Even voltmeter might not tell you everything, unless you have one with memory.
The computer is much faster than human eye, or meter inertia. Meaning it might abort the start in 1/10 second (actually I don't know how fast it is, but it is possible
)
So I would get heavy jumper cables and try it again. The #10 wires commonly sold are good for 4-cylinder Fiats.
Or new battery.