Increasing top speed by overvolting?

Hello, I recently converted a fatbike to a fat-ebike. However, top speed isn't quite where I want it. My top speed is about 27mph on flat ground. I don't think there is a speed limiter, When I lift the motor wheel and hit the throttle, the speedometer shows about 34mph, so unless watts is being limited at higher speed, there isn't a speed limiter. It was a 1500w brushless motor kit with a 48v kt controller 22A continous, 45A Peak. Seems like my top speed should be higher with this setup but maybe not.

I was thinking about getting a 72v kt controller so that it's compatible with my lcd display and keep current the same at 45A by limiting max current through my display. The only problem is I already ordered two 48v 14ah 18650 battery packs from China a while ago and they should be arriving soon(I have been using 4 8ah lead acid batteries connected in series for the meantime). So, if I want to go to 72v, I would either need to disassemble the battery packs and reassemble them as 20s, or get two dc voltage boost converters rated for 1800w and connect them in parallel which is easier said than done due to having to implement load sharing, otherwise one would take all the load and burn up. Going the converter route would also be less efficient, take up more space, I wouldn't be able to monitor voltage on my display as voltage would be kept constant and brake regen won't work. I also plan on installing a temp. sensor in the motor.

Any ideas? Any feedback is appreciated.
Fyi, my end goal is around 40mph, this motor doesn't have to get me there alone as I plan on adding a 2nd motor in the future but it needs to have enough rpm to hit 40mph which it probably will after overvolting it.

What if I connected my two 48v batteries in series?
I just came across this link. https://electricbike.com/forum/foru...lting-a-48v-1000w-motor-with-96v-10ah-battery

What would I need to look for in the controller to see if it can handle 96v? Capacitors, and what else?

I think I'm just going to order a 72v kt controller and reassemble my battery packs.
Only problem I have now is my kt lcd8h display is only rated for 36v/48v setups. I don't know if I can directly hook up 72v. I could use a voltage divider/voltage regulator to power the display but then I'm not sure if the display would communicate correctly with the controller. Any thoughts?

I'm pretty sure the Kt displays contain an onboard voltage converter to power the LCD circuits and the onboard uart for serial data.

If you drop the voltage to the display, the unit will return that voltage to the controller. where it's used tp power all the lower voltage circuits in the controller. It in effect, turns on the controller. Someone reverse engineered a 6FET KT controller and showed this. The power to the MOSFET's is always there, by the way. Schematic attached.

In my opinion, your scheme for reducng the battery volage to the display would work, if you used an active circuit to lower the voltage, like another LM317T. A resistor divider might not pass enough current to run display and also the controller.

View attachment S06S_schematic.pdf

Cool, thanks for the info. I guess I can use a voltage regulator, I wanted to avoid that at first so the voltage at the display isn't held constant making it easier to tell how much range is left and have the controller cut off under low voltage. I suppose I could just wire up a voltage divider to a Arduino, and then to a lcd to display remaining range. I could also maybe set up a jumper on the controller so a display isn't necessarily needed and add a relay with a regulator so all I need to do is flip a switch when I want to change options and the controller will perform normally otherwise.

I don't understand why kt sells a 72v controller but none of their displays are listed as being compatible with 72v. Kinda makes me want to try it but I don't want to risk frying my display.

Your speed Unload VS loaded is about right for a 48V kit. You should pick up a bit more speed from a Lithium battery over your lead acid battery, because of less voltage sag under load, but not much. Running both 48 volt batteries in series would get you over 45mph.
But 40mph takes roughly 3 times more power than 25mph. About 2300 watts of power vs the 800w or so you use at 25mph. You will also need a high voltage controller. Two 48V 13s lithium batteries fully charged in series is 109.2 volts.

You're also going to have another problem. Most Fatbike tires come apart at high speeds. I've had many simply fall apart in a couple miles above 45mph, and often the best are worn down to the thread around 100 miles. I'm not talking about cheap tires either. I've run Surley, Maxxis, Vee Rubber, 45NRTH, Kenda, Sterling, etc, etc, etc.

It's not a lack of quality, it's that the construction and rubber compounds used are optimized for slow speeds and high flexibility, but the trade off is high speed rigidity, stability, tolerance for heat, and ability to shed heat.

