I have to define the current needed to trigger a squib (you can imagine this as a fuse). The parameters I have are its impedance (1.5 ohm) and the energy necessary to fire it (4 mJ/Ohm).
Is there any way to calculate the current needed (let's say I will connect it to a 1.5V battery)?
I didnt consider it this way, but it is the same thing... echo47s answer tell you that you need 1Amp for 4milisecs to fire it, but my answer tells you that you need 4mA for 1 sec to fire it...
I've never worked with squibs, but I would expect the 4mJ energy spec to be based upon a particular applied voltage stated in the data sheet. 1.5V seems low, but maybe it's sufficient. The squib won't ignite if the applied voltage (or current) is too low, no matter how long you wait.
A 1.5V battery doesn't deliver 4mA into a 1.5Ω load.
That's better info. It says the device will never ignite below 0.3A, and will always ignite above 0.6A. Those values probably assume certain environmental conditions such as ambient temperature. 1.5V should ignite this device.
Your 4mJ/ohm device also has such parameters, but it's anyone's guess what the values are.