Full Power to Lightweight - Bosch 800Wh replaced with 400WH compact

Thanks! So, were the adapters glued on then? Or how are they fixed?
The lower one in the image has a C Clip, and the top one stays in place due to the cables which are stiff.

I agree, it will make sense to glue them in place for the final version.
 
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PS Je cherche maintenant à imprimer avec du PA12-GF par SLS en utilisant cette société :

SGD | Service expert d'impression 3D en nylon SLS - Délais d'exécution rapides


Beaucoup plus cher - environ 75 £ pour imprimer un ensemble de supports, mais la résistance dans tous les plans semble raisonnable.

Chatgbt compare les deux :

Choisissez PA12-GF (SLS) si :​

  • Vous voulez une force constante dans toutes les directions
  • Le support est structurel ou critique pour la sécurité
  • Vous avez besoin de tolérances dimensionnelles strictes
  • Vous êtes d'accord avec des pièces plus lourdes mais plus durables
  • Vous envisagez de faire une production par lots ou de faire appel à un service bureau

Choisissez PA12-CF (FDM) si :​


  • Vous avez besoin d'une monture légère
  • Résistance principalement nécessaire dans le plan X/Y (direction de la couche)
  • Vous construisez des pièces personnalisées ou uniques
  • Vous avez accès à une imprimante FDM haut de gamme (comme Markforged, Raise3D ou Prusa)
  • Vous souhaitez un délai d'exécution plus rapide à moindre coût
I had a look on my side in case you end up sharing the plans :) and I found some individuals offering printing services in PA12-XX. It might turn out cheaper.
 
that’s a cool idea! Dropping from 800Wh to 400Wh would definitely make the bike feel snappier on short rides.

Has anyone actually tried running a Bosch Compact 400Wh on a Full Fat Gen 5? I’m curious about fit and whether the system limits power output. Would love to hear real-world experiences before trying it myself.
Yeah, me.

Look back in this thread yourl see a bunch of my posts on the subject.

400wh is awesome for weight and balance of the bike. It feels a lot like a normal bike.

Also the motor does not derate with the 400wh in. Its full power.

400wh 21.5kg 180mm bike with yhe ability to punch 100nm if a thing of beauty. Such a special bit of kit.
 
I have started looking at glue and it is not easy to find a good glue for nylon / high heat, so I am going to insert a nut and bolt to hold the boxes together. First draft of the top mount...

Screenshot 2025-09-24 at 17.37.59.png Screenshot 2025-09-24 at 17.37.45.png
 
I have started looking at glue and it is not easy to find a good glue for nylon / high heat, so I am going to insert a nut and bolt to hold the boxes together. First draft of the top mount...

View attachment 168520 View attachment 168521
Looking good. From a design improvement perspective get in there with the radius tool on your your cad program and slap some nice radii on there. Square edges can chip and break and hang up on stuff. Also a slight taper to the mating parts so the can be a firm but easy fit. Tolerance is everything when designing. The magic tolerance allows the part to fit together without binding up or being too sloppy.
 
Lower mount, which is still missing the holes for the battery cover. I need the Trek battery cover to position the holes, so not finalised despite the names.


If anybody has any ideas on how to stengthen it internally, I am open to ideas. I am thinking the strenghening could also help hold the top power connection in place.

The cables need to go between the power connections as per the screenshot below.

Screenshot 2025-09-24 at 14.49.35.png
 
Managed to make it a bit stronger with some internal cross bracing.

Now I just have to figure out how to screw the two parts together.

Screenshot 2025-09-25 at 21.26.06.png
 
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Is this likely to be sufficient, or keeping going ?

View attachment 168989

Like to put a small fillet on every corner and hole. That way you reduce stress risers, parts more easily mate and there's less chance of chipping and damage of the parts.

Here's a quick example.


1759366775453.png
 
Trek have stock of the battery cover, so I should get it later this week or early next week.
 
Battery cover has arrived and it fits an old base I had lying around.

I will print the parts tonight, and test tomorrow with PLA.

Latest thinking is to print with PA12 using MJF, as it is strong in all dimensions and has a reasonable price to print - around £50
 
Printed and it all fits together :)

I need to make some small tweaks now that I have put it all together for the first time.

I hope to finalise the designs in the next couple of days, and I will then send it off to the printers.

When completed, I will do a post with links to the final designs, the bosch parts, and the different screws and bolts that I have used.

Screenshot 2025-10-13 at 22.43.12.png Screenshot 2025-10-13 at 22.43.05.png Screenshot 2025-10-13 at 22.42.57.png
 
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Tweaks made, and the battery attaches to the bike :)

Now it is time to select a printing company and filament - I still can't decide.

I might need to fit some foam between the battery and the cover on the Trek Rail - I will only know on a rough track.
 
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Tweaks made, and the battery attaches to the bike :)

Now it is time to select a printing company and filament - I still can't decide.

I might need to fit some foam between the battery and the cover on the Trek Rail - I will only know on a rough track.
Nice.
Have you tested it on the road to feel the difference (or not)?
 
