I think a flat 20% chance would work, although i would be happy with 10% rising to 20/25%.
I'd also like to remove being respawned at your corpse considering most of the time it gives you a free revenge kill with the other player also having no time to react to you coming back to life. This could also be balanced by adding a 5 second timer before you're able to respawn instead.
Making it an actual item that takes up an inventory slot could also balance it out, for example if it takes the same slot as a jetpack, you would have to pick between the two. it could also then be dropped, and if someone picks up your card after you die before you respawn, you lose the chance.
Just a few ideas, not really sure about the last one but the item definitely needs some adjustments.

I think a flat 20% chance would work, although i would be happy with 10% rising to 20/25%. I'd also like to remove being respawned at your corpse considering most of the time it gives you a free revenge kill with the other player also having no time to react to you coming back to life. This could also be balanced by adding a 5 second timer before you're able to respawn instead. Making it an actual item that takes up an inventory slot could also balance it out, for example if it takes the same slot as a jetpack, you would have to pick between the two. it could also then be dropped, and if someone picks up your card after you die before you respawn, you lose the chance. Just a few ideas, not really sure about the last one but the item definitely needs some adjustments.