My 8 plus battery keeps draining even though I have changed the battery

The phone, and the battery, is getting on for five years old - or at least a couple if it's a replacement - and is probably well below optimum.

Check in Settings > Battery > Battery Health to see what figure is given. If it's something like 80% it is time to get a new iPhone, or a new battery.

Kwesiforson ~

About genuine iPhone batteries

Repair Provider Verification

...Found by searching Apple's Knowledge Base.

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Who changed the battery?

If you did it yourself, or had it done by an unauthorized service provider, your phone is no longer supportable.