Wheelchair inefficiency

[SamTexas]

I got my first exposure with electric wheelchairs and mobility scooters while testing/improving my BIL's rides. The first thing that hit me hard is the EXTREMELY high energy consumption. On level ground, these vehicles pull about 175 watts cruising at 3mph. That's 58wh per mile!!! At that rate I would be traveling at 25mph or faster on my 9C2807 bicycle.

What gives?

[salty9]

I would check the mechanical power train. Dragging brakes, gear box, bearings, etc., (things running hot). I am not saying there isn't an electrical problem but I don't feel comfortable giving advice.

[dogman]

Brushed motors? Plus gearbox drag. You see the same thing in a 4x4. While it might get a stellar 12 mph on the road at 40 mph, put er in granny and creep for very long. Hey, 20 gallons of gas took me 10 miles. Woah!

At some point, you are too slow for efficiency. But it's got the torque I bet.

[SamTexas]

Brushed motors: Yes, two of them, one for each rear wheel. Assuming the worst: Twice as inefficient as brushless motor.
Gearbox drag: I don't know. Do they have a gearbox? Multiple automatic gears or are we talking about the typical one gear reduction like a geared hub?

I don't have a reason/incentive to open up the motor so I don't really know what's in there.

But purely from a theoretical pov, only 7 watts is needed to propel that wheelchair at 3mph on level ground. Let's double it for brushed motor -->> 14 watts. Let's also add 100% for the 4 small, fat and rubberized wheel -->> 21 watts. Let's add another 50% on top the total for the "horribly inefficient" gearbox -->> 32 watts. 32 watts is only 18% of the actual power (175watts) drawn. Where does the other 82% go? The motor temp is warm (NOT HOT) after 1 continuous hour ride at WOT. If that 82% was converted into heat, the motors would be BURNING by that time.

Anyone here have experience with motorized wheelchairs or mobility scooters? If so, please help. NO, believe it or not, this horrendously high energy consumption is typical with these motorized wheelchairs and mobility scooters. Nothing is wrong with my BILs. I just want to understand why. I suspect it has something to do with the braking systems. The vehicles do not have brakes, the motors handle the braking when you let go of the throttle.

[dogman]

Just the weight? Aren't they made extra heavy to avoid tipping? Or is it just the sla's that are heavy?

[SamTexas]

I discarded the 50lbs AGM batteries and replaced them with a 10lbs, 7S 22Ah (~600Wh) laptop LiPo. So the new total wheelchair weight is about 100lbs (I can put it in the car trunk by myself). Not light, but not that heavy. I weigh 135lbs so the total weight is only 235lbs.

[REdiculous]

The motor temp is warm (NOT HOT) after 1 continuous hour ride at WOT. If that 82% was converted into heat, the motors would be BURNING by that time.

Surely WOT is in the motor's "butter zone" though..

If it's geared for 12mph@3600rpm, 3mph = 900rpm....I think most brush motors love 1200rpm (or more) and basically suck below that, so that's something to consider.

I wonder about the grease in the gearboxes and if there's no freewheeling that'll hurt your efficiency a lot, I bet. *shrug*

[JennyB]

I don't know much about powerchairs personally, but this guy seems to know what he is talking about. Even so, he uses a 3Kwh Headway pack for a "real world" range of 45 miles.

[dogman]

Mabye it's just that normal wheelchair and power scooter use pattern is 90% short runs from a stop?

You'd still expect better range after lithium, but that assumes you used good stuff. Laptop cells wouldn't be good stuff. But 22 ah of it should have been enought to not hammer em with a high c rate too much.

[SamTexas]

You'd still expect better range after lithium, but that assumes you used good stuff. Laptop cells wouldn't be good stuff.

Laptop cells are excellent when properly used. In this case the max current draw is 0.5C, way below the 1C rate for laptop. Please stop pretending that you know something about laptop cells. It's getting old.

[REdiculous]

It's not the C-rate we were talking about anyway.

