> how does building a battery pack out of individual cells scare you? You drive next to one every day. Tesla uses 18650 batteries in early models and now has switched to 32650 and larger batteries but that's literally what they used and stitched together lol.

Early on a number of Tesla cars caught fire until they corrected it:
https://en.wikipedia.org/wiki/Plug-in_electric_vehicle_fire_incidents

When the Boeing 787 dreamliner was made, they had batteries on the planes catching fire:
https://en.wikipedia.org/wiki/2013_Boeing_787_Dreamliner_grounding

Companies like Tesla or Boeing rely on a team of engineers and advanced robots to make batteries. Even then they still had issues with fire early on.

If you will be welding 286 batteries - if you get one of them backwards or have a bad weld - it can cause overheating and lead to fire.

Running a 300 watt appliance depends on whether or not it's native DC. If you have to use an inverter to convert voltage, easily add 20% on top of that. If you want some safety margin, add another 20-25% to factor in battery degradation with time.

300 watts x 8 hour = 2.4 kw hours x 1.2 for inverter losses = 2.88 kw/hr x 1.25 degradation margin = 3.6 kw/hr.

3.6 kw/hr / (3.2 volts x 4) = 281.25 amp/hours at 12.8 volts.

3 deep cycles marine batteries is 303 amp/hours, easy to charge and wire, and costs $240:
https://www.walmart.com/ip/180350522

The main caveat is at 45.4 lbs each it will weight 136 lbs.

4 LifePo4 big boy batteries is $367 (random retailer for reference):
https://www.aliexpress.us/item/3256804647596547.html

A 300 watt load / 12 volts = 25 amps. If we over-spec to 350 watts / 10 volts low battery = 35 amps. Add $15 for a BMS:
https://www.aliexpress.us/item/2255800625909430.html

35 amps = 9 AWG wire.

A nickel strip is good for 5-7 amps. Stack 3 or so max if you have a high powered spot welder and you're at 21 amps.

Teslas work at 300-400+ volts, not 12 - that's how they do it.

18650s are possible if you target 5-7 amps.

400 watts (overspec if you ever want to charge your phone or something) / (5 to 7) = 57-80 volts = 72 volts = 24S battery with 18650s.

2.88 minimum or 3.6 kw/hr ideal / 72 volts = 40-50 amp/hour battery = 22-28 cells in parallel.

With 1,800 mah LifePo4 you're looking at 528 - 672 18650 cells.

With lithium-ion you'd be looking at 20S x 12-14 cells in parallel = 240, 260, or 280 18690 3,500 mah cells - consistent with your math of 268.

And at 72 volts, you could use anything below 22 awg wire.

A car aux power port typically has a 10-15 amp fuse and is good for 120-180 watts. 20 amps tops could push it to 240 watts. 300+ watts is beyond what that's rated for.