I'm a big fan of overvolting. Be careful though because you can fry your electronics&motor if you overdue things too much.
Familiarize yourself with a motor analyzer tool where you can input motor specs, battery, controller information and test "what if" overvolting scenarios. I use ebike.ca's motor simulator tool.
My 36V motor is rated for 500 watts and controller is rated for 1000W.
I overvolt with a 12S pack (50 volts off the charger) and dump as much as 800+ watts into it carefully monitoring heat and run-time.
With stock 10S 36V pack (42V off charger) I get 22 MPH max.
Overvolted I get over 26 MPH.....not too shabby for an e-trike on a "500W" motor.

You can get a little bit more speed by using a controller with field weakening capabilities. I have no experience with this so I dont know how much speed you can gain but it might be worth to dig for some info on it.

I would definitely not feel safe going 40 mph on a fatbike though They are usually not constructed for high speeds. I ride a heavy steel cruiser frame with mc rear wheel/tire and heavy steel fork with 26" 3.5" wide street front tire with beefy 200mm discbrakes, and anything above 30 feels a bit sketchy even for this bike with a much more stable geometry for speed than a fatbike. To feel safe at 40 mph with bicycle components I would need a beefy downhill front fork with dual 200mm discs and beefy downhill wheel. What do you think will happen when you hit a pothole at 40mph with your fatbike front wheel? Best case it bounces and you have a chance to bring it under control. Most probably your wheel will instantly deform throwing you off the bike and you wake up in the hospital.
I love high speed ebikes, just make sure you have components that can handle the forces involved.

maanebedotten said:

You can get a little bit more speed by using a controller with field weakening capabilities. I have no experience with this so I dont know how much speed you can gain but it might be worth to dig for some info on it.

I would definitely not feel safe going 40 mph on a fatbike though They are usually not constructed for high speeds. I ride a heavy steel cruiser frame with mc rear wheel/tire and heavy steel fork with 26" 3.5" wide street front tire with beefy 200mm discbrakes, and anything above 30 feels a bit sketchy even for this bike with a much more stable geometry for speed than a fatbike. To feel safe at 40 mph with bicycle components I would need a beefy downhill front fork with dual 200mm discs and beefy downhill wheel. What do you think will happen when you hit a pothole at 40mph with your fatbike front wheel? Best case it bounces and you have a chance to bring it under control. Most probably your wheel will instantly deform throwing you off the bike and you wake up in the hospital.
I love high speed ebikes, just make sure you have components that can handle the forces involved.

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I don't plan on going 40mph constantly, just want the ability to go that speed and use it when necessary or when it is ideal to do so.
I usually stay at 25-30 right now. I connected another lead acid battery in series to make 60v until my lithium batteries get here. I can now hit around 35mph fully charged even with about 10v of voltage sag. When I get my 72v controller and lithium batteries set up I should be able to hit 40mph easily.
http://www.bikesdirect.com/products/motobecane/fat-bikes/fatbikes-boris-brut-sprung.htm
This is the bike I'm using but with a 180mm rotor in the front I believe and a 26'' 4.5 wide inch tire in the front and about a 5 inch in the rear. As long as you can lock up the brakes then you have enough rotor, then all that matters is how much grip and weight you got that determines how fast you stop. Feels pretty stable with this bike all things considered. The front springs are pretty incredible, 120mm of travel and it just floats over everything. Then again all my weight is in the back until my lithium batteries get here. I haven't encountered a pothole yet on the bike trails I ride but it is possible in the rare times I ride on the street to go from bike trail to bike trail. I feel like its safer going 40mph with traffic than going 20mph, I'll just have to keep an eye out for potholes vs getting hit by an idiot driver. I live in one of the surrounding communities near Houston so drivers are the real hazard out here.

That looks much better suited for speed than most fatbikes I have seen. Decent fork that probably has good enough damping, through-axle hub that should be able to take a beating. I have tried a similar tektro hydraulic brake though, and no way could I lock the front at high speed with 180mm rotors, going to 200mm helped, but I still ended up going back to my TRP Spyre mechanical brake. I have not done proper "scientific" testing, but it feels more powerful and controllable to me than the tektro did. It also seems to keep better power when heating up on long descents. My bike is a lot heavier than yours, I dont have the strongest grip and prefer using 2 fingers so I probably need more bite and heat shedding than most though.

