Can a 7Ah UPS battery start a car by itself?
Short answer: Probably not.
Long answer: Maybe. For a start, it depends on the size and type of an engine you're trying to start. Diesel engines require more power to start, due to their high compression ratio and of course, larger engines (more displacement) will also require more power, for obvious reasons. The engine in this video is 1.6l inline 4 petrol, so it shouldn't require that much to start, but for a 7Ah battery it's still quite a heavy load. As said in the video, some power is wasted in the wires, as they aren't that thick, really (2.5 or 3.3 sq. mm, not sure exactly), but due to them being short, the voltage drop isn't that bad ("only" about 0.5-0.7V at 100 amps) and the starter on this car is rated at 800 watts, so at 300 or 400W it's receiving in this video, I don't think that giving it another 50-70W is gonna help enough to actually start the engine if we let's say "used thicker cables". But if we used a new battery and not one that's 2 or 3 years old + thicker cables, then maybe, but I can't say for sure. With a smaller engine (like 1.0l or 1.3l) it could work the way it is in this video, perhaps. Also, if you're just trying to jump start your car, you can certainly do it this way and that's true even for larger engines. Just connect the UPS battery in parallel with your car's battery and wait for a minute or a few minutes - while you're waiting, the small battery will charge the large one which will then be able to provide enough current to start your car (some current will also be provided by the small battery if it's still connected while you turn the key). If your car's battery is not that discharged (only so much that it barely won't start), you may not even have to wait before you try and successfully start it.
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