35 stone user on 21 stone capacity chair

[StephensonVoice]

Chair manufacturers seem to think the world is made of tiny people. I would love some advice on creative solutions for a very, very large person. (Yes, I need to cut my weight in half. Won't happen quickly, if it ever does. Apologies. Insulin response problems on top of poor self-control.) I think I want fast walking speed, and tree-root-buckled sidewalks are likely as close as I am to getting to off-road travel.

I have trouble standing and walking any distance. Soon I will need a chair, but I cannot afford those with an official 42 stone/600 lb/272 kg capacity. I have a background in light manufacturing, programming, and electronics, so I thought I would try to build my own. I am looking at this project, a self-balancing mobility device: https://www.instructables.com/SITWAY/



I just bought a parts-donor chair, a Pride Jazzy 600, which is in great condition besides needing two new NF-22 55 AH batteries (if I want to put it into service as manufactured, instead of stripping it for parts). This chair is rated for 21 stone/300 lbs/136 kg, and I am 35 stone/480 lbs/220 kg. I doubt the frame will break or the tires will pop, but I expect the range and speed to be at best a third of what they claim, given that I'm half-again what the poor motors and batteries were expecting.

So... if I want to keep the chair 'stock', can I do anything to improve the battery situation? I would assume that getting four of the stock batteries and adding the second pair in parallel would help with the range, but that speed would be unrecoverable.

And what can I do to keep from burning out the motors? Short of adding water-cooling to the motors, I kinda doubt there is much that can be done to protect them. Maybe just drive like a grandma?

And if I want to build the self-balancing chair above and strip the chair for parts, what can I do to make the project stout enough to carry me? I would assume that ultimately I will need 2kw motors instead of 250w, for example, but thought I would use these to get a feel for the building and experimenting - and that I should save money for the new motors between now and the time I burn these out.

The project calls for a 25A motor controller, but a 100A version is available. Would that be a good choice (either for this project or for when I beef things up later)? Or am I completely missing the important bits? I would love to get some thoughts. And I am not above throwing some dough at someone who might want to invest some time with me and the instructions for the project I'm considering.

Thanks!

-Tim

[StephensonVoice]

Can't edit. The same 25A controller called for in the project plans has two bigger brothers: they are 32A and 60A versions that have the same control-side inputs but pass more current. I misremembered it as 100A, above.

[Burgerman]

Where to begin.

Pride stuff is a waste of time.
250W motors is basically a guide to how much energy you can give them continuously before the smoke comes out. And a guide to where they work most efficiently. But it is almost unrelated to the power or torque they can actually use. Or make.

A low impedance 250W motor can draw much higher current and be able to make huge torque,
A higher impedance (typically 2 pole) motor can be rated at a higher watts like 450 or whatever and actually be far less powerful (much lower current draw at any given load/voltage). And so is much weaker. So dont be fooled by reading watts... Its efficiency and impedance that matters.

A typical GOOD 4 pole powerchair motor may easily draw 4 or 500A at low or stalled RPMs. Except that the actual amount of current it pulls is limited and controlled by the chairs power module. In the case of a decent system, that will be 120A per motor.

So since most motors are pretty much easily able to pull 120A at stall, even at tiny joystick movements (because of a thing called load compensation where the controller uses measured current and a positive feedback loop to increase this). Then the motor will be at the 120A limit as you turn in place. Or as you climb a threshold or whatever. Especially with faster motors or heavy users. I am 20 stonne and mine bounces of this 120 limit hundreds of times per day.

So that means your motor may be taking 120A, x 24v and sucking up a crazy 2280 watts peak. Somewhat more than the rated few hundred watts! But... Its normally much less. Why?
Because at STALL the motor may be pulling 120A at say 8V. The actual voltage that this happens depends on the motor impedance. Obvioulsly a cheap high impedance motor may only JUST reach 120A at 24V. So it takes way more power (in watts) compared to a low impedance motor. So a 4 pole motor with low impedance may produce 3 times less heat,take 3x less battery power (less battery amps) and the SAME torque (Amps = torque) as the higher watts being fed to the high impedance 2 pole motors.