Did you weight it by chance with all the add ones?
I have printed in PLA which is a fair bit lighter than PA12, so weight now would not be a fair measurment. I will try to weigh the mounts, bosch parts, nuts and bolts before I assemble.
 
Some more photos...

The battery mount sticks out a little bit in the middle section when on the bike, which can be solved one of two ways.

1. Make one of the mounts about 1mm longer. However, they are 72mm in total, so should be the right size.
2. Make the mount glue to the battery, via some foam between the battery and the mount. This is what Trek does with the 600wh and 800wh battery, so I think this is what I am going to do.

Screenshot 2025-10-15 at 11.12.09.png Screenshot 2025-10-15 at 11.11.55.png Screenshot 2025-10-15 at 11.11.49.png Screenshot 2025-10-15 at 11.11.43.png Screenshot 2025-10-15 at 11.11.34.png Screenshot 2025-10-15 at 11.11.27.png
 
There is a 3rd option, which is to make two versions of one of the mounts. One being 1mm longer - I will do this.
 
I have printed in PLA which is a fair bit lighter than PA12, so weight now would not be a fair measurment. I will try to weigh the mounts, bosch parts, nuts and bolts before I assemble.
I'm asking as the PLA is not feathers. Wondering what is the engoal gain.
 
I have ordered the parts in MJF PA12 - they should arrive next week.

Final piece of the puzzle is to find a good glue.
 
I have ordered the parts in MJF PA12 - they should arrive next week.

Final piece of the puzzle is to find a good glue.
I did some experimenting a while ago and found modified polyurethanes, like Sikaflex for windscreens for example, are the best adhesive options for low surface energy materials like plastics.
 
Weight
Battery_______2,043 grams (Plummet above)
Mounts________161 grams (Scales)
Bosch BCH3901___ 25 grams (Manufacturer)
Bosch Adaptor____21 grams (Manufacturer)
Nuts and Bolts____20 grams (Scales)
Glue___________ 2 grams (ChatGBT)

Total_________2,272 grams

Comparison
800wh___3,900g (Manufacturer)
600wh___3,000g (Manufacturer)

800wh with trek mounts and cover = 4,258 grams
400wh with my mounts, bosch parts and cover = 2,507 grams, a weight saving of 1,751 grams.



Bosch 400wh battery + one of the screws included with the mounts
OR

Bosch BCH3901 - 100mm version

Bosch Battery Adapter 2 Smart System

M4 Steel Hex Nuts

M4 12mm Hex Steel Bolts

https://www.ebay.co.uk/itm/166098821085?var=465613598768

Nylon Glue - I went with Loctite HY4070 due to this article

Upper Mount - please read the disclaimer

Lower Mount - please read the disclaimer

Printing - MJF PA11

MJF PA11 - make use of the discount vouchers, and select the $1.50 shipping option at the checkout.

I do not know if this will be strong enough. If I get anxious using it, I will be printing with SLS PA6-CF/GF which is more expensive. I decided not to use FDM printing, as it is weak in the Z axis. I went with PA11 instead of PA12 as higher impact resistance.

Estimated Cost
Battery_____£260 to £330
Bosch Parts__£35
Printing MJF__£40

Glue_______£25
Nuts & Bolts__£5

Total_______£365 to £445

As a comparison, Bosch 600WH BBP3860 is £725 on ebay


Trek parts with cost
Trek RIB 2.0 Downtube Battery Cover - W5318794 - £20
Trek RIB 2.0 Upper Battery End Cap - W5322238 - £15
Trek M4 Button Head Screw - 10mm -
W1052364 - £4

Total £39


Assembly Instructions

I highly recommend doing one or two practise assemblies without glue first, so that you understand how the components fit together, and how the new battery fits into your bike. Obviously do not cycle the bike at this point.

The screenshots below illustrates how the base and lid go together.

Step 1 - attach the lids to the 400 wh battery at both ends, using the M4 12mm bolts and one screw from the mount that came with the 400wh battery (which lid and hole requires this screw will be obvious, hint: the hole you can't see in the photo below). I recommend using threadlocker because the bases will be glued together, so you will not be able to retighten them at a later date.

P.S. The lids are not symmetrical, so orientate them correctly when attached to the battery cover.

Step 2 - slot the cable of the BCH3901 through the big hole in the solid end of the lower base, and hold it in place with the printed o ring. (you could add a dab of glue to hold it in place to stop it rattling) (The o ring is optional as the BCH3901 will need to be glued in place to prevent rattling)

Step 3 - glue the bosch adaptor to the lower lid, as per the 2nd screenshot below.

Step 4 - put the nuts into the lower and upper bases, and attach to the external fittings - in my situation, with the Trek W1052364 screws, attach the Trek W5318794 and Trek W5322238.

Step 5 - place the battery with the lids already attached, into the battery cover with the bases attached, and glue the base and lids together.

P.S Steps 4 & 5 are going to be a bit challenging, and will vary by battery cover design - I am not an engineer, but it should work if you're patient, and think about what you're doing.


Screenshot 2025-10-21 at 22.00.06.png Screenshot 2025-10-18 at 18.02.32.png
 
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