The Actual Mountain Wheelchair Motors are Working!

…plus lots of other updates

It’s been a little quiet on the blog over the last week or thereabouts so I wanted to I’d like to share a number of things that have happened during that time…

Spokes

After dropping off the motors and wheel rims at Halfords in Llandudno, the guys were able to take all of the measurements, but weren’t able to locate the correct spokes from any of their suppliers. Just like me, they did some digging around and the only suppliers they could find were in China. I’ve since ordered the spokes from China, but of course they’re all celebrating the Chinese new year at the moment so it’s going to be a number of weeks before they arrive.

Battery Pack

The first 48v battery pack is ready. After bottom balancing all of the cells, I was impressed as they were charging as most of them remained within 0.001v of each other, and there was just one other cells which was 0.002v behind the others. Fully charged, this is a 56.5v battery:

Interestingly, John Williamson (Aka Burgerman) of www.wheelchairdriver.com has been reading through my posts, and based on his background working with charger manufacturers in an advisory capacity for 25 years, he’s suggested that bottom balancing is the wrong way to go about this. Thanks for taking the time to read my posts John, I think more research is required on my part.

Grinding away the Dropouts

With much trepidation, I spent a large part of yesterday grinding away part of the BMX forks to get the motor axles to fit. Looking at them though, I’m really worried that the dropouts on the forks (the part where the axle will sit), aren’t going to handle the torque produced by these motors. I’ve had to file so much away, that there’s far less metal on the forks now than there used to be. Originally, I imagine an engineer would have calculated how much metal needs to be there:

Snapped dropouts is a common problem with e-bikes:

To overcome this, you can buy and attach what’s called a universal torque arm. This arm takes the torque produced by the motor and transfers some of it higher up the forks where they are stronger:

My only problem with this is that I think they look untidy. I gave it some thought last night and have made some rough sketches of a purpose built “torque arm” that will bolt to the frame. If I can get those made up at a reasonable price that’s what I’ll do, if not then I guess I’ll have to go with these unsightly universal torque arms.

At least the motors look cool now that they’re sitting in place and Ada’s really pleased with the purple dust cups which she chose.

Motor Controllers

Six motor controllers arrived from China yesterday and at first glance I’m really happy with them. They’re far smaller than I expected them to be which means they’ll be easy to fit within the mountain wheelchair frame.

The main problem I have with these controllers though is that the Chinese user manuals have been badly translated into English. Trying to understand what the manual is trying to say is rather difficult.

Whilst I was waiting for the controllers to arrive, I was however able to produce a wiring diagram to provide power the 6 motors:

Once I had the wiring diagram and knew what I wanted to achieve, I then spent days searching through online components for the parts I needed. Trying to find a simple thing like a 48v switch (200a and waterproof) has been impossible in the UK so I’ve had to resort to getting parts shipped from China again.

Actual working Mountain Wheelchair Motors

Nonetheless, and this is quite a big moment, I’ve just managed to get the actual mountain wheelchair motors running for the first time!

You’ll have to excuse the temporary wiring, but, Woohoo! It’s working!

 

Mountain Wheelchair Motors have been left at Halfords

This was the scariest moment so far in the mountain wheelchair build…

Last night, I dropped off all of the motors and wheel rims at Halfords Autocentre in LLandudno.

This will be first task that I’ve had to get somebody else in to complete and I felt extremely nervous walking away, leaving all of the wheelchair parts in somebody else’s hands – What if they don’t do a good job of it? What if they drop one of the motors? What if something gets damaged? Do they realise how important they are?

To be honest, the two people I spoke to were very reassuring, and the girl at the counter added to the label “Guard with your life”.

They were kind enough to let me take a photo for the blog – “bye bye motors, it was nice knowing you”

And, they even said they’d take some photos as they carried out the work which was pretty cool.

If you’re reading this Will, then I’d just like to say a big thank you for offering to do it. In the end I thought it best to go to a store to do it because it meant that if they did get damaged then I’d be able to recover the costs.

All in all, I’m a little nervous, but at the same time extremely excited. The wheels were one of the first things I started thinking about when I began this journey. Once the motors are sitting in their rims, they will be the first part the mountain wheelchair which will be finished. No more research, no more designing, done, finito!