Have you opened the gearbox(es)?...

I'd clean 'em and use petroleum jelly instead of tractor grease or whatever it is they stuffed in there. Even if it's new-ish, you might want to clean it anyway since there's bound to be some filings mixed in if it's past the break-in point. *shrug*

Are you sure it doesn't have normal brakes? I thought I read somewhere that they're normally electrically-held, so if you lose/cut power the brakes engage. Anyway...if there's always some hold current, that could be adding up? *another shrug*

Isn't Amberwolf the ES powerchair expert?

[SamTexas]

I haven't opened the geared boxes yet.

Brakes: The brakes are inside of motor/gearbox unit. I believe it's ON by default. When power is applied the brakes are released via some sort of solenoid. So that's one source of ineffieciency: It's take power (don't know how much) to free the wheels.

[REdiculous]

That's what I was wondering about the brakes. I was remembering right then, I just didn't explain it well.

Who needs brakes when you're going that slow anyway?...I'd remove 'em somehow!

[SamTexas]

Who needs brakes when you're going that slow anyway?...I'd remove 'em somehow!

Actually brakes are very important even at 3mph. Keep in mind that these are heavy vehicle (100+lbs + rider), so they would accelerate quite well on inclines and the handicapped riders are incapable slowing them down.

[Burgerman]

http://www.wheelchairdriver.com/BM-MK3- ... rchair.htm Chair home designed/built
http://www.wheelchairdriver.com/BM-MK3- ... rchair.htm Battery...
http://www.wheelchairdriver.com/BM-MK3- ... rchair.htm Charging. I did a lot of development and testing for hyperion so love these chargers! Theres some of me in them!
http://www.wheelchairdriver.com/BM-MK3-roboteq.htm Roboteq...

I am the designer/builder of this powerchair, that uses a 3k watt/Hr 6p 13s battery...

You were interested to know where the inefficiencies are?

Well unlike a bike it uses its motors to steer. Or turn on the spot. It does this all day long eating 20 to 40 amps typically on carpet or grass...

But even in a straight line at full power. It travels along a NON FLAT surface. The C of G is ahead of the drive (steering) wheels. So even the flat footpath isnt actually flat. Its leaning towards the gutter. The egde of the road is the same but more severe. To keep the chair traveling straight, against its natural tendency to head for the gutter, takes power. It does this because of casters at the front of course. Or elsewhere. So most of the time you are eating amps just to NOT head for the gutter! You dont really feel it because steering is just natural. At times its so extreme you drive one reverese current and the other one extra to compensate. Masses of wasted energy.

Plus a 70 percent efficient motor, is only 70 percent at one speed... And they all drive through gearboxes that waste loads more. Like up to half at some loads/rpms. Especially the worm drive gears. And the brakes (2 of) take .7 amp each... They are energised (Off) any time you move. They are only ever on when actually stopped. So you dont roll away on a hill/ramp.

I used that big battery to get extreme range on a powerchair. But I do beaches and big hills! As well as better speed (13 s cells rather than eight). Because I want speed, torque, range for a very aggressive busy user!

Theres 5 pages, one on charging, one showing the battery details, one showing the dual channel roboteq 150 amp per channel controller, on up to 50v. All OTT for a power wheelchair that totally nukes the all day performance of any bought ones...

Hope you all find it interesting!
And I do a lot of battery stuff. And LOVE the headways. They are lower resistance than it says on the tin.
Regards Burgerman.

[amberwolf]

Brushed motors: Yes, two of them, one for each rear wheel. Assuming the worst: Twice as inefficient as brushless motor.
Gearbox drag: I don't know. Do they have a gearbox? Multiple automatic gears or are we talking about the typical one gear reduction like a geared hub?

Usually the brushed motors run thru a gearbox, usually a right-angle one. I used these mtoors to run CrazyBIke2 and got much better performance out of them than a powerchair does, because I ran them thru the bike drivetrain so they didn't take so much power at startup and low speeds.

