Need help with oem dual batt controller

[BobTrek]

The bike came with the second battery mount on the frame and one under the seat, the controller
has a wire ( 3 conn. ) to the frame battery and the under seat has the controller in the mount ( Hailong )
When the battery is installed under the seat the open contacts of the frame mount have full voltage,there
is a mystery box in the frame mount that takes the 3 wires in and has two out to the battery connector pins ( diode ? )
If I disconnect the plug to the frame mount the bike won't power up at all no lights ect.
It does work if the battery is installed on the frame though.
I would like to run with the 2 batt. but worried that there could be a problem with that diode box.
Or run with just a larger batt under the seat but then still stuck with a possibly defective frame mount...
As usual no name on the controller that is traceable and the seller does not know anything about how it works.

Thanks for any idea about this.

 

[E-HP]

A few picture might help.
https://endless-sphere.com/forums/viewtopic.php?f=1&t=14748

 

[BobTrek]

Pics of controller and box in second batt holder.
Hub is a Bafang RM G0F0.500/750/1000.D/DC
The controller says 25km/h but can be set higher in the menu. Suppose it is 25mph or it means 2500 Mph !
Seems limited to 41kph, no idea of the amps though.
My first Ebike.

 

[BobTrek]

Problem is that there is only a 3 pin connector to the module but shows 4 in and 2 out ?
What I would like to do is remove it and just use the single batt mount under the seat.

No Ideas ?

 

[E-HP]

I can only see 2 wires in your pics, so no clues.