Adding a second motor as a booster only

[rickjames8]

Hey all - I recently moved from the US to France and have moved to a very walkable town. There are two places I go where I'd like to take a scooter from time to time, so I bought myself a used 36v 500W i9-Max scooter. However, there is one huge hill to get to those destinations. At the steepest bit, it's a 11% grade for about 150 meters, and I'm 190lbs, so it doesn't quite get me up the hill.

So I looked into upgrading the motor, but based on what I was able to search on in this forum, upgrading the motor would mean replacing too many parts and at that point it would be cheaper just to build something else. Which is what I was looking into, when an idea occurred to me.

Could I just add just a second (rear) motor to the i9 Max? It has a front-mount 36v 500w motor. I'd just add a 36v 250w-ish rear motor. Here's why I think it might work. I don't need to speed-control it as I don't need it to be in sync with the front wheel. I'd never ride them both at the same time, except for the 3 minutes or so that I need to get a little kick going up the portion of the big hill. So I'd actually just hook the motor up to the battery using a heavy duty relay, and kick it in with a pushbutton switch. Like the e-scoooter equivalent of adding a nitrous button. If I understand the risk, it's the potential to burn out the motor. But at €50 for a used motor, I'm willing to risk it. Are there other reasons why this would not work?

 

[amberwolf]

Unless it's a brushed (two-wire) motor, you have to use a controller (even if the controller is built into the motor).

If it is a brushed motor and you direct connect it to the battery, there is NO current limiting, so if the load exceeds the motor's ability, it will overheat and probably fail, as you note the possibility of.

But if the load exceeds what the battery can handle, then depending on the excess and the hardware limitations, it will either just shut off the output (assuming the BMS can detect current and safely shut off) or it will damage the BMS FETs (usually by shorting them so it can no longer turn off and protect the cells against damage), and may also overheat the cells, interconnects, wiring, etc, which can lead to a fire.

The battery on the scooter is probably already at it's limits with the first motor, so you probably have to carry a second battery *just* for the boost motor, and it has to be able to handle the full current that motor will draw under whatever load you place on it, which will probably be much higher than the watt rating of the motor would suggest; it may be dozens of amps. (you'd have to test it under actual conditions to see what the real current is, and if the current is too high it will probably damage whatever battery you are testing with, if the protections in place are inadequate).

 

[rickjames8]

Awesome! This is just the information I was looking for.

In the back of my head, I didn't think something so simple could work, but I didn't know exactly why.

I'm going to go ahead with my original plan of trying to build out a more powerful 48V scooter. I'd already bought the frame of an old Viron scooter (no motor, batt, or controller) for €60 and I'll add parts as I find them. I just bought a 48v 800w motor off our version of Marketplace here in France for €70. Trying to keep a reasonable budget for this project (under €250).

I haven't given up completely on the 'booster wheel' idea for the i9. I think I'd use a second controller if I tried it. It seems unlikely that the battery would overheat with such a short use, but maybe I'm wrong? I just google-measured, and the hill is 120m long with a rise of 15m: 12 Rue Savary to 36 Rue Savary

 

[amberwolf]

THis page

Calculate the Slope

Calculate the slope in degrees and percent, the distances in length and height and the total distance.

rechneronline.de

shows that's a 12.5% slope, which is pretty steep. If it's only 120m long, then if you are taking three minutes to up the slope, then according to this

Speed Distance Time Calculator

Solve for speed, distance, time and rate with formulas s=d/t, d=st, d=rt, t=d/s. Calculate rate of speed given distance and time. Find mph, miles per hour, km/hour.

www.calculatorsoup.com

you're only going about 2.4km/h, which is about half the typical walking speed.

At that speed it doesn't take too much energy to go up that slope, even if we assume a high rounded number of 250lbs for you plus scooter plus anything you're carrying. I would guess based on this cycling calculator

An interactive model-based calculator of cycling power vs. speed

An interactive calculator that lets you explore the relationship between cycling power and speed.

www.gribble.org

that the actual battery/motor power is probably less than 250w (since that always ends up higher than the cycling pedal power to do the same thing, which comes out to about 120w), and I put 20% for drivetrain losses since hubmotors are less efficient at lower speeds and higher loads. If I used 50% losses, it would take at least 400w of power and probably more than that for the battery/motor.

In that event, going at a faster speed, whatever the normal speed of the motor is, would be much more efficient, and use less power to do the same work.

The simulators at ebikes.ca give a more ebike/scooter oriented version of this, though it doesn't break down the various losses like this page does (headwind, rolling resistance, gravity, drivetrain, etc), it just tells you the total watts at the mootor, probable losses between battery and motor, etc. But it can help you figure out if different motors will give you better results for your specific riding conditions, and then you can find motors like the ones that work best in the simulator (or those actual motors), to then build hte best scooter to do the job you want done.

But the best thing you can do to find out what power is required is to install a wattmeter between your battery and controller, to measure the actual watts, voltage drop, amps, etc during the worst-case parts of your rides, so you can be sure of building a system that is more than capable of those.


Your first post said there was (also?) an 11% 150m long slope, and that would probably be equivalent in energy needs for the same speed, assumign the later data is not just a correction of the initial data.

 

[rickjames8]

Yeah, it's a steep one! I typically walk it a couple times a week anyway, so I'm getting used to it (and getting fit), but the scooter will be a nice luxury for the times I don't want to or don't have the time to walk it. Its the same hill I first mentioned, I just miscalculated the slope.

>if we assume a high rounded number of 250lbs
Pretty close. Maybe more like 230lbs

>it would take at least 400w of power and probably more than that for the battery/motor.
So the i9 Max is rated at 500w, and doesn't make it all the way up. If we're assuming 20% loss, that still takes us to 400w.

> The simulators at ebikes.ca can help you figure out if different motors will give you better results for your specific riding conditions
I'd played with that before and had trouble reading the results, but I'll give it another go. Ok, finding the closest items to what I would buy, it seems that I'd need about 725w, so the 800w motor that I got should do the trick?


>the best thing you can do to find out what power is required is to install a wattmeter between your battery and controller, to measure the actual watts, voltage drop, amps, etc during the worst-case parts of your rides
I have a wattmeter, but at the worst-case part of the drive, I'm at a standstill, so I'm not sure what it would show.

Thanks again for the help. Really appreciate it!

 

[gromike]

On my DD hubmotor bike, the 500w (as labeled), motor gets fed up to 1400w.