It’s not only optional, you get paid good money for providing the service. But once you opt-in you are under contract to provide the power when they come knocking.
> What happens if your battery is drained out at the time?
My guess is nothing.
> What happens if you hack your battery to say it ran out?
My guess is nothing, unless they catch you in which case they probably sue for breach of contract.
In that case you're probably financially on the hook for whatever their costs are for you not fulfilling your contract, or whatever damages are specified in the contract.
What happens if you hack your battery to say it ran out?