I have had a battery get into the runaway situation doing night circuits out in the training area to a large-ish pad; we smelt it and noticed the load meter reading v. high about the same time as another aircraft which was also in the pad called that there was 'steam' coming from the battery vents.
Turned off the battery and shut down, got out and stayed well away from it for half an hour or so and then had a look;
By the way it was steaming and carrying on before that, I'm sure it could have exploded if left to keep going.
We used to be told basically this:
Symptoms - high ammeter / loadmeter readings, venting, smell, bat hot lights if fitted.
Action - isolate battery by switch, land ASAP and leave it at least 30 min, preferably overnight.
There was also some talk of physically disconnecting it (leads) but maybe not so smart because you'd be going near it while hot.
So definitely not something to be taken lightly. I also remember seeing a photo of a Huey with the nose hatch area blown apart by a thermal runaway, that's probably on the net somewhere.