The spell say to use at any time

So if I have only one life, and I just lost ca battle can I use it before I lost that life?

You have to cast the Healing Spell BEFORE you lose your last life. So in your case, you have lost your last life in a battle and therefore have died so you cannot cast the Healing Spell. Think of it this way, can a dead person cast spells? The answer to this is of course NO.

Ditto to what WombatMan said!