98 foreman 400fw new to the forum and Honda

So I bought a new battery 220cca amp I also put a new rectifier on it I put a volt meter to it and it’s charging at 12.7-12.9 when at idel and when I rev it up it stays at the same volts when I turn on my light bar and speakers it drops to 12.4-12.7 what could cause this thank you