Is it time for a new battery for my iPhone 4S?

Just got a second hand iPhone 4S with iOS 7 I had iOS 7 on my last one which I lost anyway to the point I'm talking 1-2% every minute or 2 my brightness is down, location services are off, Bluetooth off etc 3G is ON although before I could easily get 7-8 hours on 3G I'll be lucky if I get 4 on this using it a lot, should I get a new battery?

**Also the other day it cut out at 20% a few times though it's going down to 1% now**

How old is the phone? After a couple years, the battery is likely to show diminished capacity, although it will still work for much longer, depending on how willing you are to tolerate reduced battery life. If it's not lasting long enough to please you, just get it replaced. It's not free but it's usually cheaper than buying a whole new phone (then again, a bad battery can provide a handy excuse to get a better phone, if you want one).

If your contract is up, then just get 4S on a new contract, 4S is free on a 2 year contract so you can get a free upgrade that way. Thats better then putting money into an older version, you can even trade it in now and get something back for it.