There is no battery fire warning EICAS message in the 787 or for that matter any Boeing airplane that I know of.
Likely that is true, but it would be very unusual to omit a BATT HOT caution, or similar measurement of unusual temperature from a modern airliner.
On my fight deck, unusual smell and a battery temp caution would lead me to believe my battery is getting warm. Couple that with a recent history of on-type battery over temp incidents and my minds made up