Generally these things are geared really low, so they can move really hefty loads slowly but definitely, so no one gets "stuck" because it won't start moving from a stop under most normal conditions.

Another issue is that these are heavy beasts and the tires are usually "airless" foam-core tires, rather than pneumatic. So there's more rolling resistance (sometimes a LOT more). Part of the weight is meant for ballast, to keep the chair from tipping in sharp turns or on slopes like ramps or crawling over a curb (for those capable of it).



Not all the motors used are brushed, though. These days there are a number of brushless hub motors in use. I have one of these to power a new bike build; testing thread is here:
viewtopic.php?f=30&t=32838
It's massive, and I don't yet know what kind of power usage it has under load, but it is less than 2A without a load on it at 40V+ full throttle using an EVAssemble 12FET.


For various reasons like steering/etc., I think your low expected wattage numbers are off, but I haven't checked them against anything (including the powerchair I have here). The reason is like you say, there's no huge heat buildup from inefficiency, so the power must actually be mostly going into the motion of the vehicle. Typically there are two 300-350W motors per chair for the smaller ones, and two 600-700W motors for the larger ones, when they are the brushed/gearbox kind. I'm not sure what the rated wattage is for the brushless monsters like I've got in that thread above.

REdiculous is probably right about the RPM stuff, and Burgerman expands on that. But I wouldn't change the grease in there unless it's trashed. I've changed grease in one to new stuff and it made no difference to anything, except that the thinner stuff I tried leaked thru the bearings into the motor itself as soon as it warmed up. That was a huge mess.

Burgerman is right about the steering and resulting power usage due to slight inclines, etc.

I suspect it has something to do with the braking systems. The vehicles do not have brakes, the motors handle the braking when you let go of the throttle.

They usually do actually have brakes of a sort, but only parking brakes, usually, to lock the wheels in place when stopped. BUUUT: As Burgerman points out, the brakes use notable power because they must be energized to allow the motor to spin at all; a solenoid is used to pull the clutch out, basically like an air conditioner compressor in a car. They are done this way so that a total power failure will not leave the unit rolling down a hill, but will instead lock it into place, which is usually safer for someoen that can't do anything about the situation.

[SamTexas]

I'm glad to hear from you Burgerman. I have already looked at your website.

Well unlike a bike it uses its motors to steer. Or turn on the spot. It does this all day long eating 20 to 40 amps typically on carpet or grass...

Why is this method inefficient? Isn't it just a matter of stopping or slowing down or reversing one of the motor? On my actual test, I was going straight forward for at least 95% of the time.

But even in a straight line at full power. It travels along a NON FLAT surface. The C of G is ahead of the drive (steering) wheels. So even the flat footpath isnt actually flat. Its leaning towards the gutter. The egde of the road is the same but more severe. To keep the chair traveling straight, against its natural tendency to head for the gutter, takes power. It does this because of casters at the front of course. Or elsewhere. So most of the time you are eating amps just to NOT head for the gutter! You dont really feel it because steering is just natural. At times its so extreme you drive one reverese current and the other one extra to compensate. Masses of wasted energy.

I'm not following you at all. How is this any different from any other 3 or 4 wheeled vehicles?

Plus a 70 percent efficient motor, is only 70 percent at one speed... And they all drive through gearboxes that waste loads more. Like up to half at some loads/rpms. Especially the worm drive gears.

I would be extremely satisfied if it was indeed 70% efficient. All indications so far tell me it's less than 50%.

And the brakes (2 of) take .7 amp each... They are energised (Off) any time you move.

Thanks for the hard numbers. So 0.7A x 45V = 22 watts per driving wheel or 44 watts total of waste. Did you actually measure that current at 45V? Or was it measured at 24V before you upgraded the battery?

---------------------------------------------------------------

So far we have 2 confirmed sources of wastes:
1) 70% efficiency for the motors
2) 0.7A x 2 brake release (on either 24 or 45V)

Great, but they are still far far far below the actual consumption. What else?

