Joe has a Nexus 7 tablet and is worried about what will happen when the battery wears out. Leo says that the Nexus 7 can be opened and the battery can replaced. There's an article on. iFixIt showing how to do it. Joe is lucky because many mobile devices, like the iPad and iPhone, are designed so users can't replace that battery.
What about using an external case that has a battery built in? Leo says sure, there's plenty of options there. However on some devices, it won't work if the main battery is dead.