general power questions

[giguana]

lead-acid batteries go from about 36v idle to 31v under load, and from 39v at full charge to 33v at nearly empty, how do metal hydride and lithium batteries compare generally?

how does torque rating perform relative to power usage? a high-speed low torque motor going uphill uses full power to go very slowly, so is it fair to say that a high-speed motor is generally very inefficient uphill and uses lots of power?

is the best power to distance ratio always at the minimum speed, or is it best to keep a bit of momentum to keep it at its optimum pace?

 

[coops]

Hi giguana

My 36V Ezee NiMH was about 41.5-42V hot off the charger when new IIRC & down to 36-36.5V when it cuts out at full discharge, the low voltage cut out on Ezee controller about 31 or 32V I think?

I haven't used lithium, but think they have a slightly higher voltage freshly charged (they are 37V 'nominally') though they have been less suitable for supplying high currents, power supply cutting out under continuous heavy load/high power needs, due to either voltage drop in the controller or their 'management' system cutting out power. The Ezee lithiums now have a different management system intended to reduce occurrences of cut-out.

Regarding torque, as I understand it a direct-drive hub motor (no gearing) like crystalyte 400 or bionx would be less efficient & use rather more power at low speed than a geared hub motor like the 8fun in the Ezee Quando for example (for the same wheel size & max rpm of motor) so yes a high speed ungeared motor would use more power & less efficiently going uphill, but on the flat or only marginally uphill might be marginally more efficient due to zero loss through gears .

I'd say that with either motor type its best to keep some momentum if possible because highest efficiency is always at higher motor revs, but on hills and/or at higher speed that must be balanced with the power required to go that speed and climb a hill, so in pratice there is an optimum speed for any given bike, motor, rider & battery combination which in all honesty you'd probably have more success finding it by experience & intuition as by any calculation you might try! Don't forget to regulate your tyre pressures, check your chain & keep you battery cosy & warm too!

I tend to try to keep a constant speed & throttle setting, more or less, on the flatter bits & for gentler uphill gradients I tend to try to keep the throttle constant & pedal a bit more, even if speed drops slightly: that way I've a better idea of the power usage & my overall range. Range wise, I know I can get about 30 or more miles at 15mph & 15-20 miles at 20mph or so in my gently undulating terrain, so I can juggle speed & range accordingly. The speed you use on hills & how much pedal assist you give too is a much bigger factor with respect to range than speed/pedal assist on the flat.

Forgot to say, you can use the motor as much as you like on fast downhills since little power used at max revs anyway, also NiCad batteries are apparently the best for high power use because they can give all their power in a very short time - except they contain toxic cadmium, so use is being discouraged.

Hope thats useful & correct, and a happy new year to you .

Stuart.

 

[Honk]

Hi giguana
Range wise, I know I can get about 30 or more miles at 15mph & 15-20 miles at 20mph or so in my gently undulating terrain, so I can juggle speed & range accordingly.
Stuart.

I understand your'e using 36V batteries but how many amp hours are they?

 

[coops]

Sorry for that important omission Honk & thanks for the reminder: its a 9Ah standard Ezee NiMH battery, not sure if thats exact or rounded up or down though, but approximately right anyhow .

Stuart.

 

[flecc]

eZee NiMh is 9 Ah as Coops says, 324 Wh, and the Li-ion version 37 V 10 Ah, 370 Wh.

The Li-ion Manganese voltage droop varies according to duration of load. Similar to the NiMh at first, if anything less droop, but as the high drain continues as on a long climb, the voltage graduaklly drops until cut-out point at around 32 volts is reached. I believe this is due to the manganese cathode limitations more than anything else.

The current drain on Hall effect hub motors is maximum at and below the point of maximum torque, gradually reducing above that until the point of maximum efficiency at or about the maximum speed. Typically the maximum torque point is about 8 mph on a Euro/UK legal bike, but on high speed versions which do about 20 mph unrestricted like the eZee Torq, it's at about 12 mph. That's the main cause of the difficulty the Torq can have with steep hill climbing, the speed being at or below the maximum torque point so the current drain at maximum.
.

 

[giguana]

thanks for all that information, it'll be interesting to see what the future holds in terms of motor innovation.

I thought I would actually have to arrange lithium cells to have a lot of ventilation, I will have to read more about monitoring bms

 

[Jeremy]

The current drain on Hall effect hub motors is maximum at and below the point of maximum torque, gradually reducing above that until the point of maximum efficiency at or about the maximum speed. Typically the maximum torque point is about 8 mph on a Euro/UK legal bike, but on high speed versions which do about 20 mph unrestricted like the eZee Torq, it's at about 12 mph. That's the main cause of the difficulty the Torq can have with steep hill climbing, the speed being at or below the maximum torque point so the current drain at maximum.

