Combination of voltage and amperage

[Arbol]

I am looking for to build an e-bike satisfying European legal requirements, but offering decent performance.

Basically, there are three motors I know of in Europe that offer 250W "legal": the new Bafang BBS-01 in 36V 250W form, the Q85 / Q100 and I believe there is another Bafang hub 250W called SW or something similar.

What I see is folks have been trying to improve performance out of these motors by two means: either using 48V batteries / controllers which provide more voltage, or using controllers providing more amperage (and keeping the 36V batteries). Most of what I have read is regarding Q85 / Q100.

It seems the Q85 / Q100 accepts relatively happily 48V, since internals are able to cope with that. Amperage is harder: KU63 / 65 provides 12A maximum, and there have been reported problems with the usage of the KU93, providing 22A max. Some people have "soldiered the shunt", providing approximately 17A or 18A on the KU63, and that seems safe (I assume this is with 36V; with 48V, probably that would be too much for the motor, right?).

Power is voltage times amperage. But it seems increases in each component have different behaviors on the e-bikes: an increase in voltage improves both max speed and torque. An increase in amperage improves torque only.

What I would like to ask is real world experiences on improving performance on these motors, especially the Q85 / Q100, which is the one I know the most. I believe there are no reported experiences with the BBS-01 yet on this issue:

- Is an increase from 36V to 48V battery safe for the Q85 / Q100, if one keeps the KU63 unchanged? And if one soldiers the shunt on the KU63?
- Why is the improvement from KU63 to KU93 worse in reliability terms than the improvement of voltage? It is just the increase is higher (22A/12A is an increase of 1.83x; 48V/36V is an increase of 1.33x; 17A/12A is an increase of 1.42x, so the improvement by "soldiering the shunt" is similar in size terms to improvements in voltage) or that increases in amperage tend to be more unreliable in general than improvements in voltage? I ask this because I would prefer to make improvements via more amperage, since one can continue using smaller batteries, and using the relatively high continuous discharge amperage of LiFePO4 batteries. Instead, by increasing voltage, one needs more bulky batteries, but the high discharge amperage capabilities of the big LiFePO4 batteries remained underused, which is suboptimal
- Maybe this is crazy, but could one use a 24V battery with a high discharge amperage rate to keep power under reliable limits, small sized batteries and a decent torque? (granted, max speed would suffer, but I do not care much about it)
- Can anybody make a guess of which could be the best improvements for the BBS-01?

Finally:

- Which is the sweet spot?

Thanks.

Edit: if one used say a 6 FET 25A Infineon controller with a 24V battery (eg Cell_man), max power would be 600W (24*25). Of course, assuming the controller accepts a 24V battery, which is not obvious (I guess one could program the LVC). Instead, with 36V and "soldiering the shunt" of a KU63 one gets similar power, 612W (36*17). If this last combination is reliable with a Q100, couldn't it be a 24V / 25A combination also be reliable, since power is similar (even a bit lower)?

The reason for this odd combination would be that in Europe, a motor cannot help further than 25km/h. As a consequence, what matters to me is what the motor does between 0 and 25km/h, so what matters is torque. That a 36V or 48V battery could provide higher max speed is immaterial to me, since regulations do not allow me to use that increased max speed (unless a 24V cannot even reach 25km/h, I do not know that). Also, I care much more about the ability to climb hills rather than max speed: within the city, I will never use high speed (too dangerous). And for weekend trips, higher speed means simply arriving a few minutes earlier, which I do not care much. But torque means being able to go to places (slopes of say 20%) that I cannot dream of right now, without a motor (but these places exist nearby!).

 

[d8veh]

You first need to take a step back. EN15194 requires that you use a motor with a "nominal" power of 250w, which means it has to be marked 250w. The Q100 is marked 250w to 350w whatever that means, so it doesn't comply. You can buy other motors (some more powerful) with EN15194 stamped on them whatever that means, but it looks convincing enough. The Bafang SWXK, SWKB and other SWX series are marked 250w, so if compliance is important, they should be OK. Also, you can buy "250w" versions of the Bafang CST motor that are stamped 250w. They can handle a lot of power. The new BBS01 crank-drive was designed to comply, so it would be a good solution.

You seem to have got most things figured out. You can't take a small motor and force large current through it. What happens depends on a lot of things, but the main bad effects are heat and direct mechanical failure.

The BBS01 has an internal controller. You have to do a bit of dismantling before you get to it, so there's not much info on it yet. Also, if you fry it, a replacement would be tricky. I reckon that a set of 12S lipos would be a worthwhile improvement in performance. The TMC crank-drive is similar to the BBS01. I tried it a lot with more current than standard. Up to about 18 amps made a proportional increase, but more than that didn't make much difference to the torque. It just made the motor get hotter.

How much voltage and current you put through a hub motor depends on the design of the motor. Basically, the heavier the motor (more copper in it), the more power it can handle. You should use a combination of winding speed and voltage to get the speed you want, and then current to get power up to the limit of the motor design.

Crank-drives are different because it's the gearing that determines the speed, so you should plan the cadence to match your own comfortable range, and then use current to increase the torque. If you don't pedal , it doesn't matter much how you increase the power, but generally high voltage has less losses than high current.

There's no advantage in using a low voltage battery. What counts is the watt-hours in it. You can get very small 100v batteries that are the same size and weight, and have the same number of watt-hours as a 24v battery. A higher voltage battery uses proportionally less current, so you don't need such a high discharge rate, although with lipos going up to 50c, discharge rates wouldn't be a problem for either.

 

[Arbol]

Thanks for the detailed reply. You confirm my suspicion that "more voltage is better than more amperage".

