Ok make way stand aside ...
This is my bag..... I know a little about this.
Ok alternator on engine is designed to charge a specific battery and run electrical systems that are necessary. This is why alternators over the last 30 years have grown from 35 amps and 50 amps to 90 amps and up to 200 amps and over.
Modern engine management systems can consume loads of power now days, ( pumps, ecu s, injectors, radios, air bag systems ad blue pumps ect.
Battery technology has changed and so have invertas
The cheapest way of doing this is to kick the living poo out of the system you have already and sacrifice a bit of battery life then invest in a better battery. ( A lead acid battery destined to take partial charges and operational current. ) may be the cheapest option. Charge managment then gives better longgevaty. A proper system could cost a lot to build and run.
You don't get oat for nawt.
But I can help.
Other wise you can end up spending loads for no gain. Trust me.
A bit more info is needed to get the best from your system
What size inverta and what are you running from it? and what 12 v load do you want? I.e. What is the load you want to achieve?
What driving hours do you do?
When do you want to run this load during driving or at rest.
I know it's all boring stuff but my job is generators, hybrid solutions, solar and wind turbines and batteries. That's what I do and I have over 150 bits of kit on the hinkley point power station job in Somerset. This is my bag baby.
You can message me if you like and I will issue a phone number
Services are free to mates.
I now have over 50 years experience in these matters