Help needed. I just ordered 4 - 450 watt panels, 2 - 100 ah 12 volt batteries and a150-60 mppt. I plan on connecting the panels in parallel and series. What else do I need to get 12 volt for use if I’m charging at 24 volts ?
You need a 24V to 12V DC to DC converter or charger.
See the Victron page linked below. A converter has a constant output voltage. A charger has a proper 3 stage battery charger algorithm but can be set up to work as a converter with a constant output voltage. There is a wide range available depending on how much 12V power you need. They can be set up in parallel. The new Orion XS 1400 can supply up to 50A at a nominal 12V. If you need short bursts of high power it can help to have a 12V buffer battery.
You said I should use a buffer battery. Should I just connect to the start battery?
No, I said a buffer battery may be useful. Yoy have to decide what average and peak loads you need to cover. If for example the max load is 16A and the average is 12A then just get a 20A converter no battery. If peak load is 90A and the average is 12A then a 20A converter and buffer battery may be better thsn 2 x 50A converters.
If you use the starter battery and things go wrong you could run the starter flat. You need to be very careful doing this.