It should go to the termal sensor. 

anyone knows how to install the built in sensor, and what kind of sensor it used.

cheers

If the original battery is dead, i suggest you strip it and just replace the cell. 
You'll have a precise part number printed on the original cells and reuse the connectors and sensors.

It's what i usually do for Beo6 remote batteries.

Unfortunately, I don't have the original one, that's the problem.

anyone there have a spare dead battery for BS3, I would like to reuse

the female plug with wires & the sensor and I will pay for that. 

Cheers!

here on German Ebay for only 30 € for tinkerer ....

https://www.ebay-kleinanzeigen.de/s-anzeige/b-o-bang-olufsen-beosound-3-an-bastler/1308300306-172-9473

good luck 🙂

Hi, this is the BS3 battery connection schematic. The NTC sensor temperature is 8,5Kohm at  27°C.  It is not necessary to have the female connector. Insert insulated wire into the three pins of the male connector and stop them with blu-tack or hot glue.

Internal view