New Andy kirby 15 min conversion kit looks good

[Woosh]

Yet another go no distance battery, I suppose it is ok if you have 3 spare batteries

The woosh kit for Bromptons has two battery options: 36v 10ah and 36v15ah. That's enough for a whole day.

 

[Chris.mitu]

The controller is placed inside the same battery bag on the woosh kit.

but when folded the bag looks out of place and appears to destabilise the bike. I'd prefer a permanent battery box

 

[Woosh]

You can have a 36v13ah small frog battery mounted on the front luggage block.

All three solutions let you fold the bike and carry the bike normally albeit ther extra weight of the battery makes carrying more difficult.
The centre of gravity is a bit on the side of the fork but because the weight carried by the frame, it does not feel 'off centre'.

Small frog battery (36V13AH) mounted on the front luggage block:

when folded, centre of gravity is also a bit on the right side but because the weight is carried by the frame, it does not feel 'off centre':

With 36V 10AH bag battery: controller and battery are in the same bag:

 

[Chris.mitu]

What is the size of the controller? I wonder if it would fit inside something like this?

 

[Woosh]

yes but why do you want to move the controller out of the bag? The cables are made just long enough to follow the existing brake and gear cables and eliminate excess. Moving the controller out of the bag will require longer cables for the motor, LCD and PAS. More risk tangling them when parking, folding and unfolding.

 

[Chris.mitu]

yes but why do you want to move the controller out of the bag? The cables are made just long enough to minimize excess. Moving the controller out of the bag will require longer cables, more risk tangling them when parking, folding and unfolding.

My question is related to the controller that comes with the frog battery in the hard case. At the moment it seems to be held in a phone case off the centre. I'm wondering if replacing the phone holder with the hard box in the link and mounting it roughly in the same place below the handlebars would make it look more purpose built.

 

[Woosh]

I got you now. It seems from first look straightforward what you want to do but in practice, not practical. The Brompton needs to have the gear and brake cables able to independently move when you fold and unfold. If you cable tie anything to them, your new cables need to move with them very closely, hence the bag needs a bit of movement not to pull on anything when you fold the stem quickly, something a rigidly mounted box can't do.
If you mount the controller in a box rigidly fixed to the stem, you will need to add extensions to the battery, LCD, motor. Not practical.