iPhone 5s battery draining

I have an iPhone 5s on IOS 12.4.6.. I know this is an old phone but it has no damage and over time it seems the battery does not hold its charge which is to be expected however over the last 3 days the phone has gone from 100% to 1% over night and I have to keep charging during the day. I have looked at the battery usage in settings and over the last 10 days for the first 7 days the battery usage has been less than activity, but the last 3 days the usage has been more than activity. I was thinking it was about time I invested in a new phone but this morning the phone has gone from 100% to 75% overnight. Is this a battery problem, is it worth replacing or has this phone had its day. Nothing has changed apart from normal updates for the apps that I use, no new apps etc. Advice please.