89 fth ave 318ci please read if you know a bit about it's problem?

My gauge goes to Discharge until i accelarate then to charge.I figured it was the battery so i bought a new one.Well the new one seems weaker then the old one.The belts sqeel a bit.Should i buy some of that belt adhesive? I don't think it's tha alt cause like i explained it charges at a fast idle or when driving faster,let off the gas and it goes right back down into the discharge zone with the red warning light.