Can someone explain athenes to me?

I'll explain it with an example. Sona has 0 AP and uses rank 5 Hymn of Valor on one target - that ability does 200 damage before you take into account MR, so Athene's stores 40 blood charges (20% of the pre-mitigation damage Hymn does). Sona then uses her W and consumes all her blood charges, healing her ally for 40 HP from Grail. If Sona has 250 charges stored and heals her ally for (random number) 180, she will consume 180 blood charges to heal for that amount, leaving 70 extra charges. The reason there are extra charges left over is because Grail will not heal beyond the value of the heal/shield that activated it.

Make sense?