So long as everything goes well of course :S

*Update*

Received a call from Halfords the following day to say that they had done all of the measurements and found that they needed an odd size spoke which they couldn’t obtain from their suppliers. They did however email me a link to some on ebay, but they’d need to be shipped from China. The good news is that they’ve done all of the measurements and at least now I know what size spokes to get.

I’ve been into Halfords to collect the motors as the motor controllers will soon be arriving and I want to start playing with the electronics whilst I’m waiting for spokes to arrive.

First look at the Mountain Wheelchair Motors

So the 6 motors which will be used on the mountain wheelchair arrived this morning and I couldn’t help but open them up to take a look inside to make sure they were up to spec’.

Externally, the build quality looks great are the dimensions are perfect for the wheelchair. To get inside the motors you need a special tool usually, so I was really pleased when I undid the screws on the housing and it easily lifted away.

With the housing off, the first thing I noticed was that if I turn the axle, the motor spins much faster. 5 times faster to be precise. So that’s the first thing ticked; the motors will be able to spin at their optimal speed thus improving their efficiency, reducing the heat they generate, and all the while producing more torque.

Motor Windings

The second thing I noticed was that there are in fact 32 magnets with 16 pair poles, unlike many of the cheaper motors which have fewer magnets. This should result in a quieter motor with improved torque.

The motor also has a 20T winding (all of the copper wires which are wrapped around the “magnets”):

These copper coils are what create an electromagnetic field when electricity is passed through them. As the “magnets” turn on and off in the right sequence, it turns the motor. Typically, the hub motors on ebikes have 6 to 10 windings (the amount of times the copper is wrapped around to create a magnet). A bike with 6 windings will travel at a higher speed; because there is less copper for the electricity to pass through it gets where it needs to be quicker. A 10 turn winding motor will therefore go slower because it takes more time for the electricity to pass through the coils. What this Chinese company have done for me is 20 turns which means that although the electricity will take twice as long (in simple terms) to pass through the coils, the resultant electromagnetic force will be twice as strong, thus resulting in a motor which has far more torque and travels at a speed which is more appropriate to a mountain going wheelchair.

Planetary Gears

Although I was happy with what I’d seen so far, I must admit I was a little disappointed with what I found when I extracted the motor from the housing; plastic Gears!

Presumably the gears are made from ABS or Nylon and should last some time, but I can see myself having to service them in the future. At least I’ve got a 3D printer though so might be able to make the gears myself. I would have preferred steel(?) gears but I imagine these plastic ones will at least be quieter and cheaper to replace.

Freewheel Mechanism

Finally the other thing I wanted to have a look at was the freewheel mechanism. Bicycles are intended to roll down hill without the rider having to pedal constantly. For this to happen geared e-bike hub motors have an internal freewheel mechanism. For the mountain wheelchair, this presents a problem because when put into reverse, the motors would just spin freely without turning the wheels.

I’ve read about some people overcoming this problem by welding the freewheel mechanism, but the company that made these motors for me came up with a far more elegant solution. I believe it’s called a Woodruff Key?

Verdict

So all in all, I’m a little sceptical about the plastic gears, but other than that I’m really happy with these motors; they’re lighter than direct drive hub motors, consume less power, they have internal temperature sensors but are less likely to overheat anyway, they’re well engineered, and importantly they should provide more than enough torque to get this wheelchair into the mountains.

In Other News

Now that I have both the hub motors and the wheel rims, I need to get the two things attached to each other. I’m not sure what, but something in the back of my mind tells me that it’s quite difficult to balance a rim on a hub. Add to that the fact that I don’t know what size spokes I need, I figured it was time to get some external help.

The first thing I did was ring West End Cycles in Colwyn Bay. The quote they provided over the phone, for all six wheels, was nearly £1,000! Just for putting some spokes on! Perhaps I’m just not appreciating how difficult it is?

I then rang Halfords in Colwyn Bay. They wanted £25 in labour per wheel, and although the guy on the phone was struggling to find the best price for spokes, he was able to find some which would cost about £35 per wheel. This puts the total quote from Halfords at £360.

