undervolting question

hey all, i ve just got a new sensorless 48v 500w Bafang BPM front hub motor. now I m kind of thinking i'd like to run my cargobike at 36v instead of 48v. if my motor is wound for 48v (code 12-i think, 24" wheel)what effect is running it at 36v gonna have?

ie i ve hear that running a 36v motor at 48 makes a lot of torque. so is running 36v on a 48v motor winding gonna make way less or hopefully it won t make much performance drop?

In some cases, a 48v controller will have a low voltage cutoff set high for 48v, making running the bike on 36v impossible. Some "48v" controllers are just 36v controllers being run on 48v. Only one way to find out, try it on 36v.

Same amp controller, 36v will be less total watts, and the lower voltage of those watts will make it slower. Often 4-5 mph slower.

Watts is determined by the old equation, volts x amps = watts. A different controller with more amps, could get you the same watts for climbing the hills, but at 36v. But it is the tends to make the motor hot way to do it. At some point, you cross a line and too much amps just cooks a motor. It depends on a lot of variables what happens in the real world.

A better option could be lacing the motor into a smaller rim. But that can be opening a different can of worms, low pedals hitting the pavement. etc. But 20"rims might work if you have disk brakes.

A Code 12 Bafang BPM produces about 34kph max at 48V and 24in wheel, so expect 26kph at 36V.
If you have an ecitypower controller, the LVC for 48V is 35.75V which is perfectly acceptable for 10S lipo or 12S lifepo, if a bit on the safe side. For other you have to figure out.

Basicly, yes, you can undervolt the Bafang. It will have less power and speed. But the controller may be an issue. It should be fine for Lithium type batteries but the cut off will be higher than normal, so you won't be able to use 100% capacity. Of course, you should always avoid using 100% capacity of a battery, but the 48V controller won't give you the option if you should ever need it.

thanks for that, i understand a bit more now. i actually have a holmes hobbies mini stealth controller that i ve never used. it's just 6 fet 36v but i ve heard of people running them at 48. i m not sure what the lvc is on it. i m about to buy another Ping. i had a 48v 15ah ping on my last setup and i loved it for it's reliability and output but it was too fast really. i crashed it at full throttle, almost broke my hip, tore all the wires out of my hubmotor and snapped my front fork. i thought maybe 36v would be more sane for me provided i can still haul up hills. my bike is a cargo style bike and weighs close to 50lb before the motor and battery are added. if my motor is going to give similar results to running a 36v500w then i think i ll just run 36v thru my 48v500w motor.

What I'd be worried about is current. The bike is still going to try to pull the desired power, but will compensate for this by drawing more current, right?

One time, one of my four batteries was not connected properly giving massive voltage sag - no-load voltage was still 44v, but on throttle, who knows how much it dropped? I didn't know this at the time, but it just felt sluggish and definitely something was wrong, but I kept riding, hoping I could get home before diagnosing.

Low throttle on flats was fine. I actually went about 4km. However, hit my first hill, I twisted the throttle to max, and my 35A fuse blew. 35A! I've been riding with a 20A fuse for ages with no problems, and it blew a 35A fuse... Probably meant my voltage was sagging at least 40%,

I suspect if you constantly undervolt the system, it will work, but who knows how much you'll be shortening the life of components, by forcing it to over draw on current to compensate?

I could be totally wrong, and happy to be corrected, but that's my take on it.

thanks for your take, Sunders. this is basically my concern. i m starting to think the best solution might be to go 48v but run a cycle analyst so i can govern my speed and current draw. i m just not sure if the cycle analyst speed sensor will work with my greared bafang. either way i ve got to slow down.

There are other ways to skin that cat, but might cost you a lot more than you planned. Basicly, start over.

I've become a huge fan of the 9 continent direct drive motor, but in slower windings that go slower at 48v. You still get a lot of power to climb a hill, but unlike fast winding dd motors the slow motors don't get as hot. I find them ideal for two applications. Cargo and commuter bikes that need to climb hills, and dirt trail bikes that want to climb mountains.

I have a 2810 winding motor on my cargo bike, and one of my commuters, and find them ideal, 20 mph speed on a 48v pingbattery, but able to climb 10% grades no problem at all. 20 amp stock controllers. You'll pull about 1000 watts going up steep hills.

This is the only place to get one I know of, a guy you can trust. http://endless-sphere.com/forums/viewtopic.php?f=31&t=31146

Here is my cargobike, with 9c 2810 on the rear.

dogman said:

In some cases, a 48v controller will have a low voltage cutoff set high for 48v, making running the bike on 36v impossible. Some "48v" controllers are just 36v controllers being run on 48v.

Click to expand...

this may be too obvious to mention, but I run my geared MAC at 24V, I just had to program the controller LVC. grab a programming cord from LYEN and you can set your LVC to whatever.

I tried using the CA to limit the current and found it to be a bit twitchy. there may be a way to adjust the seetings for perfection, but I couldn't find it. at 24V 26" wheel I get about 16mph on the MAC.

Maximum current will normally be more or less the same independent of the voltage.
I'm not sure, but I thought the 12 FET Ecitypower controllers are 36/48v controllers. They seem to behave different to other controllers in that there is a significant difference in power depending on what voltage you run them at. I think maybe the software runs a different set of parameters depending on what voltage it detects. I use one of these controllers at both 36v (Lifepo4) and 44v (12s lipo) and one time I forgot to charge half my lipo pack and the voltage went very low (maybe low 30s volts), but I didn't get LV cut-off from the controller.