I think you will find that the main battery is used to start the car from stopped. The hybrid battery is only used to restart the engine in the idle stopped state. It is also used for powering other ancillary equipment.

You should only ever need to jump start across the main battery. I would seriously caution against trying to charge the hybrid battery, which is Lithium-ion, using any kind of external charger. There is a high risk of serious damage

 

Thanks Ian. Good advice and is what other sources have been telling me.

 

This is the first time I have heard that someone try to jump start a car by the mild-hybrid, under the seat battery.... I have the same 2024 Swift-Hybrid and yes it has a normal battery (next to the engine) and you can jump start the normal way - however I don't know why the battery will die so early....

 

Lithium under the seat should not be touched by all accounts