No service on water damaged Iphone 4

My Iphone 4 has water damage. DIrectly after it had contact with water, I put the phone into a bowl of rice for 24 hours and everything was working fine. It worked perfectly for a couple of days, then it started randomly turning off while I was out and wouldn't turn on again until plugged into a power source. When i would plug it in, the "low battery" symbol would appear, but it would then turn on and say it has 70% battery or so. It would then take several hours to charge 20%, and would randomly turn off and turn back on and randomly get signal off and on. Now, I cannot take it off the charger without it turning off, and it lost it's ability to get service completely. Should I replace the battery before buying a new phone or is all hope lost?

Please help!

Thanks