That is not a sentence. It is a subordinate clause. The present tense places you in a living moment. However, that moment can be in the past, in the present, or in the future. The present tense can be timeless. The time frame used by a speaker has as much to do with what is going on inside his head as it has to do with reality. Examples: "Three plus two equals five." This was true, is true, and will be true. It is timeless. "Tomorrow I shop for vegetables." The present tense can describe the future. In the speaker's mind, he is already shopping. The word "tomorrow" jumps him into the future. Now that he is there, he can use present tense. You only need to say "tomorrow I will shop for vegetables" if you want to emphasize how certain you are that this will happen. "Yesterday I ran into Mike. He says "don't I know you from somewhere?". I say, "no". This is technically wrong. If you are taking an English test, you should use past tense. However, people do talk this way. "Yesterday" jumps the speaker into the past. Now that he is there, he describes what he is seeing. In your clause it would be possible, if you wanted, to use tenses. You could have said "If your battery will die and an hour of commute will still be remaining, ..." but nobody talks like that. It sounds weird to use future tense to describe something timeless.

In addition to the other explanations, you could also say: "When your phone battery IS dead, and you have an hour left on your commute." It has basically the same meaning.

The comic is talking about a relationship which is always true. When my phone died, I had nothing to do. (In the past only) When my phone dies, I have nothing to do. (Always. Past, present and future) When my phone dies, I will have nothing to do. (My phone battery works now and I’m telling you about the future after it stops working) When my phone is dead, I have nothing to do. (This also a good way to describe a situation which is always true, and could be used in the comic.)

This is a common structure. It's essentially a description of a situation. I think that's why it's all put in the same tense. It's not how you would say it; more like that way a news report would describe it. Imagine the the clause is preceded by: This is the situation . . . . . . when I don't understand a sentence and ask on Italki. . . . when I'm hungry and have nothing to eat. etc.