Hello was hoping somebody could tell me what size lithium i need to run 6000rms? Also would it be better to run a larger lithium or a smaller lithium plus cap bank?
If it's from a second battery you take worst case scenario on voltage, which is about 12v. So 6000w / 12v = 500 amp continuous discharge current which should be your primary concern. There's not very many needs for caps out there, this definitely isn't one of em. Something like a SLIP40AH or equivalent would be right on target for handling the upper limits of that load without significant vdrop. If you've got a decent AGM under the hood you can probably knock a little bit off the requirements of the lithium battery.