Battery problems - totally errant indication, and randomly failing

My iPhone 5S is about 18 months old, so I realise the battery is going to be flakey.

However, some days the phone dies (claiming battery dead) when on anything as high as 40%. I then connect a charger, it goes through the usual powering on, and the indication will pick up from where it was when it died (e.g. 40%!).

I've tried running it down completely and then charging without disconnecting, but the problem remains.

Any suggestions? I'm reluctant to pay for a new battery without exploring further.

Thanks,

Ed