What can I do if Auto battery voltage is going up and down

i just replaced the battery and starter. when i drive the battery surges and when i brake it goes back down. this is not right what else do i need to do or get??

I'm a Parts Professional, not a Tech but will ask some of my Tech Customers about your problem once I understand it fully.
Exactly what do you mean when you say, "The battery surges"? Batteries don't surge. Do you mean the voltage in the car is going up and down? IF the voltage is going up and down, I'd go down to your local O'Reilly Auto Parts and have them do a "charging system" test on the car. Have them do the test at idle AND at about 2,000 RPM. IF it looks odd, remove the alternator and let them check it on their machine.
Keep in touch and I'll do all I can to help. Please remember, I'm a Parts Professional, NOT a Tech but I'm willing to try to help you get your car fixed..