Would the furnace be able to contain it just fine, or would it explode, or would the flaming battery juice melt through the hull of the container, or what?

You don't burn the coffin during cremation

I just had a quick search about temperatures in a crematorium. Around 800 °C. A structure that can withstand that sort of temperatures is probably also able to withstand exploding batteries. If I remember the Galaxy 8 disaster, they go pop and then it's just mostly a chemical reaction releasing heat and gas. 30 batteries might be a lot. It may not end up destroying the place but it will possibly leave it in need of repair.

Why do you want to create this sort of fireworks show in a confined space where under normal circumstances nobody will be present to witness it?

There actually exists a study done in Japan, combining both a survey of crematoriums and experimental tests using a crematory retort and sample medical device batteries.

The results found that the device explosions were audible, but did not damage the experimental crematorium. In the survey, however, 92% (in 2012) and 51% (in 2013) of questioned crematoriums reported "damage" from medical device batteries. The cause for this discrepancy is unclear; it's possible those answering the questionnaire considered scorching, scratches, or metal residue to be "damage", while the test only looked for "structural damage".

30 phone batteries is a lot more than 8 medical batteries, but it's still not a whole lot. Compared to, say, powerbank or scooter batteries, which can go up rather more spectacularly, the effects are more limited.

tl;dr - it'd probably be noisy and some refractory bricks might need replacing, but it probably wouldn't blow up the retort.

As a result, you’d end up with a bemused mortician the next time they used the coffin, and that’s about it.

I've witnessed many lithium battery fires with my own eyes, as part of my job (long story, not as interesting as it may sound), and while there is a lot of heat and energy involved, to the point where it competes with a hand grenade in terms of energy density, the energy release isn't nearly as instantaneous (except from special circumstances regarding primary cells, but that's also a long tangent that isn't as interesting as it may sound).

The main concern is the offgassing and the heat. I am not a corpsologist, but I believe offgassing shouldn't present a problem in a crematorium. I hear they have advanced hitech gadgets such as "chimnseys" and "filters". (Citation needed)

As for the heat, lithium battery fires reach 1100 degrees C under optimal condition and while that is, to use a technical term, hot as fuck, anything built to withstand cremation temperatures every day for years should be able to withstand lithium fires that one day. It will certainly get hot, but probably not destructively so.

As a final note, yes, there will be pressure release, but the pressure wave is most likely negligible compared to the structural integrity necessary for a crematorium to carry its own weight. As briefly mentioned before, the pressure release of a battery is not instantaneous, and this is by design.

He could put them in his prison wallet.

What? A whole coffin?!

Depends on his fortitude and dexterity roll.

You don't even get in a coffin these days. You get taken straight from the place you died into the oven.

He autosucceeds at the fortitude roll, as an undead. The Dex is harder, because of rigor mortis

? Usually they are intended as part of the fuel needed to burn the body, at least here in DE.

There is usually a small gas explosion risk but I imagine that the conditions would never be right inside a furnace.

Usually this isnt part of the actual service, at least not in cremations I've been to. It happens afterward, so all the batteries would do is damage a bunch of equipment and potentially set off the fire alarm during the wake.