There are several in depth reviews of Brymen meters (among many other brands) on
https://lygte-info.dk/info/DMMReviews.htmlThe reviews include measurements of when the battery low indicator came on and at what point readings became unstable. The list of Brymen meters reviewed doesn't include the BM78x series, but for the five reviewed two (BM27s and BM869s) remained stable until shutdown or an error message is given. The others (BM235, BM525s and BM829s) gave what seems to be quite a bit of buffer between the voltage where the battery low indicator comes on and the reading become unreliable.
As I said there is no data for the BM78x series in the reviews, but in general it looks like Brymen takes care to show the battery low indicator well before the meter starts giving questionable readings.
BM235: Meter works down to 1.7V where meter turns off, battery symbol show at 2.5V
Reading is stable until about 1.8V where it increase significantly (5.000V -> 5.206V)
BM27s: Meter works down to 2.3V where meter turns off, battery symbol show at 2.5V.
Reading is stable until meter turns off.
BM525s: Meter works down to 4.0V where it says "InErr", battery symbol show at 6.8V
The meter reading is stable down to 5.7V on battery.
BM829s: Meter works down to 4.0V where it says "InEr", battery symbol show at 6.7V.
The meter reading is stable down to 5.6V, then it will show too high value (It was 36% to high at 4V)
BM869s: Meter works down to 5.6V where it says "InErr", battery symbol show at 6.3V.
The meter reading is stable until it reports "InErr".