My first build, 1000W, 2000Wh

[michael.jahrer]

Hi,

Basis is an German Kalkhoff all-round men bike 28", which I bought used several years ago for 150€ or so.
Focus was cheap, good power, durability, quality and minimalism.
The build took me several months of buying, planning, making, testing.
I had some prior experience in building different electric powered equipment.
At the end I am pretty happy with the eBike, it can accelerate silently to 50kmh ~ 30mph with the 1kW motor at full load, while being fully functional.
The controller that comes with the kit does not provide regen and is loud (square wave), I tested the bike a few days with this one and an other battery.
The VESC is much better, smaller. I set max. current to 20A to and from battery. The rest is standard auto.
Regenerative braking works good, at steep hills it send back -15A to the battery.
I notify the added weight at the front wheel when riding over bumps, it is not that smooth as it was before.
The original front/back light is still functional switchable, power supply is the 5V output of the VESC.

2000Wh battery (the "highlight"):
I figured out that the angle of the frame is nearly 60° at the crank, this was the kick-off for the large battery.
High capacity cells were available at nkon for ordering. BAK N21700CH-58E - 5750mAh (~20Wh).
13S8P would be a nice number to break the 2kWh barrier
First I bent a stainless steel band int right shape and I welded 16x M5 screws for the mount to the frame.
While mounted I spot welded the cells to the complete 13S8P pack. This is important that every cell fit its perfect place.
Front, back panel were TIG welded while the band being mounted. Then I carefully removed weld residuals by grinding.
After post welding all spots I added a balancer connector, no BMS.
Complete battery has 9.08kg, cells pack alone 7.63kg.
When attached to the frame the construction fit is very strong. The box is rigid, no movement at all.
I covered it with a bathing cap to be rainproof.

After the first longer ride at full throttle the VESC (which is mounted on an aluminium 4mm sheet) does only get a little warm.
Hub motor also got warm. But not dramatic.

I ordered several components from aliexpress, battery from nkon, metal and epoxy sheets from amazon.
Other parts like throttle, brake, connectors, cable etc. were found in my garage.

Anti-spark switch: https://de.aliexpress.com/item/1005007582347642.html
Battery meter: https://de.aliexpress.com/item/1005005356586078.html
Speed controller VESC: https://de.aliexpress.com/item/1005004275900738.html
Front wheel hub motor: https://de.aliexpress.com/item/1005009015247450.html
21700 cells: https://www.nkon.nl/en/bak-n21700ch-58e-5750mah-11-2a.html

Best Regards from Austria,
Michael

 

[chuyskywalker]

You've done a great job.

Can you clarify what you mean when you type "accu"? There's some language barrier happening on that one.

 

[E-HP]

You've done a great job.

Can you clarify what you mean when you type "accu"? There's some language barrier happening on that one.

Per Google AI, so who knows how accu-rate.

"Accu" is a German word for a rechargeable battery, a shortening of "Akkumulator".

 

[michael.jahrer]

ouch
yes I mean battery sorry

 

[E-HP]

ouch
yes I mean battery sorry

Looks like you’ll be able to cover a lot of distance with that setup! Is the bathing cap the final solution, or temporary. It blends right in; I wouldn’t have noticed if you didn’t mention it.

 

[michael.jahrer]

yep, should be very good range.
I also like the regeneration feature. This enables me to get back some energy when going downhill.
yes I don't know if the bathing cap will break. time will tell. this was a quick and creative solution.
I have no other idea how to cover that battery case right now.

 

[SlowCo]

A lot of battery and I love the bathing/swimming cap idea to cover the box.
I hope the green water bottle isn't needed as fire extinguisher but just as a drink on the looong rides with that much range.
Have you installed a decent torque arm on the motor axle connected to the front fork as I can't see one in the photos? If not, please do so as it would really hurt if the fork breaks due to the torque.

 

[michael.jahrer]

A lot of battery and I love the bathing/swimming cap idea to cover the box.
I hope the green water bottle isn't needed as fire extinguisher but just as a drink on the looong rides with that much range.
Have you installed a decent torque arm on the motor axle connected to the front fork as I can't see one in the photos? If not, please do so as it would really hurt if the fork breaks due to the torque.

good point thanks. Yes the fork is probably the weakest point. I noticed that when riding over bumps. It doesn't feel good.
No there is no torque arm installed. The front hub motor kit does not include that.
I missed this completely, will check how I can make it.

 

[michael.jahrer]

yo,I spend a few days thinking about how to make a good torque arm.
I found the "Grin V7 Regen", but that is too expensive.
Search led me to this thread: torque arm picture thread
The design I choose is in the first post.
Clamping force is incredibly strong. I clamped it with 2x M6 12.9 screws per side.
I plan to drive more often with this bike, also to and from work (3km). It took me the same amount of time that it would have taken me by car.
It is fun, even in the cold.