Finally, I’ve got one more person to try who I discovered lives just around the corner from me. When I’m not so full of cold I’ll ask for him to have a look and take it from there.

Motors for the Mountain Wheelchair Arrived

This is just a very quick post as I was excited to answer the door this morning and see three large boxes plastered in Chinese stickers. The motors had arrived!

We’re all full of flu at the moment but despite how energy sapping it was, I couldn’t help myself but lay the parts out on the floor. Tada! It’s (almost) a real Mountain Wheelchair Rocker Bogie Mechanism!

OK, it’s not quite a real mountain wheelchair rocker bogie, but you get the idea. Importantly, I’ve been able to find 6061 aluminium tubes with the exact measurements I needed to fit over the bicycle forks and accommodate the bearings which allow the rocker bogie to pivot. This means I’m almost ready to start actually building the real mountain wheelchair!

In Other Mountain Wheelchair News

…I discovered last night that Dan, of Coastal Welding, is willing to grant me access to a new workshop which he’ll be opening soon, complete with tube bender! This means that I could potentially get the curves that I need in the frame without having to spend thousands of pounds on my own tube bender. And unlike me, Dan actually knows what he’s doing when it comes to metal work. if Dan goes ahead with his workshop then it could be pivotal in getting this wheelchair made. If you’re reading this then no pressure Dan 😉 Give me a shout if there’s anything I can do to help.

Battery Charging for the Mountain Wheelchair

The post man arrived today with more goodies for the mountain wheelchair; another 8 LiFePo4 batteries, a 48v LiFePo4 charger, a 3.65v LiFePo4 charger and a new multi-meter.

I’ve had my existing multi-meter since I was in high school, more than 20 years ago. It’s lasted all this time, never failed me, and so I’ve never thought to replace it until now. Following on from a previous post where I discuss bottom balancing LiFePo4 battery cells, I needed a multi-meter that would give readings that were accurate to +-0.001v, which my old meter wasn’t capable of. Now armed with two multimeters I realised I had a problem…

In the post mentioned above, I’d connected the wheelchair batteries to a set of motors to try and discharge them. It took days (literally) of leaving two 300w motors running to get the battery cells down to about 3v each. As there were 8 batteries in the pack, I needed to get the pack down to about 24v (8 * 3), and as was to be expected, the voltage remained stable for days then suddenly started to plummet. At this point I disassembled the battery pack and started to discharge them individually.

Discharging Individual Cells

There are lots of ways to discharge an battery, some of which are far safer than others, but I used what to hand; about 350 Light Emitting Diodes:

Incorrect Multi-Meter Readings

Whilst in the process of discharging the batteries, I noticed that my old multi-meter is giving different readings to my new one. To see which one was wrong, I got hold of a third one and lo-and-behold, the multi-meter that I’ve been using for the last 20+ years is reading higher voltages than it should do. Hooked up to a 3.65v charger, the old one is giving me a reading of 4.09v!

I’ve no idea how long it’s been like this but it’s likely that I’ve been using the wrong measurements for years. Oops!

The good news is that at least now I know I have one that is accurate and I can go about getting all of these battery cells to exactly 2.750v.

Anyway, I’m digressing somewhat; the reason for writing this post is because I wanted to share a modification that I’d made to the 48v LiFePo4 charger.

Factory LiFePo4 Charger

In my previous post (see link above) I explained that after bottom balancing all of the cells to 2.750v, I will then wire all of the cells in series to give me a 48v battery. To this end I purchased a 48v LiFePo4 charger. From the factory, it charges the batteries up to 57.6v (which is about normal for a 48v charger).

If there are 16 cells in the pack, then that means 3.6v per cell (57.6 / 16). This presents a problem because as was shown in my previous post, the charge/discharge curve for LiFePo4 cells is very steep at both ends. As a cell starts to reach its maximum charge, it suddenly starts to shoot up. If one cell were to reach 3.6v before the rest of the cells in the pack then its voltage would increase faster than the others and likely result in permanent damage to that cell. The absolute maximum voltage these cells can safely handle is about 4.2v but I’ve decided to aim for 3.5v to give the whole system a bit of “room to breath”. This meant that I needed to modify the battery charger.

