hi i changed out the battery in my 2004 mustang mach 1 the dealer gave me a hand me down battery and i had a new red top optima i put in and also the pos connecter was bad so i cutt the power wires from the old one and put them in the new one which has 2 8 gauge ports and two 4 gauge ports on it for wires to go in and ever sence i have done that when i shut the car off the battery gauge stays on along with the gas gauge and now when i fill up the gas gauge takes awhile to go up to were the gas is and it use to jump right to it but all other gauges go back down also the service engine light is on and stays on and the ABS light i have tryed to disconect my battery 5 times to see if it works and nothing can someone please help me out with this thank you

 

and also if i let my car sit for just one day without starting it the battery will be dead the next morning

 

am I understanding this correctly that you put two mis-matched batteries, with one being used? I want to help but Its hard to tell with the paragraph being all one run-sentence like it is. Be happy to help but if I need a translator to go from text-engrish it will be much harder...

 

no what i did was i took the old one out and i put a new one in and after i did that is when my gauges stayed on.

 

did you get your alternator tested?

 

post a pic of the battery terminals you installed. Curious if you have them on there tight enough.