All the components of a purpose-built electric bikes are designed to work together and normally come with a guarantee. In addition, they are normally designed to be ridden on the roads.
Not all batteries are created equal
A Sealed Lead Acid Battery (SLA) costs a fraction as much as a Lithium-Ion battery. But they also hold a fraction of the power. They are also much heavier, larger and more susceptible to damage.
Whether you choose to build your own bike, buy a conversion kit or buy an off the shelf bike a Lithium-Ion battery is a far better choice than a SLA battery.
The motor on an eBike needs to be able to withstand some heavy-duty use. Especially if you expect to be riding it in rain, mud or sandy conditions.
A better-quality motor will be more resistant to water and dust and also feature more durable components. See the related question: Can I ride an electric bike in the rain?
Hub motors, in particular, have to be able to withstand every bump and jolt of the road. If you buy a kit make sure that the hub motor and wheel have a solid construction and are water resistant.
If you expect to be using your eBike primarily for shorter distance commuting on good quality roads a lower cost hub kit motor may well meet your needs.
Another key difference between low-cost bikes and higher end bikes is whether they are ‘street legal’ in your jurisdiction.
In most parts of the US, Canada, Europe, UK, Australia, New Zealand and Japan electric bikes need to have speed, pedal and braking sensors to be ridden on public roads. Most cheaper conversion kits will not have these sensors. This means you could get into trouble with the Police or face serious charges if you are involved in an accident.
If you buy a conversion kit ensure that it will allow you to comply with electric bike regulations in the area in which you intend to use it.
All the components on a good quality purpose build electric bike have been designed to work together. This means the bike should be well balanced and efficient. It also means cable plugs and connectors should make a secure waterproof connection out-of-the-box. Chargers, batteries, motors and mechanical components will also have been designed to complement each other.
Purpose built electric bikes usually have a stronger frame with more ‘braze-ons’ to allow motors, batteries, sensors, gauges, and controllers to be safely mounted. Because an eBike is typically 7kgs heavier than a similar traditional bike the forks and frame are subjected to greater forces when you hit a bump. In addition, electric bike motors apply greater force to the frame/forks/chainstay when you accelerate. These attachment points are strengthened in a purpose-built eBike.
Selecting an Electric Bike Conversion Kit
It is easiest to select a conversion kit which includes all the components you need to convert your bike. Whilst it may cost a little more, in the long run, you are likely to be much better off.
Some top-end conversion kits have all sensors built into the rear wheel hub. This makes converting your own bike a whole lot simpler, safer and more reliable. However, these conversion kits are not cheap.
See the related question: What is the best type of electric bike conversion kit?
There is nothing to stop a keen home-builder designing and building their own electric bike using components individually selected. However, if you go down this track take the time to do your homework and learn some basic electronics and bicycle mechanics.
Some of the most exciting eBikes are built by enthusiasts. If you have a passion for creating something cool building a new eBike can be the most rewarding project. Go for it!