Modified LiFePo4 Charger

Rather than have a charger that tops out at 57.6v, really what I wanted is a charger that tops out at 56v (3.5 * 16), so I went ahead and opened up the charger. My luck was in as I found that it had a very small adjustable VREF relay. By making a small adjustment to this, I now have a charger which tops out at exactly 56v and is perfect for charging the batteries safely.

For anybody else wanting a charger which tops out at 56v, I purchased this one from Eclipse Bikes. The adjustable vref can be found on top of a small blue relay next to the green/red lights and is labelled VR2 on the circuit board. Obviously this will void the warranty and more importantly; the charger is running off 240v mains supply so there is inherent risk involved. For me it’s been a pleasant surprise and I now have a means to charge the mountain wheelchair without damaging the batteries.

In Other News

The 6 hub motors previously ordered have successfully passed UK customs, and I’ve also gone ahead and ordered 6 motor controllers to go with them. When they arrive I’ll have everything I need (apart from a few small pieces) to make an actual moving vehicle. Yeah, I’m making progress with the mountain wheelchair!!!

Time to start building a Mountain Wheelchair!

Some really good news for the mountain wheelchair project…

I’ve purchased a welder which is capable of welding aluminium, and… wait for it… found somebody who is not only willing to spend a day teaching me how to use it, but also to look over my welds when it’s all done. I couldn’t have asked for a better outcome, especially when you consider that Dan (of Coastal Welding) not only manufactures parts for DMM climbing, but also has experience of building custom wheelchairs!!!

Although it was the cheapest I could find, the welding equipment has still cost about £1,000 once gas, fittings, accessories and supplies have been included. So long as it lasts long enough to build the frame, then I’ll be happy.

*Update*

It lasted less than 5 minutes! I was in the process of building myself a steel welding table thinking it would make things easier, and give me a chance to practice with the welder. I did the first tack-weld and the welder threw in the towel.

The welder has been returned to the seller, fixed (loose connection apparently) and has now been able to finish all of my tack-welds. This means that I can actually start building the mountain wheelchair!

Other News

In other news, the motors have made the journey from China and are currently in Northampton. Exciting!

LiFePo4 – Power for the Mountain Wheelchair

It became apparent quite early in the project that batteries for the mountain wheelchair would need considerable research. In a previous post I estimated that the wheelchair would need a 3 tonne battery to get up the mountain. Luckily, the project has come a long way since then.

When I built the first test platform for the mountain wheelchair I decided to use lead acid batteries (think car battery). This was partly because I’m already familiar this technology, and partly because they’re inexpensive; the four batteries on this prototype were £20 each.

The problem with lead-acid batteries is that (1) they’re heavy, and (2) the way their voltage drops over time isn’t particularly useful.

Voltage Drop

Take a look at the blue line in the graphic below, this shows how a lead acid battery’s voltage drops over time.

If you imagine for a moment that this battery is powering a wheelchair motor, then you would expect that over time the battery’s voltage would drop. In fact, looking at the blue line for the Lead Acid battery, you can see that the voltage drops in quite a linear fashion, so there’s nothing really surprising here. Batteries discharge as you use them right?

The problem with this is that the power available to the motor steadily gets weaker and weaker over time. On the mountain wheelchair, it would mean you’ve got lots of power to begin with, but as soon as you start driving, the available power drops in a linear fashion. Half way up the mountain you can no longer get over the same obstacles which you could at the start.

LiFePo4 Batteries

Now take a look at the red line on the graphic above, this represents LiFePo4 batteries. As you can see, the LiFePo4 batteries lose power in a far less linear fashion; they maintain a constant, high voltage for a long period of time and then quickly drop off at the end. This is great for the the mountain wheelchair because it means that you’d have fairly constant power for the duration of the trip. If you managed to drive over a boulder near the foot of the mountain, chances are you’d be able to drive over a similar boulder on the summit.

Not only this, but LiFePo4 batteries are also typically half the weight of lead acid batteries, so not only do you have more usable power, but you also won’t need as much power because the wheelchair will be lighter.

