G'Day Andrew,
You are absolutely right, of course, and its a valid comment to make. I confess to making the assumption
that up to a full 12V could be used through the heater.
Even with some power tanks these days having multiple voltage outlets (3V, 6V, 9V typically) you could of course make a simple nichrome wire heater suited to one of these voltages as well - nice, neat simple solution!
As you say, though, you just need to make sure you don't put excessive voltage (and hence current and heat) through it.
Al.