Will soon need to replace the 2 x 100Ah lead acid batteries used in the car audio set up in one of my vehicles.

Toying with the idea of getting into a DIY Lithium build instead of new lead acids.

I know nothing.

Seems I can get 3.7v 200Ah LiFePO4 cells from Alibaba at a reasonable price but thats were the trail ends.

I assume I will need some sort of BMS, but what ?

Will the 250Ah alternator in this vehicle charge 2 of these batteries or should I ehtier add a second or upgrade to a 350Ah alternator ?

Many more question but lets start there.

You will need a DC-DC converter. Something like this.

https://www.victronenergy.com/dc-dc-converters?view=card

Your alternator can't provide the voltage needed to charge the lithium battery. Therefore, you'll need a DC-DC converter to ensure the battery receives the appropriate charging profile. I don't anticipate that you'll require more than 50 amps for charging. How much power does your car audio system draw?

It's critical to use the proper size cable with proper crimps.

You will need a BMS to roll your own battery.

4 minutes ago, Mr Awesome said:

You will need a DC-DC converter. Something like this.

https://www.victronenergy.com/dc-dc-converters?view=card

Your alternator can't provide the voltage needed to charge the lithium battery. Therefore, you'll need a DC-DC converter to ensure the battery receives the appropriate charging profile. I don't anticipate that you'll require more than 50 amps for charging. How much power does your car audio system draw?

It's critical to use the proper size cable with proper crimps.

You will need a BMS to roll your own battery.

Not sure what the draw is to be honest, Ive not clamped it ( 2 x 2000w RMS mono sub amps/2 x 800w RMS 4ch amps).

The current alternator seems to be feeding the car (100Ah) and two audio (100Ah) batteries just fine.

Has 1/0 AWG wiring for all 3 batteries (Earth and Positive) wired in parallel.

8 minutes ago, Ralf001 said:

( 2 x 2000w RMS mono sub amps/2 x 800w RMS 4ch amps).

How many horse powers is that ?