I see it two ways. a) He knocked the phone off when he was away from you because if he did need to contact you he'd be able to tun it on again and get through to you, and than wasnt bothered about whether battery died or not the next day because he was with you anyway, and as it was, battery wasn't as low as he thought it was. He came home when he said he would and was all sweaty and beery so wanted to, as he says, freshen up for you, hence the shower.
or
b) he turned phone off cos he was with another girl and got in shower to wash her off him before he saw to you. And reason he is so anti cheating is cos he's a cunning b4stard who thinks you'll buy his pack of lies.
I have a tendency to over think things though and see problems where there are none!! I really hope he hasnt cheated on you. If it's praying on your mind ask him straight out.