Here are the results after and before:

 

[Tommy Lyn]

Hi, First off, congratulations! This is a seriously impressive DIY project. That 2000Wh battery pack, in particular, shows incredible craftsmanship.
I have two small suggestions regarding safety, just to help make your awesome beast even more durable and secure for the long haul:
1. Charging Safety (No BMS)
You mentioned that you didn't install a BMS and are only using the balance connector.
A battery pack without a BMS is very prone to overcharging individual cell groups due to slight imbalances. This is a significant safety risk. Please be extremely careful, and never forget that it's charging. It's best to monitor it closely or use a high-quality balance charger that actively manages every cell string to keep it within safe voltage limits.

2. Nickel Strips & Vibration Resistance​

I noticed the spot welds on your battery , as seen in your original post,It looks like you've used wide, rigid nickel strips with many spot welds, making for a very "stiff" connection. As you noted yourself, the bike experiences bumps and high-frequency vibrations during normal riding.
This type of rigid nickel connection can struggle with constant vibration. The stress is concentrated on the welds, which can lead to fatigue, cracking, and eventual failure (breaking off). The worst-case scenario is a loose strip falling and causing a short circuit, which is extremely dangerous.

Suggestion: If you ever (and hopefully not!) need to repair it or you build a new pack, you might want to consider using a different style of nickel strip,the H-type or "honeycomb" style.These strips have much better flexibility (toughness). Their structure is designed to allow for slight movement and thermal expansion between the cells, which is excellent for absorbing vibrations.The structural design is more reasonable for this application, distributing stress much better and making it far less likely to break off, significantly increasing safety and longevity.
Again, huge credit for your amazing skills! You've already tackled the critical torque arm issue, and I hope these small suggestions are helpful.

Ride safe and have fun!

 

[michael.jahrer]

thank you for the tips.
yea a good and compact 13S BMS would be nice for the long term safety. I'll take a look.

currently I charged it only once with a high-power charger (40A).
during this charging I noticed cell voltage differences due to the "bad" cell connections (4.15V vs. 4.08V)
These 0.2mm nickel stripes are not suitable for higher currents.
I think during drive with ~20A they will do the job.

 

[Nix Liou]

Seconding the concern about the BMS- without a BMS you aren't protected from overcurrent situations, and most BMSes will also have temperature probes to protect against thermal issues. I would recommend the JBD s004sp14 smart bms: It's quite small, compatible with 7-14s, and bluetooth compatible with full adjustability of the safety parameters.

Additionally while the tape between cell groups is good, I would definitely recommend just using cell spacers to basically completely eliminate the risk of a cell to cell short due to vibration. It wouldn't add much size, and would allow a consistent layout and better structure.

Some of the spot welded strips are a bit short, with welds on the very ends. Probably okay, but welding on the edge of a strip can cause a more fragile weld.

On the positive terminals, it's generally good practice to add an extra insulating ring, though not strictly necessary as cells usually have a plastic protective ring under the PVC. Barley paper rings precut for 21700 cells are just few dollars though, and add a non-meltable insulation.

It looks like you just did one weld per strip per layer, but I would recommend doing at least two, ideally 3. Just so that even if one pops off eventually you still have others, and each weld has less force on it.

Overall though, pretty good build quality, I've definitely seen way worse for a first time

 

[michael.jahrer]

thanks for the recommendations.
Yes cell connectors are not the best for higher current. Maybe I'll rework them.
There is some mV sag in the cell connectors when running higher current.
Therefore cell voltage readings are not accurate in higher current conditions.

I have the BMS you recommended ordered and it got delivered, hopefully I find the time to integrate it.

So far it runs well. I use it nearly every day. I added a better light because I almost drive in the cold dark.

My bigger concern is the Regen when braking (=battery charging for a few seconds).
We have here in Austria 0° C temperature in the morning (around freezing point). And the bike is always outside.
Regen can reach 0.5C (20A, the pack has ~45Ah) a few seconds when braking from 50km/h.
This can cause lithium plating?! Since I am using here high capacity cells with low current rating.
I'll switch now to manual braking to protect the battery.
Discharge with 0.5C should be no problem in the cold.

Is there some general recommendation for charging in cold conditions with low current cells ?
Like here: https://www.akkuman.de/shop/mediafiles/PDF/18650 Lithium/147793_en.pdf
...
0°..5° : 0.1C
5°...10° : 0.2C
...
??

 

[Nix Liou]

thanks for the recommendations.
Yes cell connectors are not the best for higher current. Maybe I'll rework them.
There is some mV sag in the cell connectors when running higher current.
Therefore cell voltage readings are not accurate in higher current conditions.

I have the BMS you recommended ordered and it got delivered, hopefully I find the time to integrate it.

So far it runs well. I use it nearly every day. I added a better light because I almost drive in the cold dark.

My bigger concern is the Regen when braking (=battery charging for a few seconds).
We have here in Austria 0° C temperature in the morning (around freezing point). And the bike is always outside.
Regen can reach 0.5C (20A, the pack has ~45Ah) a few seconds when braking from 50km/h.
This can cause lithium plating?! Since I am using here high capacity cells with low current rating.
I'll switch now to manual braking to protect the battery.
Discharge with 0.5C should be no problem in the cold.

