E-Bikes, how do they work?

You see many articles about ebikes, but not many give you specifics of how they work so here at the Folding Bicycle Club we give you the info you need to make an educated decision.

First of all, road legal.

Road legal is very clearly set down in law. The bicycle must be a pedal assist. It must not have a throttle control (such as a twist grip or lever) and it must have a motor rating of no more than 36V 250W and both this, and the battery rating must be permanently marked on the bicycle.

This means many of the ebikes you see are illegal – Indeed my own Fiido D2S is illegal unless modified (unplug the throttle controller is the way to achieve this).

So what are the numbers? (Skip this bit if you don’t want to know the maths)

36V is the voltage of the electronics. A higher voltage system allows you to generate more power for less energy – but there are some compromises. Most cycles use exactly the same cell in the battery. The 18650.

The 18650 numbering actually refers to the size of the cell – 18mm diameter and 65.0mm long. It’s storage capacity is around 2600mAh, or 2.6Ah with a 3.7V voltage.

Doing the maths you find that you need 10 in a line (known as series) will give you 37V. Even though this is slightly higher than the rated 36V, you will generally see between 35 and 37V on a fully charged battery.

Because of the nature of batteries though, a 36V battery made of 10 cells will only see the voltage increase though; your storage capacity will still be exactly the same at 2600mAh.

When wiring in parallel on the other hand (connecting all positive terminals tougher and all negative terminals together, then connect all these to a single device) the voltage stays the same but the storage capacity gets added together; so two cells in parallel would get 5200mAh and three would get 7800mAh. You’ll notice that these figures are quite familiar if buying pre made phone chargers based on 18650 cells – 2600, 5200, 7800 are all figures quoted on the boxes.

The problem is that the output of the cells is STILL 3.7V and only clever electronics will change that (and at the cost of capacity sadly).

So how do folding bikes get their power?

Well imagine you build a battery, putting say 6 cells in a parallel block, and then connecting three blocks together? Similar to the pack above.

Well you’ll get 6 X 3.7 Volts at the outputs so about 22V total

Then you’ll get 3 X 2600 storage capacity, so about 7800mAh

So I’m throwing these figures around, but what do they actually mean?

Well the voltage is how much actual electricity is delivered to the motor, whilst the storage capacity is how the long the battery will last – or how many miles you’ll do on a charge.

So a 36V motor might get you up to around 35km/h if not restricted because of local laws, a 48V motor might get you up to around 45km/h and a 72V motor could see you touching 65km/h theoretically. The voltage is basically a guide to speed, although that’s not quite how it works – the actual power output of the motor is expressed as watts on the motor housing. It’s all a question of amps! Yes. We’ve seen that before…

All three figures are related by an equation:

Amps X Volts = Watts

Well imagine your motor is 250 watts, powered by 36V battery. You’d have to drive it at around 6900mA to get a full power output. The H in the 7800mAh is “hours”, and thus you can probably guess that my little Fiido can drive it’s motor at full 250 watt power for around 1.2 hours.

Equate that to time and you get around 21 miles of range at 16mph, but that’s driven only and with no human input. What we sometimes call “moped mode”. When using an E-Bike you can input as much personal power as you want by pedaling harder – the lazier you are the shorter your range essentially.

How do they know? (No more maths – promise)

E-Bikes use a clever controller. It can be in the battery, on the frame, in the frame, or on the handlebars. These electronics take inputs from various systems around the bikes and tell the boost system when to kick in.

The first sensor is the Hall sensor. Hall is capitalised because it’s named after Edwin Hall who first noticed the principle. It’s used all over the world, in cars for the crank position sensor, but also as you approach automatic gates and traffic lights – have you seen the lines across the road? That’s the same effect.

The hall sensor usually sits around the crank and senses the pedals turning using ferromagnetic effects. When the electronics see the pedals turning they know to engage the motor.

The second sensors are usually on the brake levers. The moment you apply your brakes it kills the motor until you pedal again.

Finally, the cleverer systems might have hall effect speed sensors attached to a wheel, or may have brake force sensors which instead of disengaging the motor just slow it down.

If your E-Bike isn’t road legal you’ll possibly also have a throttle. This will likely be a twist grip like a motorbike, however on simpler systems it will just engage full power all the time and not be variable.

All these are connected to the brains of the system which will then give the user feedback and input through a dashboard. The usual input is limited to 2-5 power assist levels.

What are they like to ride?

If you’ve never ridden one before I’d suggest starting on the lowest power level until you get the feel for it.

A 250 watt motor is capable of simulating the input produced by a strong person, so imagine two of you pedaling, but the weight being carried by the bike is that of just one person.

When you start to turn the crank you’ll generally feel the motor kick in fairly quickly. If you’re on high power it can feel a little uncontrolled if you aren’t expecting it.

If the ground becomes slippery the hall effect sensor on the pedals can become confused and will keep driving the wheel at full power even when there’s no grip – thus leading to what bikers call a low side. In poor conditions, mud, and ice it’s better to back off the assistance and turn it down to L or M – but you can ride in snow – I know! I’ve done it!

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