There is a wealth of information on the message boards you can read through. Here’s a good place to start -
If you give us more info, we can probably give you some better direction e.g. what car do you have? What are your driving goals - speed, distance, hill climbing, range? If you want all, could get expensive and require changes to gears and motors. Budget?
Folks on here have come up with some pretty straightforward, drop in options (again, not knowing the specifics of your car). For example - New 22s 82v 90ah+ Gem battery Available now - #4 by Inwo
That kit, you’d just have to connect b+ and B- and you’re running. Depending on your car, could need charger programming and a magic magnet to remove the speed limit. 2016+ are a little trickier to get over 25mph.
Lithium is a game changer with these cars and really unlocks their potential. When I bought my 2016 (upgrading from a 2000 with lithium and an AC setup… which I wouldn’t recommend doing in a pre-2005 car $$$), I had zero interest in driving it until I got a chance to install lithium and mod it to go faster than 25 mph.
I don’t know anyone on here that’s successfully installed lithium and regretted it.