According to the hub motor simulator on the ebikes.ca site, maximum torque for all of the motors listed seems to be at zero rpm, which is exactly what I'd expect for a motor of this type. Max efficiency is, of course, close to max speed. Here are some examples:

Crystalyte 405, 20" wheel, 36V 8Ah NiMH:

Max torque = 0 rpm, zero torque = 460 rpm = 27 mph (near enough a straight line between the two points).

Max power (and hence max current) = 230 rpm = 14 mph

Max efficiency = 390 rpm = 23 mph


eZee 26" wheel, 36V 8Ah NiMH:

Max torque = 0 rpm, zero torque = 280 rpm = 22 mph (near enough a straight line between the two points).

Max power (and hence max current) = 180 rpm = 13 mph

Max efficiency = 230 rpm = 17 mph

Jeremy

 

[flecc]

I usually express it as above, as the point of maximum power/torque, since that is what is most useful to e-bikers. The maximum torque alone at zero speed isn't of any practical use on a two wheel bike of course, and until maximum power is reached as well, typically at about 8 mph on most e-bikes, 12 mph on the higher speed bikes, the torque has limited usefulness in practice.
.

 

[Jeremy]

I don't disagree, I was just querying the term "below the point of maximum torque" as it seemed to me that this couldn't exist, hence my confusion. I assume that what was meant was "below the point of maximum power", which makes more sense, at least to me.

Jeremy

 

[flecc]

Of course Jeremy, no disagreement since you're quite right anyway, my post just clearing up my meaning, I do at times in here on this issue refer to power/torque but just get lazy sometimes.

In posting I prefer to interpret theory in practical terms since that's what's most useful. For example, my mention of about 12 mph on the higher speed Torq motor refers to the point where the steep climb in the power curve flattens out to start being really useful, rather than the actual peak which, as you've expressed, is later. The same of course in my reference to 8 mph.

Equally I express the point of maximum efficiency in approximate terms as being near maximum actual speed rather than theoretical speed, so near enough for practical use. For example the eZee Torq motor maximum 22 mph in a 26" wheel is only just reached in a 28" wheel with ideal conditions in practice. A 26" wheel falls somewhat short of that in flat road neutral conditions, so the difference between maximum speed and maximum efficiency speed is quite small.
.

 

[Jeremy]

The 22 mph figure I quoted is that for zero torque, so the motor isn't delivering any power at all at that speed, hence the difference.

My guess is that maximum speed on motor only, straight and level in nil wind, for an upright style bike would be a bit less. The kreuzotter power calculator shows that about 19.7 mph is the theoretical top speed for the eZee in a 26" wheel at 36V, but this assumes near perfect conditions.

Jeremy

 

[flecc]

Yes, that roughly matches practical experience in the 28" wheel, just about 22 mph if everything is perfect, but nearer to 20 mph much of the time.

The same motor in the 20" wheel Quando spins that up to 17 and even just 18 mph due to excess voltage when a Li-ion is fresh off the charger, but soon falling back a bit. Good while it lasts, and that's why I converted my T bike eZee NiMh battery to 39.6 volts with three extra cells, since that maintains the extra bit.
.

 

[Tiberius]

I have the Cycle Analyst on my Torq (700C wheels) and I was testing it out on the flat the other day.

Max speed without pedal power was 21 mph, taking about 380 W.
If I then pedalled hard I could get it to go up to 22 mph, but the interesting thing is that the power taken from the battery dropped by 100 W.
I tried this in both directions, to allow for wind, and got more or less the same numbers.

With a serious wind or a gradient helping, the electric power taken continues to drop off with speed rapidly - I remember seeing about 60 W at 27 mph, but to be honest I was concentrating on steering instead.

It is actually difficult to maintain over 20 mph for any time, 18 or 19 is more comfortable. So the number 22 is a top speed not a cruise speed.

Nick

PS. For the record, I have access to a large flat off road tarmac area for testing.

 

[flecc]

All my experiments with both the Torq and T bike (28" wheel), plus the Quando experience (20" wheel) with the identical motor has suggested the ideal compromise wheel size is 24". That would comfortably run at 18 to 20 mph all the time, depending on charge level, while retaining much of the Quando's prodigious hill climb ability.
.

 

[giguana]

apparently a Yesa eBay person says that their batteries last five years and 1000+ cycles...five years? I thought lithium was two!
and apparently the operating temperature is -20 to 75 C'...

 

[flecc]

apparently a Yesa eBay person says that their batteries last five years and 1000+ cycles...five years? I thought lithium was two!
and apparently the operating temperature is -20 to 75 C'...

In theory that can be true for Lithium Iron Phosphate ( LiFePO4 ), but for Li-ion manganese, cobalt or Li-poly, two years and 500 charges is more like it with the very best examples such as Panasonic's. Not that temperature range though, quite ridiculous even in storage, since freezing damages them.
.