What does this mean in reality?
It means you need what are called HD motors. Definitely not 2 pole ones. nd definitely not "fast" motors. These HD motors are 4 pole low impedance motors, mated to a LOWER SPEED gearbox. HD is for moving larger mass with lower wasted heat in the motors Twice the speed means you need to double the motor Amps at every speed or manoever.

So at your weight you really need to throw away the motors you have.
And since everything pride is locked down and not user programable you best get rid if the rest of the chair too.

Speed...
Next problem.
A 6mph chair, 4 pole motors with a 120A controller that is PROPERLY programed is just about capable of working properly at my 20 stone weight. Just. And that is with "full sized" grp24 80Ah low impedance batteries. Speed + weight = lots of wasted heat and battery power. And inadequate torque. And controler thermal current rollback issues. Meaning at your weight faster otors are going to be overwhelmed with inadequate torque and too much heat.

If I fit 8mph motors, the control system cannot produce enough Amps for adequate stall torque in extreme situations like trying to turn in place in my hallway... It stalls out. Sits at a measured 120A and refuses to turn. But its worse than that. It is uncntrollable and steering responce gets dngerous on the steeper ramps into things like trains and even the ramp to my van. It runs out of Amps for steer responce as its already maxed out.

Also then - because the amount of current needed to move at EVERY speed, every turn, is increases with gearing then the range suffers and group 24 is no longer adequate either. Because of a thing called peukert. The range sufferes horrendously out of proportion to the increased motor current. Then that means the battery average depth of discharge also increases daily causing short battery service life too. Its also why your 22nf batteries, at your weight is a joke even on a 6mph chair.

So theres a lot to think about.

HD low impedance motors are usually 4 or at best 5mph for good reason.

Also, the chair you buy needs to have a group24 battery area ideally, as that allows you to fit 8 lithium cells of 230Ah... Far more in keeping with your weight. These offer HALF THE WEIGHT of lead, 5 times the range. Your weight will kill the range of lead to the point where they will be useless. But lithum will save you 45lb of mass.

And realy if you want user programmability R-Net control systems are the only game left in town.
So questions???

[StephensonVoice]

Dear Mr. Burgerman, here is what I took away from your post:

• You are incredibly generous in investing your time in your long and thoughtful reply, and very gentle to a noob who set a foot wrong - thanks!
• I did not do my homework very well: yes, I tried searching, but there's a lot of basic information (motor/battery theory, for instance) that I still need to learn;
• I bought a chair that MAY be appropriate to use 'stock' for my petite wife, but which won't work for me (motor, batteries, or locked-down brand);
• I need a chair that has (or, for my project, to buy) a pair of good, low impedance, 4 pole, HD powerchair motors, and lower speed gearboxes;
• I need a control system that is able to produce gargantuan amounts of power - for actual safety concerns as much as any other reason;
• If I want programmability, an R-Net control system is my option; and
• I need to plan a big-boy group 24-Size battery and fill that Group 24 space with 230 Ah of Lithium cells.

Seems I need to go roll some more drunks in the park or work extra hours, on top of doing a lot more reading. Sigh.

I will be digging through this goldmine - I mean website - to find more appropriate motors. And since I already paid for the chair and am picking it up today, I will probably replace the batteries with stock, and keep the chair in storage for my wife's use when she gets to that point. Or flip it to someone else, now with new batteries, and put the money back into better motors.

Thanks again for your efforts on my behalf!

-Tim

[Burgerman]

Correct.

I need a control system that is able to produce gargantuan amounts of power - for actual safety concerns as much as any other reason;

You may think a 120A control system is gargantuan.
It isnt.
It is the minimum required for me at 20 stone with 4 pole 6mph motors, rear drive chair (high torque is especially essential with rear drive).

