Tuesday, 22 October 2013

How to make a really cheap electric bike.

How to make a really cheap electric bike.

Before you start, you need to be able to use a soldering iron, and bike spanners. If you
can't, then go learn.

You must also learn about the care of Lipos. They are dangerous, no question. I'd say that
each of the Lipos that I'm using is as dangerous as a pint of petrol. Which means that if you
don't do anything stupid, you'll probably be OK. But some people do treat petrol stupidly, and
get hurt. Likewise Lipos. So, learn about the safe handling of Lipos.

The two most important things for safety; 1) don't over-charge them (your charger should ensure that doesn't happen), 2) don't short circuit them by connecting a battery's positive to its negative (I designed my connectors to make that pretty much impossible). And I'd add a third, important not for safety but for keeping the batteries working,  3) don't let the voltage of any cell fall below 3.0 volts (I use 3.2 volts as a minimum).

First, you need a bike. If you're reading this, you probably already have a bike and you can
convert that. If you haven't got a bike, or don't want to mess with the fine machine that you
already have, you'll need to buy a bike.

I just checked Ebay; second hand bikes go for £20 and upwards. You'll want one with gears;
three at the pedals and five or more at the rear. You might want suspension; that will cost
extra. You might pay as much as £50 for a really good second-hand bike. You might need to
replace the brake pads (about £1 per pair).

You'll need a back rack. Not a "seat post rack", you want one that will be carrying the weight
of the batteries. That will be £11 or so. You'll also need at least one pannier, the kind that
hangs down from the back rack. £8.

That's the bike hardware, total cost £39.

Now you need the motor. Ebay again. This will probably come from China, and will probably be shipped by sea, unless you want to pay a fair bit extra for airmail. Here's what the wheel looks like, after I put on an inner tube and a tire, and installed it in the bike.

The wheel should come with cables and a small controller box. I mount that box on the back rack, as you can see here. Then I wrap the protruding cables in duct tape, to improved the water and mud resistance.

Here you see the controller on the back rack, with a pannier hung from the rack.

The orange cable is a 13 amp household flex, taking the power from the controller to the motor, because the motor I had came with cables that were too short.                              Usually, the cables won't be too short.

You want a 250 watt motor, because that's what's legal in the UK. That can be 24 volts, 36 or
48, it doesn't much matter which. I see a 24, 36 and 48 volt 26 inch wheel on Ebay for £153,
including postage. This is the biggest cost, but for that you get the wheel, and also the
controller, headlights, a key switch, a throttle and some other stuff that you probably won't
use. Notice that the wheel is 26 inches; that's great if the bike you're converting is also 26
inches (26 inches is the commonest size). It's very bad if the bike has a different wheel
size. So, obviously, make sure you get a wheel the right size.

How much is 250 watts? It's about a third of a horse power. The average human who is fairly fit, can do 50 to 150 watts for an hour, but if you're looking at an 8 hour stint, 75 watts is about it. So you can reckon you're riding a third of a horse. Or you're being towed by three people.

Also, notice that there probably isn't a disc brake disc on the hub of the wheel you're
buying. That means that they bike you're converting, should have caliper brakes (i.e., not
disc brakes). If the wheel you're buying does have a disc for a disc brake, then you can have
a disc brake. I hope that's clear! One bike I converted had a disc front brake. No problem, I
installed a caliper brake there, and that works fine.

Should you get a front wheel or a back wheel? A front wheel is very easy to install. A back
wheel will come with gears (5 or 6) which you'll want to be the same number of gears as on the bike (and you'll probably need to adjust the gears after you've installed the wheel). The
advantage of having the motor on the back wheel, is that a lot more of your weight is on the
back wheel, so it'll grip better. But having said that, I've always used front wheel motors,
and I don't have a problem with grip. When you're needing to put in a lot of effort, you're
using your pedals and that always drives the back wheel. And I think that maybe having part of the motive force on the front wheel (the motor) and part of the motive force on the back wheel (me pedalling) means I have a two-wheel drive, which has some advantages over a one wheel drive.

24, 36 or 48 volts? I got 24 volts because I already had some 24 volt batteries. The advantage of 48 volts, is that with 24 volts the motor is pulling 10 amps (and can pull a bit more for short bursts). But with 48 volts, the same power (250 watts) pulls 5 amps. So you could use thinner wires. But you won't, because the wires that you get for mains electricity can carry 13 amps, so they'll work here too, and there's no real saving in using thinner wires..

Now you need a battery. It took me a long time to realise that I should go to the model
aircraft and car community, because that's by no means obvious, or at least it wasn't obvious to me. They runs models that are really powerful, with powerful motors and heavy-duty batteries. Hobbyking is the place to go. Everywhere else seems to be double their prices. I was very surprised about that.

Let's assume you got a 24 volt wheel. You might think that you want a 24 volt battery. But
it's not so simple.

Lithium batteries (Lipo) are 3.7 volts. So 3 in series is 11.1 volt, 6 in series is 22.2 volts
- not enough, you see? But. When fully charged, thay're 4.2 volts, so six of them are 25.2
volts. Which sounds like enough ... but it very quickly won't be.