---------------------------------------------------------------

Do you have an actual energy consumption figure for your super wheelchair? I would love to see it.

a) total ride distance in miles
b) average wh/mi for that ride (a)
c) top speed during that ride (a)
d) average moving speed during that ride (a)
e) total weight (rider + super wheelchair)

Thanks. Looking forward to your reply.

[SamTexas]

Usually the brushed motors run thru a gearbox, usually a right-angle one. I used these mtoors to run CrazyBIke2 and got much better performance out of them than a powerchair does, because I ran them thru the bike drivetrain so they didn't take so much power at startup and low speeds.

Do you have the energy consumption figures for CrazyBike2? (wh/mi at mph?)

Not all the motors used are brushed, though. These days there are a number of brushless hub motors in use. I have one of these to power a new bike build; testing thread is here:
http://www.endless-sphere.com/forums/vi ... 30&t=32838
It's massive, and I don't yet know what kind of power usage it has under load, but it is less than 2A without a load on it at 40V+ full throttle using an EVAssemble 12FET.

Do you have plan to measure its power usage? I'd be very interested in seeing the number.

For various reasons like steering/etc., I think your low expected wattage numbers are off,

Not sure I understand you here. What do you mean by "off"? 175 watts is what I saw at 3mph. Is that too low or too high in your expectation?

Typically there are two 300-350W motors per chair for the smaller ones, and two 600-700W motors for the larger ones, when they are the brushed/gearbox kind.

Mine (my BIL, actually) is lower than that. 180W motors. The controller limit appears to be 12A, so max power is 288 watt.

[Burgerman]

Burgerman wrote:
Well unlike a bike it uses its motors to steer. Or turn on the spot. It does this all day long eating 20 to 40 amps typically on carpet or grass...

Why is this method inefficient? Isn't it just a matter of stopping or slowing down or reversing one of the motor? On my actual test, I was going straight forward for at least 95% of the time.

24v poweerchair. Takes about 20 battery amps to turn on the spot. Measured on a thin carpet. Because the casters are fat, have a fat bloke sat on top! And remember that this is happening all the time. You are using motor power to steer or even to correct your course all the time. In a car or bike the motor eitther pushes or pulls. It doesent spend its time doing both manoevering and going nowhere, wasting amps, but not asctually covering distance.

Try and turn a power wheelchair set in free wheel mode on the spot with an adult sat in it and you will get the idea. Its bloody hard work! Think amps! When you were going forwards at max speed in a straight line, and on a LEVEL surface it is using the least power. Now add steering corrections (eats chunks of power as iy turns your mass directly) and add a little left to right slope. This is the killer. And power consumption goes through the roof.

Driving along the edge of a road SLOWS the chair, and can take 60 to 100 amps. Remember that the casters have say 1/3rd the weight. And want to naturally head to the gutter. To stop this, while traveling in a straight line you push really hard with one wheel, and at times even pull with the other. To "twist" the chair against gravity to travel straight.

Burgerman wrote:
But even in a straight line at full power. It travels along a NON FLAT surface. The C of G is ahead of the drive (steering) wheels. So even the flat footpath isnt actually flat. Its leaning towards the gutter. The egde of the road is the same but more severe. To keep the chair traveling straight, against its natural tendency to head for the gutter, takes power. It does this because of casters at the front of course. Or elsewhere. So most of the time you are eating amps just to NOT head for the gutter! You dont really feel it because steering is just natural. At times its so extreme you drive one reverese current and the other one extra to compensate. Masses of wasted energy.

I'm not following you at all. How is this any different from any other 3 or 4 wheeled vehicles?

Because the car resists sideways movement. The caster doesent. It heads down hill... So to drove along a transverse slope like a pavement or road requires continuous motor power to head straight. Or even to KEEP straight when you stop or are stopped. Thats why they need brakes to lock position when you stop.

Burgerman wrote:
Plus a 70 percent efficient motor, is only 70 percent at one speed... And they all drive through gearboxes that waste loads more. Like up to half at some loads/rpms. Especially the worm drive gears.

