question

The single 120Ah 12V lead-acid battery is massively undersized for a 3000W/12V inverter.

Use fuses on all positive wires connected to the battery.

Panel Voc is 21.6V so 21.6Vx3 = 64.8V => plenty of safety margin to the maximum 100V allowed by the 100/50 charger.

Isc = 6.24A, one string so 10AWG (~6mm2) should be fine for PV wires (since it's for a car, I assume they are not extremely long). Use a 10A fuse on the positive wire (CH10x38 gPV type) or one such fuse on each wire (one on positive wire, one on negative wire).

You can also add a DC disconnect switch, too (between the fuse(s) and the solar charger) - should be rated for at least 100V DC / 15A (for some safety margin). Should disconnect both the positive and the negative wires from the panel array.

Use a small connection box for these.

On the other hand, many connect the panels directly to the solar charger. Your choice.

The 100/50 solar charger can output 50A current to the battery/inverter, but with a 300W PV array, the maximum is about 25A. So you can use this 25A current to size the wires between the charger and the battery (and the fuse on this positive wire). 6 AWG should be fine (again, assuming not excessively long). You can even use 7 AWG (10mm2) since you won't reach 50A. Use a 32A fuse on the positive wire between the charger and the battery.

For the wires between the battery and the inverter, if sized for 3000W, that means about 250A. Massive current. Massive wires. 6 AWG is tiny for this.

Not really worth considering a 3000W inverter for 12V. Even for 24V, 3000W it is kind of big. This is 48V area.

You could/should include DC busbars in your design.

So sorry for the long delay @Seb71, it wasn’t because I'm not extremely appreciative, but rather, your response really threw me by revealing a classic case of 'fools rush in' on my part; as in, I believe I possibly could run/draw 115-140A for 6mins from the Fullriver - but i'm likely to prematurely degrade the battery fairly quickly (now that I've read-up on a few things).

So I'm in a position where I'm desperate to get confirmation on the design now with your added suggestions—and whether changing to two Lithiums in parallel (200amps continuous) would allow me to run a device drawing 115-140A for 6-10mins?

Worst case scenario, I will keep all that I have above, and just run my camp gear without this large load - but if it is possible to use 2 x lithiums to power the 140A, however - it may really entice me to make the switch.

Thanks again for all your help - Peter

So the maximum load is under 1400W.

Still too much for a single 120Ah 12V lead-acid battery.

Two 100Ah LiFePO4 in parallel (or better in series) should be able to supply that power for 10 minutes (and even more).

LiFePO4 batteries are expensive. Most people use them with a BMS, which is also expensive. If you sink that kind of money into this system, really think about switching to at least a 24V system. If you already got the 12V inverter, sell it. The 3 panel string Voc is still high enough for a 24V battery. The charger can use a 24V battery, too.

You could also consider using the solar system (with the cheaper 12V lead acid battery) only for small loads and getting a generator for that 1400W load which only runs for 10 minutes.

Or if that load is an oven, change to propane.