I took one of my old 24 volt batteries apart, and it had seven cells. 29.6 volts when fully charged. So that's what I thought I wanted. But it's not so simple.

You can't get seven-cell batteries. Mostly what you can get, are 4 cell and 3 cell batteries (and less), so maybe you could put a 4-cell in series with a 3-cell?

I thought about this, and I thought about the possibilities for making mistakes, and I
thought, suppose I use two 4-cell batteries in series? That's a nominal 29.6 volts, and 33.6
volts when fully charged. Will the 24 volt equipment on the bike be able to handle this
highter voltage? I thought it would, and it did. So that's what I'm using; two 4-cell
batteries in series.

Hobbyking offer 5 Ah, four-cell Lipos (product ID T50004S-20HC) for £18.25, which is
blisteringly cheap. And if you leave the page loaded for a few minutes, a pop-up will offer
you a discount. I got 9% off. You'll need two of these; that will give you a 29.6 volt, 5Ah
battery pack. Most electric bikes that you buy, have twice this capacity. You can too; just
buy twice as many batteries. Or even more. I've bought 12 batteries. And if you bought a 36
volt motor, you would use three of these four-cell batteries in series. or with a 48 volt
motor, use four.

Here, you see two four-cell batteries taped together (the tape is just for handling convenience); I also tape a handle to them because I don't want to lift them by their wires, because if those wires break, I can't use the battery. I've soldered a female kettle plug to each of the batteries, because I'm using kettle plugs as my standard plug and socket. Radio Control folks wouldn't use those because they're big and bulky, but I like big and bulky for bike connectors. You can also see the small white connectors, those have wires that go to each cell inside the battery, so that the charger can charge each of the cells equally. What you see below, has a nominal voltage of 29.6 volts (33.6 fully charged) and 5 amp-hours. That would last me maybe 2 hours on the bike. Obviously how long it lasts depends on how much power you use; if you want to, you can do 90% of the work with pedalling and then the batteries might last all day. I like to use the battery a lot - that's what it's there for!

You'll also need a charger. You can't use any old battery charger for Lipos. At Hobbyking, the
ECO6 costs £13; that lets you charge up to six batteries at once. Here's my charger, I use an iMax 8. On the right, I've connected a wattmeter, costing £5, which tells me how much power is going into the batteries. The wattmeter is connected to  four male kettle plugs, so I can charge four batteries at once. You can also see the white plugs; they connect to the white plugs on the batteries, and they let the charger charge all the cells in each battery equally. The charger only has one balance port; you can see the six-way adaptor in this picture.

Here's another parallel power adaptor; that will let me charge six batteries at once.

And you need to power the charger. For that, you can use anything that gives 12 volts or
thereabouts, and enough amps for the charger; at least 5. I use an old PC power supply, zero
cost. If you buy a power supply from Hobbyking, that's £30. I use the 12 volt line (coloured yellow for positive, and the black is negative), of course, and that's connected to a couple of female 4mm connectors, which is what the 4mm male connectors of the charger want to mate with.

You'll also want a multiple balance lead so you can charge and balance up to six batteries at
once, £1.36. And you want a cell voltage alarm (PRODUCT ID: Voltage-Alarm) to tell you when any cell falls below 3.2 volts while you're riding the bike. One for each battery, so you need two, that's £5.32. I'd also suggest a car-type fuse, just for extra safety (£2, and I don't have a really good justification for including this, except that it costs very little and if anything goes wrong with the wiring while I'm out, I'm hoping that the fuse will blow rather than something more drastic happening), and a voltmeter to display on your handlebars (£1).
How can anything go wrong? Well, I've come off the bike a few times over the years; nothing too drastic, but that could damage things in an undefined way.

Get some 4mm connectors. 10 pair for £3.17, you'll need them for connecting to the batteries and the charger.


Bike           £20
Carrier        11
Pannier         8
Motor kit  153
Batteries     34    (5 Ah)
Charger      13
Misc             13   (leads and connectors)

Total           252

The cheapest electric bike you can get is more than twice that, and for a good one, you might
pay £1000. But this way, you get a base bike that's as good as you want it to be (suspension?
gears? brakes?) and as much battery as you want. I assumed 5 Ah above, but you probably want at least 10 Ah, and maybe more. If you buy six batteries, then you'll be able to charge them all at once (but see next paragraph).

If you've fully run down some of your batteries, but only partly run down others, then you
can't charge them all together, because as soon as you put them all in parallel, an immense
current will flow from the partially charged to the discharged, and that's not good - it could
even be quite bad. By "bad" I mean maybe catastrophic. So, charge the fully discharged ones,
then take those off the charger and charge the partially-discharged ones. Or get two chargers (that's what I've done).

So now, with all the parts acquired, it's time to build our bike.

First, install the wheel. That installs just like any bike wheel. Then you wire it up to the
controller; that's just a matter of plugging things together, and you can't get it wrong. The
controller will have some extra wires; there's wires to the replacement brake levers, so that
power is cut when you brake. I don't bother with that, because I can't actually brake unless I
take my thumb off the throttle. There might also be wires for a pedal sensor, which again I
don't use because I prefer to use a throttle. And maybe a cruise control - again, I prefer to
use the throttle.

