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If just before a cremation, I lined the coffin with 30 phone batteries, would the resulting lithium ion fireworks display have any meaningful effect on the proceedings?
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.