I bought a refurbished Samsung Galaxy S8 Active in excellent condition about 6 months ago, and it came with the standard manufacturer-provided power adapter and cable for charging (Model: EP-TA20JBE, labeled "Adaptive Fast Charging"). For the first 5 months it charged without problems - Fast Charging was available virtually every time I plugged in the phone and my battery typically charged from less than 20 to 100% in about 90 minutes.
In just the last few weeks though, charge speed seems to have diminished significantly, and my phone never indicates Fast Charging anymore after being plugged in, only Slow Charging, which variably estimates 4-6 hours to reach 100% from less than 20%. I've used the same cable and adapter the entire time and haven't adjusted any software settings. It seems as if this original cable and/or adapter have simply crapped out, for no apparent reason. Occasionally, I'll plug in my phone before falling asleep at night, Slow Charging is shown, but when I wake up 6 or 7 hours later the battery has gained only 1 or 2% charge (or sometimes none at all) even though the cable remains plugged in and the icon still indicates that the phone is charging.
I did a bit of forum searching on other sites and learned that the manufacturer-provided cables and/or adapters can age very quickly or otherwise be unreliable. I bought a new cable and adapter of the same type, but made by a third party, and tested it: no luck. Neither the replacement cable nor the adapter, together or in combination with either the original cable or adapter, seems to improve the charging speed.
I'm still committed to the notion that the problem lies with the cable or the adapter rather than the battery itself, since I've noticed nothing unusual about the battery's lifespan or other activity in any way - there's no excess heat either when plugged in or not; battery life is nearly as long as it was 6 months ago when I first bought the phone; I never use the phone for any games or movies or other battery-intensive apps, just Internet browsing and basic communications. Observing the circumstances as empirically as I can manage, charging speed seems to be the only difficulty. But I'm at a loss to explain why.
Should I trust the perception that the cable/adapter combination is at fault? Is it preferable to purchase a cable or adapter from Samsung that is identical to the one I'm using now, rather than one of the many third-party options? How can I regain Fast Charging ability - or, at the very least, the peace of mind to know that when I fall asleep, I'll wake up to a fully charged phone? I appreciate any insight you can provide!