The other main advantage of LiFePo4 is the amount of power (current) they can provide in a single burst. If the motors needed a sudden boost of power to get the wheelchair over a particularly large step, LiFePo4 batteries would be more capable of supplying that boost than lead-acid.

The two main disadvantages of LiFePo4 batteries though are their price and the additional care needed to look after them.

LiFePo4 Price

In the prototype shown above I had four 12v 7ah batteries. These batteries were wired together to give me a total of 24v 14ah. In total this cost £80.

LiFePo4 batteries only provide 3.2v. So to make up 24v, you need 8 batteries. The cheapest I’ve been able to find suitable LiFePo4 batteries in the UK is £23.45 each. Multiplied by 8 this is £187.60 for 24v (More than twice as much as the lead acid batteries).

That being said, LiFePo4 are good for 2,000 cycles (fully drained and recharged again), whereas lead acid has a maximum of 300 cycles. In the long run then, LiFePo4 might prove to be more cost effective as they won’t need replacing as often.

As you can see, these batteries are worth the premium and not too long ago these 15ah cells arrived in the post. Shiny!

Caring for LiFePo4 batteries

As I said above, the other problem with LiFePo4 batteries is the additional care required.

Batteries can catch fire. I imagine it would be a terrifying thing to see somebody in a wheelchair engulfed in flame part way up a mountain.

One of the reasons they catch fire is because there are lots of individual battery cells all working together. If one battery fails, then all the other batteries will keep putting demands on the failed battery until it expands, pops, and eventually has the potential to ignite.

The way to avoid this is to make sure that all of the batteries are well matched. This doesn’t mean just buying batteries with the same specification though, as over time each one will deteriorate at a different rate to the others.

There is conflicting information on the internet regarding how to make sure all of your batteries are well matched – LiFePo4 is a relatively new technology and many opinions are based on previous experience with lead-acid batteries.

Battery Management System

One method is to use a BMS (Battery Management System). A BMS monitors the state of every individual battery. If one battery runs flat, the BMS will stop the vehicle.

The problem with LiFePo4 batteries, as can be seen in the graph above, is that when the voltage of your weakest battery does drop, it will drop quickly. If your BMS isn’t fast enough, kaboom!

The other problem with this is that you have to prematurely stop the wheelchair just because of one weak battery cell.

A better solution is battery balancing.

Battery Balancing

Top balancing means charging each individual cell as much as possible until they all have the same voltage.

Bottom balancing seems to be the best option for LiFePo4 cells though. Instead of trying to balance the cells by forcing them to a certain voltage, in bottom balancing you drain the LiFePo4 cells until they’re in the steep downwards part of the discharge curve (Usually about 2.75 volts).

As I write this post, I have my LiFePo4 cells wired up to two motors which have been left to run.

There are 8 cells in total and I want to get them down to about 3.1v each to begin with. 8 x 3.1 = 24.8. Once the pack reaches 24.8v, I will begin to discharge the cells individually until the each have 2.75v.

The cells then need to be left for 24 hours for their voltage to stabilise. At which time, it might mean charging or discharging them a little more, until they are all within a +-0.001v tolerance.

Once all cells have stabilised at 2.75v, I will wire all of the individual battery cells together, and charge them as one unit, as if it were a single battery. Before I can do this though, I need to buy another 8 cells (to give me 48v in total), and a 48v battery charger.

With all 16 cells at 2.75v, then wired together in series, I will use the 48v charger to bring them all back up to almost maximum capacity. Maximum capacity for these cells is 3.6v, however, I’ll only bring them up to 3.55v in order to give them a little room to breathe. For all 16 cells, this will be 56.8v in total, and should be plenty enough to get the wheelchair into the mountains.

*Update* See the modification I made to the battery charger.

Once they’ve been charged, they need to remain in their pack. All cells should be discharged and recharged together. If for some reason you disconnect one of the cells, then you will need to go through the whole bottom balancing process again.

How much power will the mountain wheelchair have?

The current design for the wheelchair will accommodate a maximum of 112 x 15ah LiFePo4 cells.