Here is a R-Net 120A power module with the programmer connected. I did a "turn in place" in my hallway on a carpet. This screenshot shows that even without a ramp. No curbs, no hill. No grass. Just a flat floor in my hallway REQUIRED almost 120A to begin the chair turning.

One motor shows 102.2A. The other 113.7A.
They both actually touched 120 several times. But getting a screen shot on my laptop at that exact momente was not easy!
Understand that 8mph motors would need an extra 30% MORE amps to give this same movement at the same speed.
So you would need a 150A controller and they dont exist. (Well they do but thats another entire book).
Also note that 102 and 114 (rounded) is 216 Amps total being used. And battery current is not even close to this. Its 10 times lower! This is another thing that initially seems not to make any sense. But there are times when battery current actually IS 216A. But not at stall/slow speeds. I can explain this in time but its not simple to grasp.

[Burgerman]

And remember that if the all up weight of the chair doubles, (instead of a 10 stone user you have a 30 stone user approx) then the CURRENT required also doubles.

So fast motors. Bad plan.
Heavy user. Bad plan.
Weak 2 pole motors. Bad plan...
Small batteries really bad plan.
Hills. Bad plan.

A little of all of the above and disaster.

Heres a quick video. This is a very light chair. 6mph motors. Rear drive. Old chair but also demonstrates this issue.
I have a DC clamp meter on a single MOTOR cable. Remember that the opposite motor is doing the same unseen.
This is an old chair.
It only has a 100A controller. And as you can see it isnt enough. The chair works. But its not linear and hesitates when the controller gets maxed out.

http://www.wheelchairdriver.com/gopro/motoramps.mp4
What would you expect to happen if you put 35 stone on board? I can tell you. It will stall out probably not turn, or not turn in place reliably. The controller will get hot and roll back power and then the batteries will last about 1/3rd as long in range and service life.

In this vid, at the end. its slowed down so that you can see the current ramping up, while the chair still isnt moving. It only decides to turn right at 100A maxed out. It hesitates and I wait untill this happens. As you can see 100A is REQUIRED to make a chair initiate a turn in place even on a smooth hard surface.

[StephensonVoice]

...and, of course, I live in a hilly area. Sigh.

Gonna buy a Harley and call it done.

Actually, I may pursue a proper build. But I'm wrapping my head around umpty thousands of dollars rather than the $200 I spent on the Pride chair.

[Burgerman]

You will either need whatever insurance pays for where you live, or a lot of thousands. Yes disability isnt cheap unfortunately.

[StephensonVoice]

I may go back to all of the Electric Vehicle research I did ten or fifteen years ago, and just build a power chair with proper EV guts in it. Physics be damned.

For example: https://www.electriccarpartscompany.com/zilla-1k-ev-dc-motor-controller-1000a

[Burgerman]

Thats stuff is easy. But the software and control thing, including the likes of motor load compensation etc if the hard bit. You are likely looking at years of development time and endless testing. We did all that with the Roboteq controller and 45V and 150A.

Its not simple. And certainly not a fast project. The danger here is that you dont yet understand the complexities and will not until you are well and truly into it...
Theres a ROBOTEQ thread at the top of the forum. Please read all of it. Theres thousands of posts in what was seemingly a simple idea!

Now if you used FOUR mph motors, at say 48V then you would have the same torque as a HD 4 pole motor has, and it will go 8mph. Thats also at the same lower current. But while this will solve your problem its really way way more complex than it seems!

For e.g heres the control script, years on and still unfinished. https://www.wheelchairdriver.com/BM3-co ... Script.txt

And you may like to look through some pictures... https://www.wheelchairdriver.com/BM3-construction/

[StephensonVoice]

I have been talking with my dear wife about this, and we are in agreement that this is a noble project, and that I would be very wise to shut up, listen (read), and not try to reinvent the wheel. And save money up because this is not a hit-and-giggle task.

[Burgerman]

Very wise.

Gonna buy a Harley and call it done.

In the UK we call those Hardleys. Because they can hardly pull the skin off a rice pudding.