Also often forgotten thing is that aircrafts have also designed max landing weight. This isn't hard limit, but limit for normal operations. This is why they go in holding pattern to burn fuel for example if cabin pressure is lost. So not immediate emergency, but something where you want to land sooner than planned.

To put it in perspective, the A380 max landing weight is 391 tonnes, versus the 7000 tonnes of lithium batteries required to power a long haul flight.