I have a duralast battery. 700 CCA. 875 CA. 120 RC. Now, I have an aftermaket system in my car. I was wondering if this battery could be causing my car'* voltmeter to act flaky at times. It usually happens in the morning. It may happen intermittently during the day at times. If yall could help me i would appreciate it.

A better description of 'flaky' would help. Particularly any other symptoms you notice. Lights, radio, or other accessories acting up?

My voltmeter goes down toward the red when i put on brakes. Later on in the day it smooths out. Thats why this is flaky.

You either have a short in your brake light circuit (possible, but not likely) or your alternator is weak enough that when the engine loads down under braking in a drive gear, you get low output.

Honestly i think that battery is not putting out the amount of power i need for my setup. Alternator might be goin bad too. Car cranks up fine everytime though, so i dunno could be anything.

ill probably let my mechanic check it out. whats a good voltage in case i do get another alternator will?

You got some stuff a bit mixed up here.....

The battery 'only puts out enough' when you start the car. You run off the Alternator when driving.

You should have at least 14 volts at idle. It may tend to be higher for the first several minutes of driving as the alternator recharges the battery.

ok. that makes sense. if i get another alt, whats the best voltage to get?

you can buy a 140 amp alternator, and that would be your best bet with an aftermarket stereo

Bet. I will keep that in mind guys.