When I first got my Samsung last Spring, the battery, once charged up, seemed to stay around the high 90% level for ages. Now, it seems to quickly dip down to the 40s or 50s.
I don't think we've activated anything that wasn't before (it's PAYG SIM only, no internet connection - except when it's sitting in my living room and apparently by default connects to our WiFi). Is there anything obvious I should look for?
Well do you need it connected to your home wifi? If not turn it off, if you do there are several things connected to wifi use that will drain your battery, even if they are running in the background as they are constantly hunting for updates/input. WhatsApp for example - do you use that? On my phone 25% of battery use is assigned to WhatsApp even though I only use it every 3 days or so.
Unfortunately, most of the cheapo batteries available online are "compatible" ie unbranded. They will fit but often offer poor performance....ie don't last long or even overheat. I'd avoid them.
i used to have instagram on my phone, resource hog battery included
also ate all my payg credit, i have whatsapp, but in its setting i set it to only use wifi, and that's turned off..less when i decide to go online. you could do a factory reset, and see if it helps.