How do you plan to add the second motor btw? I have never tried it myself, but the general advice is to avoid putting a motor in a suspension fork so your options are limited. Considering the complexities of a dual motor setup it is definitely easier and maybe even cheaper to just buy a properly sized motor for the power you want to push.

maanebedotten said:

That looks much better suited for speed than most fatbikes I have seen. Decent fork that probably has good enough damping, through-axle hub that should be able to take a beating. I have tried a similar tektro hydraulic brake though, and no way could I lock the front at high speed with 180mm rotors, going to 200mm helped, but I still ended up going back to my TRP Spyre mechanical brake. I have not done proper "scientific" testing, but it feels more powerful and controllable to me than the tektro did. It also seems to keep better power when heating up on long descents. My bike is a lot heavier than yours, I dont have the strongest grip and prefer using 2 fingers so I probably need more bite and heat shedding than most though.

How do you plan to add the second motor btw? I have never tried it myself, but the general advice is to avoid putting a motor in a suspension fork so your options are limited. Considering the complexities of a dual motor setup it is definitely easier and maybe even cheaper to just buy a properly sized motor for the power you want to push.

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Thanks, It handles high speed pretty well. Feels pretty decent compared to my friend's Ripcurrent S. Weight distribution is horrible right now though with 25lbs of lead acid batteries on the back rack. On the plus side I can do a wheelie pretty easily 8) .
The 2nd motor will be added in the future if I ever do it. Not sure if it is necessary now. As long as I can maintain a decent speed and hit close to 40 I don't see a reason to add one. It would only hurt efficiency and the only benefit would be 2wd. I don't think I will ride in enough mud to make up for the drawbacks. If I did, my ideal setup would be a geared motor in the back and a brushless motor in the front. As long as all components share a common ground I should be able to use the same throttle and pas for both motors/controllers.

Well, I've fixed my top speed problem. Got a 72v controller and managed to hit 38mph with lead acid batteries, Lithium should help some. Display seems to work fine with 72v. Only problem now is I have two 48v battery packs that are gonna arrive today. I'm going to take apart the packs and make 1 battery pack thats arranged 20s 5p. Based on the battery specs listed on the Chinese website, this puts me at 37.5A continuous and 112.5A instantaneous.
My controller is rated at 30A continuous 60A max, but I have the max adjusted to 45A.
Do you think the discharge rate of the battery is sufficient for this. I was gonna add more cells in parallel if needed but I'd rather not mix different cells.

Do you have a link for the batteries? Chinese battery suppliers can be hit or miss. Some will use good cells and rate their packs properly, others just use whatever they can get the cheapest and pull ratings out of their hindquarters. Unless you know the specs of the cells used it is impossible to know what performance you can expect. Beware of fake cells, testing is the only way to determine capability unless you get pack/cells from a reputable source.

Do you have a spotwelder and supplies (bms, nickelstrips, kapton etc) for rebuilding the cells into a new pack?

What motor do you have? Only a direct drive hubbie does well with overvolting. Going to 72V with a geared hubbie virtually guarantees a quite short motor life.

maanebedotten said:

Do you have a link for the batteries? Chinese battery suppliers can be hit or miss. Some will use good cells and rate their packs properly, others just use whatever they can get the cheapest and pull ratings out of their hindquarters. Unless you know the specs of the cells used it is impossible to know what performance you can expect. Beware of fake cells, testing is the only way to determine capability unless you get pack/cells from a reputable source.

Do you have a spotwelder and supplies (bms, nickelstrips, kapton etc) for rebuilding the cells into a new pack?

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I bought them from a vender called Viset. They look rather new, started on may 21st so very new. The product description has been updated on their site, They now list the battery with more cells but lower capacity cells to achieve the same ah. The ratings I included are based on the same ratings that were listed in their original description. Question is like you said, are they truthful? I bought them from aliexpress for $86 a battery plus tax and shipping.

I have all the supplies but a spotwelder and kapton. I do have a huge box of unneeded silicone though. I'm using solder, I have a method that avoids adding much heat to the battery. I already split half a pack and then soldered that to a full pack making 20s 4p. Now I just need to solder the other 20 cells to add another p. Problem is I plan on using a 4 inch PVC pipe as a battery case and I can't fit the battery pack if its more than 4 cells wide. (Please see pictures.) https://ibb.co/zxpQxGnhttps://ibb.co/MVfTCmJ https://ibb.co/sQKcnV4https://ibb.co/bKkyMxZhttps://ibb.co/DVmhVF6https://ibb.co/nzRhXKZ

So, I can add the 20 cells on the far end(which I did in photos), connect them in series, then connect them in parallel to the rest of the battery. Problem is, I then wouldn't know how to connect the balance wires on the bms. Only thing can think of is to wire each of the 20 cells in parallel by adding a copper wire going across. I have already done this to 1 cell in the pictures. I'm sure there's probably an easier way to do this but I'm pretty new to Bms'. Or I can find another way to store the batteries on my bike. I was gonna mount the PVC pipe on the lowest bottom end of my bike near the center via existing bracket screws. It's in a perfect location. PVC seems like it would make a very tough waterproof case.
edit: I might just move the cells and use this as a case. https://www.lowes.com/pd/Amerimax-120-in-White-Aluminum-Downspout/1101827 I'll paint it black to match my bike. Not sure if it will offer much protection though and I have to find a way to make end caps.

