Before I start, I don't exactly have much experience of shield use; I've only even had 2 real battles, and in one of them, I didn't have any shields.
However, several months after the battle, I realised that I left the shields active, and I only realied because they were slowly running out of fuel while they were docked at my homeworld.
I realise that having a warning that shields are on would just be message spam if you constantly got it while you were in the middle of a battle, but would it maybe be possible to include a warning during the 5-day increment? That way, you wouldn't be constantly spammed with messages in battle, but you'd get a warning no more than 5 days after if you forget to deactivate shields.