I did not know there were Bafang CST 250W, since they do not appear in neither bmsbattery or greenbikekit. But they appear in elifebike and in an Austrian webpage:

http://www.elifebike.com/peng/iview.asp?KeyID=dtpic-2013-3C-C6H5.4FUVV
http://www.greenroad.at/shop/Bafang-Hinterradmotor-CST-BPM2-26-250W

I also have found a Cute CST at 250W, which accepts 10 gear cassettes (the Bafang only accepts up to 9):

http://www.elifebike.com/peng/iview.asp?KeyID=dtpic-2013-46-K3QJ.1WASD

The Austrian webpage says for the Bafang "48V is possible". Does anybody know how much amperage will accept that motor? And the equivalent question for the Cute CST?

Edit: could it be the internals of the Bafang CST 250W, 350W and 500W are the same, and just the controller changes? I ask this because if one looks at the size diagrams for both the 500W (http://www.bmsbattery.com/hub-motor/492-qswxk-front-drive-brushless-hub-motor-for-ebike.html, second picture) and the 250W (http://www.elifebike.com/upfile/dtpic/2012/8A/QXSX.8TKBR/8FC1AP_WUPK6.jpg), all external sizes are the same. Of course, it still could be that even though external sizes are the same for all motors, internal sizes (the amount of copper, etc) differ. Is it known if anybody has opened at least two different CSTs to check that? Thanks.

 

[MadRhino]

I am looking for to build an e-bike satisfying European legal requirements, but offering decent performance.

If it is satisfying EU Legal requirements, you can't have decent performance because that is exactly what EU laws are keeping you from. Build it illegal and stealthy, put a 250w stamp on it, add a legal mode switch and ride wisely.

 

[d8veh]

The 250w, 350w and 500w Bafang CST are all the same size, but thet're different inside. I'm pretty sure that the 250w and 350w are the same motor with different labels, but they don't have the grunt of the 500w one. I had a 350w one and did some tests on it. It felt the same as the 250w ones that I tried on test bikes, and didn't respond well to more current. Its top speed was about 19mph - slower than the 22mph (270rpm) 500w version, but I think 12S lipos would liven it up a bit. It has substantially more power than the Q140/M140, which is tiny by comparison. The Bafang runs happily at 22 amps. 25 amps gives it a bit more torque, but a 30 amp controller doesn't add any more. if you connected a Q140 to a 30 amp controller, it would explode. I reckon the optimum for the 250w Bafang CST would be 20 amps and 48v. I wouldn't go any higher than 15 amps on the Q140 at 48v, and even then, you'd have to be careful. The bafang weighs more than twice the Q140. Thet're completely different motors. If you want power, it's no contest. Get the Bafang. If you can get one labelled 250w, then it's legal. I know people with the 500w version with 250w labels on them, and if you wanted to be really devious, buy both and swap the internals. The one thing you can't hide is the 25km/h speed limit. If legal is important to you, then it needs to be robustly fixed. Override switches aren't allowed.

 

[John in CR]

... an e-bike satisfying European legal requirements, but offering decent performance...

That's an oxymoron, or to quote Tin Cup, those words don't often collide in the same sentence.

 

[teklektik]

Override switches aren't allowed.

Hmmm - this is interesting - although of course it makes perfect sense. Can you call out the section of the regulation that specifies this? Not arguing - just trying to grasp the details of the standard.

 

[Arbol]

The 250w, 350w and 500w Bafang CST are all the same size, but thet're different inside. I'm pretty sure that the 250w and 350w are the same motor with different labels, but they don't have the grunt of the 500w one. I had a 350w one and did some tests on it. It felt the same as the 250w ones that I tried on test bikes, and didn't respond well to more current. Its top speed was about 19mph - slower than the 22mph (270rpm) 500w version, but I think 12S lipos would liven it up a bit. It has substantially more power than the Q140/M140, which is tiny by comparison. The Bafang runs happily at 22 amps. 25 amps gives it a bit more torque, but a 30 amp controller doesn't add any more. if you connected a Q140 to a 30 amp controller, it would explode. I reckon the optimum for the 250w Bafang CST would be 20 amps and 48v. I wouldn't go any higher than 15 amps on the Q140 at 48v, and even then, you'd have to be careful. The bafang weighs more than twice the Q140. Thet're completely different motors. If you want power, it's no contest. Get the Bafang. If you can get one labelled 250w, then it's legal. I know people with the 500w version with 250w labels on them, and if you wanted to be really devious, buy both and swap the internals. The one thing you can't hide is the 25km/h speed limit. If legal is important to you, then it needs to be robustly fixed. Override switches aren't allowed.

Thanks for the detailed answer.

Three questions:

- You say " I think 12S lipos would liven it up a bit (...) I reckon the optimum for the 250w Bafang CST would be 20 amps and 48v". 12S lipos are 44.1V and I believe LiFePO4 are closer to 48V (with Cell_man's A123 being 50V). Would lipos and LiFePO4 be both OK in your opinion to run the 250W Bafang CST at 20A 48V?
- Your optimum is 20A 48V. Would 22A 48V be also OK, or it would be too much? I ask this because the shops I know selling the 250W Bafang CST have controllers at 22A, but I am not sure if they have 20A ones
- I believed a PAS overrides the issue of 25km/h speed limit (as well as the fact that it does not allow the motor to work unless there is pedalling). So, if one had a 250W Bafang CST (at 20A 48V) with a PAS, then "everything would be legal", right?

 

[d8veh]

12s Lipos are nominally 44v, but you charge them to 50v, and they go down to about 44v, so that's 47v average. On short journeys, you can say 48v.
The bikes I've been on with the 250w CST had good climbing power. They must have been running with something in the region of 20 to 25 amps at 36v. Im pretty sure that 48v and 22 amps would be OK, but I haven't tried it.