Is there some general recommendation for charging in cold conditions with low current cells ?
Like here: https://www.akkuman.de/shop/mediafiles/PDF/18650 Lithium/147793_en.pdf
...
0°..5° : 0.1C
5°...10° : 0.2C
...
??

Well, the recommendation is basically don't do it. You can potentially set up the pack with heating pads to warm it up before you go, that will help with discharge a bit as well (technically speaking, a temperature of 35C is optimal for discharge as long as you can keep it there).

Newer cells or cells with different chemistries can have much better cold performance, such as most molicel cells. With this pack I would recommend some heating solution and thermal insulation. Some bms options include a heater connection where below the threshold temperature any charge will instead go to the heaters. That might work well in this situation as it would prevent charging in the cold, condition the pack, and still allow regen... But you just got this BMS in

I've attached the datasheet for the JBD sp21, which is one BMS option with an automatic heating system (heating element not included). I believe there are variations between 80a-200a, making it pretty beefy compared to what you need. Probably there are other better options for you, but this is just an example.

 

[minde28383]

Front, back panel were TIG welded while the band being mounted.

very nice looking work. what battery box steel sheet thickness you used?

 

[michael.jahrer]

very nice looking work. what battery box steel sheet thickness you used?

thanks. Battery box ground plate is 2mm stainless steel, walls are 1mm stainless steel.
The box was the most work, I put much effort in let the cells sit in their final position during building/welding/spotwelding.
So that there is no movement in driving.I drilled two drain holes in the deepest point if there is any water. Hopefully not.

I've installed the JBD SP14S004S BMS as a plug-in between battery and bike.
App and bluetooth connection work, lets see if it brings any benefit. At least I can read cell voltages with app
Cells are perfectly balanced in middle SOC (all at 3.6xx V, diff is below 10mV).
I adjusted some default parameters. In cold weather I'll use regen braking only very little.

 

[n00b]

Hi,

Basis is an German Kalkhoff all-round men bike 28", which I bought used several years ago for 150€ or so.
Focus was cheap, good power, durability, quality and minimalism.
The build took me several months of buying, planning, making, testing.
I had some prior experience in building different electric powered equipment.
At the end I am pretty happy with the eBike, it can accelerate silently to 50kmh ~ 30mph with the 1kW motor at full load, while being fully functional.
The controller that comes with the kit does not provide regen and is loud (square wave), I tested the bike a few days with this one and an other battery.
The VESC is much better, smaller. I set max. current to 20A to and from battery. The rest is standard auto.
Regenerative braking works good, at steep hills it send back -15A to the battery.
I notify the added weight at the front wheel when riding over bumps, it is not that smooth as it was before.
The original front/back light is still functional switchable, power supply is the 5V output of the VESC.

2000Wh battery (the "highlight"):
I figured out that the angle of the frame is nearly 60° at the crank, this was the kick-off for the large battery.
High capacity cells were available at nkon for ordering. BAK N21700CH-58E - 5750mAh (~20Wh).
13S8P would be a nice number to break the 2kWh barrier
First I bent a stainless steel band int right shape and I welded 16x M5 screws for the mount to the frame.
While mounted I spot welded the cells to the complete 13S8P pack. This is important that every cell fit its perfect place.
Front, back panel were TIG welded while the band being mounted. Then I carefully removed weld residuals by grinding.
After post welding all spots I added a balancer connector, no BMS.
Complete battery has 9.08kg, cells pack alone 7.63kg.
When attached to the frame the construction fit is very strong. The box is rigid, no movement at all.
I covered it with a bathing cap to be rainproof.

After the first longer ride at full throttle the VESC (which is mounted on an aluminium 4mm sheet) does only get a little warm.
Hub motor also got warm. But not dramatic.

I ordered several components from aliexpress, battery from nkon, metal and epoxy sheets from amazon.
Other parts like throttle, brake, connectors, cable etc. were found in my garage.

Anti-spark switch: https://de.aliexpress.com/item/1005007582347642.html
Battery meter: https://de.aliexpress.com/item/1005005356586078.html
Speed controller VESC: https://de.aliexpress.com/item/1005004275900738.html
Front wheel hub motor: https://de.aliexpress.com/item/1005009015247450.html
21700 cells: https://www.nkon.nl/en/bak-n21700ch-58e-5750mah-11-2a.html

Best Regards from Austria,
Michael

That battery (akkumulator) is something else! I like the mounting system!
Also the build of the battery itself, very skilled.

 

[michael.jahrer]

I still use the bike 3 days per week regularly. Works perfectly.
I added a better light in the front + one in the back (both powered by the 5V 1.5A output of the VESC).
I increased the battery amps to 30A, so it can pull more power (1.5kW), also increased the field weakening current to 20A to spin faster.
My speed record is 58km/h on the straight (with full battery).
Here in Austria there is much salt on the streets, the chain becomes rusty, I had to clean + oil it.
And the rest of the bike becomes "salty white", also cleaned it regularly.
The voltage sag is around 4V in the cold when pulling 30A. So when battery on-cell-voltage is 51V it can sag to 47V under longer full load.