Hybrid batteries are located to the rear of the vehicle and are often found located in the boot floor, under rear seats or under the car to optimise vehicle weight distribution and ensure stability and handling of the vehicle. Therefor aren’t affected by the heat of the engine!
In mild hybrids the intergrated starter generaor replaces the flywheel, so is sufficiently cooled and the 48v battery within it is smaller than the the vehicles auxillary 12v battery which has sat in the engine bay for many years without issue!
However, an high electrical current, two fuel systems and two bombs!
Media reports vs relaity and flawed statistics, as of the end 2024, there were more hybrid vehicles on UK roads than electric vehicles according to the SMMT, yet the same related article on press reader tells a different story (below)!
https://www.pressreader.com/uk/the-mail-on-sunday/20251228/281543707282708