If when you connect everything up, the wheel spins backwards, don't worry! You can either
install the wheel the other way round, or else you can change the three power connectors for
the wheel, so they connect in a different order. Instead of yellow-yellow green-green,
blue-blue, you might try yellow-green, green-blue, blue-yellow, for example.

The controller will also have wires that go to the battery. I connect those to a male kettle
plug, and I connect the batteries to a female. That makes it easy for me to disconnect the
battery pack completely, so I can take it off the bike quickly (very useful when I want to
lift the bike over an obstacle).

 There will also be a display to show you power, on the handlebar. I find those of limited use; it's just 1, 2 or 3 blobs, os something similar. And with the overvoltage we'll be using, it won't tell you anything useful. But a voltmeter is great. You wire it in parallel with the blobby-meter, and it tells you the voltage of your battery pack, which is a pretty good indicator of how much push you have left. Below, you can see the blobby-meter, looking very like a petrol gauge, but actually pretty useless, because I'm not using the voltage that it's expecting. Blu-tacked to the top, you can see the voltmeter, which gives me an accurate reading of the battery voltage. And when I apply power to the wheel, I can see how that voltage drops as the battery comes under load.

It starts at 33.6 volts (4.2 times 8), and when it gets to about 28, the batteries are nearly exhausted, and the cell alarms will start to go off soon.

On kettle plugs - on Ebay you can get 10 extension leads for £15 (search for "kettle plug
extension").  That gives you ten male and ten females, already wired up.  Cut the lead in the
middle, and solder the lead to whatever it need to be soldered to. I already had several dozen
of these doing nothing. These should be able to handle 13 amps, and you're putting 10 amps

The battery pack is wired up thus. You have two 4-cell batteries, wire them in series. Put the
car fuse in series with those. If you have a 250 watt motor, it should draw maybe 10 amps
(watts = amps times volts), so a 15 amp fuse (or maybe a 20) is what you want. I carry a few
spare fuses. And connect that to the female kettle (female, because you would have to be
trying really hard to short circuit a female kettle plug).

It looks like this.

+kettle _____ fuse _____+battery1   -battery 1_____ +battery 2    -battery 2 ___ - kettle
I call that the driver cable. It looks like this. You can see the car fuse poking out attached with the red wires. The two short cables go to two batteries, the long one goes to the controller.

But you can't use that for charging. For that, you put the batteries in parallel. I call that
the charger cable,

So  the + of battery 1 is connected to the + of battery 2 and the + of the charger.
And the - of battery 1 is connected to the -   of battery 2 and the -   of the charger.

When charging, you use the parallel balance lead cable to connect the balancing ports of the
batteries, to the balancing port of the charger When driving, you connect each of the
balancing ports of the batteries to a cell voltage alarm. I set mine to alarm at 3.2 volts.
The reason for this is that if you run a Lipo down below 3 volts, you can find that it won't
charge up again.

The throttle goes on the handlebar (I prefer the right). The controller goes on the back rack;
I just bolt it down, wrap the wires coming out in duct tape and splodge some sealant on the
tape to help it be more waterproof. The batteries will go in the pannier. I use cable ties to
make the various cables neat on the bike frame.

And here's what the completed bike looks like. The batteries (both charged and discharged) are in the pannier; if that doesn't give enough room, I put on the other pannier.

The bike is a bit heavier because of all this. The motor is pretty heavy, 2 or 3 kilograms. The batteries are about 1 kg per pair (giving 5 AH). So if you want 30 AH (which is about what I use in a full day's cycling) that's about 6 kg. That's a lot, but when you're trundling along, the bike is carrying it, and if I need to lift over an obstacle, I unplug one kettle plug, take the pannier off, lift the bike over, and replace. So it's pretty quick. And I rarely carry the full 30AH. Usually, I do a circuit in the morning winding up back at the car, then another in the afternoon. So while I'm back at the car, I can swap out exhausted batteries for full ones.
By the way, for comparison, a standard car battery holds about the same amount of power as these 6kg of Lipo batteries. And weighs about four times as much.

If you haven't got a bike helmet, then get one. I only have one head, and I need it. I always
wear gloves; I do come off my bike sometimes (I ride on rough ground) and gloves keep my hands usable. If you'll be riding at night, get a good head torch (which can also have a red
flashing light at the rear). And a bell, to warn pedestrians that you're coming through. I
also carry a good strong bike lock and some puncture repair and bike repair tools; since
you're already riding with a pannier, you have somewhere to keep this stuff.

Good luck with your new electric bike, and have fun!


  1. I took the battery back immediately when it wouldn't charge. I was told that it was my charger and not a prob with the batteries. Even with a modified charger the batteries never took a charge. electric bikes nz

  2. use xt60 connectors - best i ever used! also use heat shrink wrap for wires much better! Cheers=)

  3. use xt60 connectors - best i ever used! also use heat shrink wrap for wires much better! Cheers=)

  4. This certainly is a cheap eBike build. For anyone that wants a higher quality ebike though. I recommend building one yourself using this guide.. since I found it so helpful.

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