To make 48v, I need 16 cells. If I then divide 112 by 16, this gives me 7 x 48v battery packs. If each of these packs has 15ah, then this gives me a total battery capacity of 105ah (7 x 15).

I don’t know how many amp/hours the mountain wheelchair will need. Some people have said that 105ah is far too much, others have said it won’t be enough. As I don’t have the mathematical ability to calculate this figure for myself (although I have attempted it here), and I seem to get conflicting opinions depending on who I ask, my plan is start off with one 16cell 48v pack which gives me 15ah. I’ll see how I get on with this and then add more packs as required.

It makes sense to start off with as little as possible, not just because it makes for a lighter wheelchair, but also because each 48v battery pack costs nearly £400! If I use up the available space on the mountain wheelchair and have 112 cells to give me 105ah, then I will have spent £2,626.40 just on batteries.

What’s next?

The other 8 batteries have been ordered, along with a 48v battery charger and a single cell 3.65v charger. When they arrive I’ll get the whole pack balanced. The hub motors have been posted and are currently en-route from Cina. One of my next tasks will be to invest in some motor controllers so that when the motors arrive, I can begin testing the electronics.

Pre-Made Trials Bike Forks

Over Christmas I spent considerable time looking at the properties of different metals to find which is suitable for building the mountain wheelchair frame. Steel is far too heavy; even this smaller “prototype” which I built earlier in the project weighs 40kg. That’s half a grown man!

Aluminium Grade

The other two likely candidates are aluminium or chromoly. In the end, I decided to go with 6061-T6 (or its UK equivalent 6082-T6) aluminium. This alloy is used in a wide variety of applications where appearance and better corrosion resistance with good strength are required. However, everything I’ve read and all the advice I’ve received have suggested that welding aluminium is not for the beginner.

First Full-Size Test Vehicle

Before building the full mountain-going wheelchair, I figured it’s best to get the rocker-bogie part made first, with some kind of platform in-between for batteries (see image below), make this radio controlled, and then run some tests. At this point it should in theory be capable of getting up Snowdon, if not then it’s back to the drawing board. Eek!

Quotes for Building the Frame

With this in mind I’ve obtained some quotes for making six sets of bicycle forks to accommodate the hub motors. Quotes have been in the region of £1,200 – £1,500.

With £1,500 just for the forks, I imagine I’d be looking at 5/6k for the full thing. In light of this, I’ve decided to continue with the same mindset that I’ve adopted thus far and attempt to weld an aluminium frame myself. Not only will this keep costs down, but it means I will have complete control of “deadlines” and be able to make modifications both during the initial build and at a later date after having conducted some tests.

Pre-made Bicycle Forks

To try and simplify some of the fabrication I went ahead and purchased 6 pairs of pre-made BMX forks. The forks have the correct dimensions for my needs, are made from the same 6061 aluminium that I plan to use for the rest of the frame, and are intended for trials riding (jumping over rocks) so should be perfect for the mountain wheelchair.

Bmx Trials
In addition to this, the recommended retail price for the forks is £119 each but I got at them for an absolute steal at £120 for all six!

Full Size Wheelchair Model

When designing the wheelchair it was important that Ada found it comfortable to sit in for hours on end. With this in mind I made a PVC mock-up of the wheelchair seat, then everything else was measured to fit around this.

With the dimensions correct in the virtual 3D computer environment, it was still difficult to get a sense of scale. In light of this I decided to make a full-size replica of the wheelchair out of PVC pipe.

As it’s made from PVC pipe and just balancing on the wheels it is, as was to be expected, a little flimsy. However, after spending so much time staring at the 3D model on a computer screen and not really having any sense of scale, building this PVC model has been worthwhile. When I first took a step back to look at the life-size model I felt both a huge sense of achievement and relief as so many of my worries melted away. With a little imagination it is (in my mind at least) easy to see this becoming a real wheelchair which is capable of getting into the mountains.

What’s next?

There’s a little tweaking still to do in terms of ground clearance (at the moment it’s probably a little too high), but the design will allow me to adjust this during the build. The important thing is that everything I’ve been doing on the computer looks like it’s going to work in the real world.