watt rate past 80% regardless of which charger you use. You are overthinking this. Just charge overnight, every night, with any charger. You can use the 5W, because it doesn’t matter how long it takes when it has all night to charge. Or the 20 watt if it is more convenient.

Your battery capacity will decline over time no matter what charging strategy you use. Following the advice posted by Bob Timmons will slow the decline of your capacity, but it won’t completely stop capacity loss.

When it falls to 80% have Apple replace the battery

It's OK, but it will take longer to charge the phone, and it won't help the battery health or life.

Please read what battery expert Lawrence Finch has to say about charging the phone.

There is never a time when any smartphone is doing nothing, so it will use energy 24/7, and the amount used will vary from day to day and minute to minute. For that reason alone you should charge your phone overnight, every night. In addition, the absolute best way to get maximum use on a charge, as well as slow the decline of battery capacity long term is to enable Optimized Battery Charging (Settings/Battery/Battery Health) and charge the device overnight.

The battery will fast charge to 80%, then pause. During the nighttime pause the phone will use mains power instead of battery power, allowing the battery to “rest”, and thus reducing the need to charge the battery quite as often. The phone will resume charging to reach 100% when you are ready to use your phone; it will “learn” your usage pattern. 

If you enable iCloud Backup (Settings/[your name]/iCloud - iCloud Backup) the phone will back up overnight also, assuring that you can never lose more than the current day’s updates. Here's more information→About Optimized Battery Charging on your iPhone - Apple Support

We already said above that......."it won't help the battery health or life". But, it's OK if you have the time and patience to constantly watch over the battery.

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