I was thinking fuses but unsure about how to proceed with that.
Would that trip the fuses on any ohm build? And would I have to run a mfet?
Your right about the first circuit yes they are on their own leg and will act individually and are only 15 amps
but on the parallel circuit they will act as one 30 amp fuse.
This is what I was taught how to increase a fuse rating and lower their resistance by paralleling them..
Fuses wiki:
The use of fuses in parallel can be advantageous. • To obtain higher current ratings than existing ranges.
So if you wanted 30a you would need 4 fuses 2 each leg for 45 amps 3 fuses each leg and 60amps would be four on each leg.
I was told do not use more than four fuses in parallel for that which dave posted they will have issues..
It's always best to use a single fuse after a parallel join. You can find threads on using shunted breakers in all sorts of applications. There's talk in some circles about using two breakers to equal on of a higher rating. I guess that's what they have and want to use it instead of buying the right one. When multiple fusing is done in commercial circuits it's usually where 220VAC or 440VAC is protected by two or three breakers or fuses, one on each leg of a 3 or 4 wire circuit with out of phase hot leads.
Before sub ohm vaping got popular a 7A resettable button fuse was sufficient to protect a mech battery from melt down. Now, lots of people are doing sub ohm without protection.
Now if I added enough fuses to let's say trip at 30amps. So what if I have a build that is lower ohms. Would it trip at lower builds if the batteries were put in reversed in a parallel build?
True, but diodes, by their nature, induce a fairly high voltage drop. Which is why people parallel PTC fuses; to lessen to resistance and induce less Vdrop in the circuit. For reverse batt protection in more complicated circuits, most people use a P-FET