In 2020 I decided to take a look at acquiring an ebike to aid my 17km daily non-electrified bike commute. During this period I’ve mostly worked from home so it wasn’t really an issue. Now that we are more or less allowed to go to the office again it’s time to get this built. First let’s look at the commercial options.
The budget e-bike
The market has a lot of relatively affordable bikes. The cost around €1000-1300, so by no means cheap, but most of them have some glaring flaws:
- Heavy frames
- Cheap hub motors and controllers
- Low capacity batteries, typically around 10 Ah
- Low end bike hardware e.g. gears and brakes etc.
As an example let’s look at a €1600 bike from the danish shop THansen currently on sale for 1100: El-cykel dame E-go.
The linked bike has:
- 24.5 kg, 54 freedom units (pounds)
- 7 Shimano Nexus gears
- V-brakes in front, foot-operated drum brake in the back
- 250W hub motor with a max torque of 35 Nm
- 10.4Ah battery (Probably a 10S4P at 2600mAh per cell)
- 2A charger
Now let’s compare the non-electrified parts to a conventional bike from the same store like the Leonora coming in at €400. This one has the same materials used for the frame and the fork. The same Shimano gear system and the same brakes. Note that the €400 is the normal retail price while the €1100 is on promotion. All prices are with the 25% danish sales tax. This would mean that a hub motor, 40 cell battery and wiring is “worth” between €700 and €1200 euros. We can safely assume these parts are sold with a markup. €1200 is more than the whole bike on promotion. €700 seems more reasonable, but you can get an electric bike for €800, batteries included, albeit less capacity. So there’s a price gap between conventional bikes and ebikes that’s not directly correlated to the cost of materials (shocking, I know).
A quick glance at EU-based shops suggests a hub motor + controller + LCD price around €140. For the sake of argument let’s say it’s €200. This leaves €500 for a battery which seems a little steep, but more on that later.
A lithium ion cell (smallest unit of battery, looks like a beefy AA battery) is usually 3.7V nominal. This means it’s 3.7V on average throughout it’s usable voltage span which is usually 2.5 to 4.2V. Lithium ion battery packs consist of a whole lot of cells packed in a case. They are wired in series in groups to get a specified voltage and then multiple groups are bundled together to get more capacity.
A 36V battery pack with 10 cells in series and 4 parallel groups is written on the xSyP notation: 10S4P. The battery is therefore 42V when charged fully and somewhere between 25V and 31V when completely discharged.
Quality battery packs are expensive partly because cells are expensive but mostly because they have somewhat complicated battery management systems (BMS) and have to be spot welded. A BMS is something you can buy for 10’s of euros depending on amp rating. A spot welder on the other hand is not something your average Joe will have access to. Good spot welders start in the 100s of euros and like all industrial equipment they can cost tens of thousands depending on the amp ratings, certifications and the like.
Good cells cost money yes, but you can buy a LOT of cells for €500. As an example the Panasonic NCR18650BD 3100mAh cell can be had for €3.7 a piece if you buy 100 or more. This means you can buy ~130 cells and get them shipped for €500. This is enough to build three decent battery packs. That’s of course without the BMS and the case. A case is ~€35 and a BMS €20-30. So €65 for electronics and case leaves you enough cash to buy over 100 cells or buy two cases and BMSes with 40 cells each.
So the conclusion is that the profit margin is very good on battery packs. So we need to learn how to build battery packs.
There are basically two types of ebike motors: hub and mid-drive. The hub motors come in different shapes and sizes mounted in the front or the back. Mid drive motors are mounted in the middle of the bike and usually connected to the pedals. The cheap ones are brushed but most are brushless. Weight distribution can be an issue with the big hub motors as they are ~5 Kg (~10 pounds). Mid drive motors have a similar weight but give a nice weight distribution since they are in the center.
There are a LOT of ebike brands but very few manufacturers of motors. In the high end you have Bosch mid drives and in the low end you get unbranded hub motors. They vary greatly in torque and speed. In Denmark I’ve seen the noname motors, Bafang, Bosch and a couple of Yamaha.
Bosch are very nice but hard to get a hold of for a hobbyist. They require custom frames. The Yamaha I’ve seen are the same. Bafang on the other hand have both the custom frame mounts AND the much more hobby-friendly bottom bracket mount.
There’s also the issue of controllers/batteries. Bosch and Yamaha have proprietary battery controllers and you need to buy the batteries from them. The Yamaha battery protocol has been reversed but the Bosch is still, as far as I know, inaccessible to hobbyists. Bafang BBS motors don’t give a shit who made your battery as long as it delivers enough volts and amps. More importantly Bafang let’s you change voltage, amp and power limits. The software is in fact open source and you only need a TTL UART USB dongle which is cheap and plentiful in my drawer of electronics.
So I would like a mid drive motor from Bafang mounted in the bottom bracket on a bike. There are three models: BBS01, BBS02 and BBSHD. The two first are more or less the same motor with different electronics. The latter being able to handle more juice. The BBSHD is an insane 1000W motor capable of delivering 160 Nm peak (Newton metres) which is comparable to a small gasoline car. For comparison the 2021 Peugeot 108 1.0 delivers 93 Nm at 4400 RPM. The slightly quicker VW Up! delivers 165 Nm.
I chose the BBS01 with 36V built-in controller because it can deliver a juicy 100 Nm and the power and assist levels can be tweaked to my heart’s content.
The plan of action
Every e-bike needs a battery pack. Every decent battery pack needs a shitload of cells and every cell needs to be spot welded. The conclusion is therefore that I need to get my hands on: a spot welder, a BMS and 30-50 18650 cells.
Next I need a motor. AliExpress has some delivered from Germany. Easy.
The actual bike
I will be using my Nishiki 7B Lite bike with 7 Shimano Nexus gears and rollerbrakes. It has an aluminum frame and stainless steel screws and such. Weighing 14.6 Kg it’s quite a bit lighter than the cheaper bikes.
Let’s acquire a spot welder.
Thanks for reading if you’ve made it this far.