John in CR said:

What motor do you have? Only a direct drive hubbie does well with overvolting. Going to 72V with a geared hubbie virtually guarantees a quite short motor life.

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I have a direct drive.

Soldering works, but why did your remove the nickel strips? You could have kept them and just soldered the series connection from group 13-14.

The pvc pipe is a good idea, but any shock to the pipe will put a lot of force on the "corners" of the pack inside unless there is room for reinforcement or padding.

Honestly though, I would not consider that pack safe enough to use. There is no spacing or insulation between cells or washers on positive end so if/when vibration wears through the blue shrinkwrap fireworks will go off.

I did use the nickel strips and combine them from group 13. The rest of the cells are from the left over 7 groups.
I bought a 4.72 inch pvc square tube off amazon just now. A little pricey but might be worth it.

I'm gonna have 4 extra cells left. Thinking of taking these 4, connecting them in series and powering a 120w silicone pad from a safe distance as a way of testing the discharge on these batteries.

With the 4.72 inch tube I should have a little bit of extra space. I can center the battery in the tube and then fill all the surrounding space with globs of silicone caulk to help with vibration. I used to do silicone contractor work for a floor comany last year so I have about 10 extra leftover tubes.

I finished the battery yesterday and went for a ride. I managed to get 25 miles out of it before the bms low voltage protection cut me off early. I was still around 65v.
Today, I was about to go on another ride, I turned my lcd8h display on but then it randomly turned off. Won't turn back on. I've been riding it for a while on 72v, maybe it's really not rated for this and I finally fried it?
If this is the case then I don't understand why kt makes a 72v controller but not a 72v display.

My motor kit also came with a jumper to plug in place of the display in case you don't have one. So, I plugged this in and went on a ride. After a very short period, (less than a mile) my power cuts off and I stop. I wasn't pushing it hard at all. Then after waiting a minute I tapped the throttle while off the bike and then I heard a hissing noise following a boiling noise. Then I smell a burning smell. I run to take cover just in case its my battery about to explode lol. Turns out it was my controller, its completely fried now.
https://ibb.co/bzPd6yK Looks like a bad capacitor. Do you think the controller had anything to do with the display? Seems a bit odd they both went belly up back to back.

Edit: Upon closer inspection, it looks like a couple mosfets blew up.

did you ever find out if the display is still working and if not what caused it to burn? I have a lcd8h 48v and I'm thinking of converting the battery to 60v.

05silgto said:

Hello, I recently converted a fatbike to a fat-ebike. However, top speed isn't quite where I want it. My top speed is about 27mph on flat ground. I don't think there is a speed limiter, When I lift the motor wheel and hit the throttle, the speedometer shows about 34mph, so unless watts is being limited at higher speed, there isn't a speed limiter.

Any ideas? Any feedback is appreciated.
Fyi, my end goal is around 40mph, this motor doesn't have to get me there alone as I plan on adding a 2nd motor in the future but it needs to have enough rpm to hit 40mph which it probably will after overvolting it.

Click to expand...

What tread are on the tires, as in which brand / make of tires. I assume they are 4.0 inches wide, probably 26" diameter. You are doing this on pavement, no need for such wide tires. If you went down to 2.8 or 3.0 wide and a more XC-oriented tread pattern, I guarantee you'll get more speed by that switch alone. And it's fine to put them on a rim designed for 4.0 tires. IMO there is a lot more quality and selection in 2.8 tires than 3.0, but those are mostly 27.5x2.8 unfortunately. If you are running 26" less to choose from in 2.8 or 3.0. WTB, Surly, Vee, some other companies make 2.8 and 3.0 tires in 26" diameter. I hated the WTB Ranger though. Also, on a practical level, relatively skinnier tires will not help the bike brake as well. Otherwise the speed and handling will be way better, and you will notice longer battery life too. Wait for the lighter lithium batteries before switching tires.