I would be extremely satisfied if it was indeed 70% efficient. All indications so far tell me it's less than 50%.

No idea of actual efficiency of your motors. Good ones are UP TO about 70, (at one best RPM/LOAD) plus gearbox losses.

Burgerman wrote:
And the brakes (2 of) take .7 amp each... They are energised (Off) any time you move.

Thanks for the hard numbers. So 0.7A x 45V = 22 watts per driving wheel or 44 watts total of waste. Did you actually measure that current at 45V? Or was it measured at 24V before you upgraded the battery?

No 0.7 amps at 12v (all are a bit different but most are around this figure.
---------------------------------------------------------------

So far we have 2 confirmed sources of wastes:
1) 70% efficiency for the motors

Ish... At ONE RPM and LOAD - PLUS the gearbox!!!b]

2) 0.7A x 2 brake release (on either 24 or 45V)

[b](12V mostly)

Great, but they are still far far far below the actual consumption. What else?

The huge loss comes from the tank steering and the forward C of G combined meaning they head downhill all the time when traveling, sucking up power needed to twist/pull and keep straigtened up all the time. You dont notice this much because the tiny joystick corrections are assisted by a fancy algorythm called motor compensation. This allows full torque at tiny input levels. But this is where the biggest amount goes. That and fat tyres, inneficient gearboxes.

---------------------------------------------------------------

Do you have an actual energy consumption figure for your super wheelchair? I would love to see it.

No. Not yet, soon! Just about to do a final build.

a) total ride distance in miles
b) average wh/mi for that ride (a)
c) top speed during that ride (a)
d) average moving speed during that ride (a)
e) total weight (rider + super wheelchair)

Thanks. Looking forward to your reply.SamTexas
100 kW


Posts: 1260
Joined: Wed Dec 29, 2010 5:34 am
Location: Houston, Texas - USA. Age 53

[Spacey]

Brushed motors? Plus gearbox drag. You see the same thing in a 4x4. While it might get a stellar 12 mph on the road at 40 mph, put er in granny and creep for very long. Hey, 20 gallons of gas took me 10 miles. Woah!

At some point, you are too slow for efficiency. But it's got the torque I bet.

Stellar 12mpg at 40mph? Damn and there's my old 2.0 litre Diesil VW Estate regularly getting 62mpg at 60mph

[Spacey]

You'd still expect better range after lithium, but that assumes you used good stuff. Laptop cells wouldn't be good stuff.

Laptop cells are excellent when properly used. In this case the max current draw is 0.5C, way below the 1C rate for laptop. Please stop pretending that you know something about laptop cells. It's getting old.

You know out of all the people on this forum it's only you Sam Texas that I notice gets a bit batey, techy, slightly rude and just plain old bad tempered. No need to jump on Dogman like that, he is as everyone is..... Entitled to his opinion regardless if he is correct or not.

Just saying that you should chill a bit......ducks as Sam Texas lifts up his now lighter wheelchair to throw at me

[SamTexas]

You'd still expect better range after lithium, but that assumes you used good stuff. Laptop cells wouldn't be good stuff.

Laptop cells are excellent when properly used. In this case the max current draw is 0.5C, way below the 1C rate for laptop. Please stop pretending that you know something about laptop cells. It's getting old.

You know out of all the people on this forum it's only you Sam Texas that I notice gets a bit batey, techy, slightly rude and just plain old bad tempered. No need to jump on Dogman like that, he is as everyone is..... Entitled to his opinion regardless if he is correct or not.

Just saying that you should chill a bit......ducks as Sam Texas lifts up his now lighter wheelchair to throw at me

I don't give a damn about what you think how I should act. I don't mince words. I say exactly what I see and observe. When you begin to pretend to know something you don't, you'll get the same treatment from me. As for labeling me all those above adjectives, I'll let it go this time. But try it one more time and I will react.

[Spacey]

Over sensitive?

