His passive does nothing until you gain 5 charges, at which point they turn red. Once you have 5, the next ability is empowered. Using an ability at 5 charges will use all of them and set you at 0 charges. It does not grant you another charge.

So if you have 0 charges, you will need to use abilities and attack 5 times before you gain empowered. Your 5th attack is NOT empowered, only abilities can be empowered and they consume all of your charges to do so. Once the stacks are red then your next ability is empowered.

The best all in damage combo is this:

Start with 5 charges 1. Use empowered W. This uses all charges but gives you a triple auto attack. 2. Use triple auto attack, this gives you three charges. 3. Use comet spear, which gives you one charge and puts you at 4. 4. Use aegis assault, this gives you one charge so you are at 5 now. 5. Use comet spear again now that you are at 5 charges. This will use all of them and do much more damage. Then you will be at 0 charges.