Thanks a lot. Now, I hope my following question is not too stupid: but where is actually the subwoofer (and the wires going to it)? It is making such a low noise that I found it a bit hard to locate (sorry, my lack of decent hearing I guess too) ;-)
Yeah, indeed 300W is not that much, but actually I was thinking about using something like that for on camping trips, where I just need to have a small fridge, and some small (usb alike) chargers etc. powered. For that, it makes sense to keep as much as possible of the car's protection circuitry in the loop.
For a battery, to take the initial inrush current, would a li-ion battery like the one for jump starting also work?
like this one:
Buy DBPOWER 800A 18000mAh Portable Car Jump Starter (up to 7.2L Gas, 5.5L Diesel Engine) Battery Booster with Smart Charging Port, Compass, LCD Screen and LED Light (Black/Red): Jump Starters - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
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(not sure whether it charges via the large 12V leads, though).