/gets popcorn whilst waiting for the 'reaction'

[amberwolf]

Do you have the energy consumption figures for CrazyBike2? (wh/mi at mph?)

Some figures from not long before I ended up temporarily decommissioning the bike, in Dec 2009:
http://electricle.blogspot.com/2009/12/ ... -data.html
Distance/speed/etc from Veloace on a PDA, and electrical measurements from a Turnigy Watt Meter.

I didn't apparnetly note down my speed, but typically at that time I'd be going about 15MPH cruising speeds, up to 18-20MPH peaks if I was lucky. Probably 10-12MPH average, at a guess. (slower than what I do nowadays, depending on traffic conditions, which is 20MPH cruising and 14-15MPH avg)

Trip to Arrowhead area mostly along canal path, outbound leg:
11.298 miles
48 minutes in motion
9.059Ah
328.8Wh
(~36.4Wh/mile stated in article but that has to be a typo or wrong math; it calculates to 29.1Wh/mile)
77.57Ap
29Vmin
37.59Vend
2437Wp

Recharged 9.4Ah at destination

Return leg:
11.4miles
48 minutes
9.28Ah
327Wh
28.6Wh/mile
?Ap (88A calculated)
27.27Vmin
36.48Vend
2402Wp

Work commute next day, both legs combined:
5miles
4.379Ah
151Wh
30Wh/mile (30.2 calculated)
97.5Ap
2915Wp

Some other data here:
http://electricle.blogspot.com/2009/12/ ... terns.html
showing about 29.52Wh/Mile. That would have been around 15MPH avg for the outbound leg with peak of 20MPH, but I noted specifcaly that I had to slow down to 9-10MPH to save enough power to get home. Later I noted that it was 9.1MPH average for the whole trip.


Note that power usage on that brushed motor system thru the chain drive actually was about the same as it is with my direct-drive brushless 9C front hub now, even though nowadays I ride several MPH faster with the hubmotor system.

Do you have plan to measure its power usage? I'd be very interested in seeing the number.

Absolutely. I now have a working welder again so I'm hoping it won't be too long before I have something to test on a bike.

Not sure I understand you here. What do you mean by "off"? 175 watts is what I saw at 3mph. Is that too low or too high in your expectation?

I mean, that your actual measurements are probably fine, but you are expecting a lower wattage usage than I think they really end up always using, for various reasons like the power wasted in steering corrections, etc. I have not yet sat down to measure the Pride powerchair I have here; I keep intending to but I have to move a bunch of stuff out of the way to even get to it, much less get it out the door to do a test run.

Mine (my BIL, actually) is lower than that. 180W motors. The controller limit appears to be 12A, so max power is 288 watt.

Ah. I guess I must have the supercharged motors.

[SamTexas]

Some figures from not long before I ended up temporarily decommissioning the bike, in Dec 2009:
http://electricle.blogspot.com/2009/12/more-power-usage-data.html
Distance/speed/etc from Veloace on a PDA, and electrical measurements from a Turnigy Watt Meter.

I didn't apparnetly note down my speed, but typically at that time I'd be going about 15MPH cruising speeds, up to 18-20MPH peaks if I was lucky. Probably 10-12MPH average, at a guess. (slower than what I do nowadays, depending on traffic conditions, which is 20MPH cruising and 14-15MPH avg)

Trip to Arrowhead area mostly along canal path, outbound leg:
11.298 miles
48 minutes in motion
9.059Ah
328.8Wh
(~36.4Wh/mile stated in article but that has to be a typo or wrong math; it calculates to 29.1Wh/mile)

Thanks amberwolf. 29.1wh/mi is still a horrible number at 15mph, but leaps and bounds better than the 58wh/mi at 3mph I experienced on my BIL's wheelchair. I looked at your above mentioned blog trying in vain to find the modification you did to the wheelchair motor for adaptation to your bike. Can you point me to the specific blog entry for that?

I assume you have (at the minimum) disabled or removed the internal brake, correct?