iPhone draining fast

HI, thanks for replying. I use my iPhone 6 very little. But I do hotspot connect to my iPad to play a regular online game and for study and minimal web browsing. For the last 6 months or so, my phone has been draining so fast that I can’t really take it off charge. It sometimes switches itself on and off, displaying the recharge/plug in symbol then restarts again, requiring the passcode and displaying a higher battery percentage. It often flashes on and off repeatedly if, for example, I’m trying to enter many digits while phone banking, which makes the job nearly impossible and yesterday it dialed 2 other phone numbers while I was speaking on the phone, I’m quite sure it wasn’t accidental. Just really odd stuff.... Not the crisp, magical phone I saved up for and thought would last me many years.... (Sorry, not a big spender 🙁) My MacBook Pro was a dream come true also... still under extended warranty but completely unusable now that the screen cracked and can no longer be used with another screen cos it stopped accepting my password, it was my whole world and I will never be able to replace either of these beautiful devices 😞. At least help me fix my phone, please... I need it case of an emergency.

Best regards, Carolyn

iOS 11.4, very little use... except hotspot connection to my iPad

Battery health 51%

K.... I guess the battery needs to be replaced.