undervolting an ebike motor for more range

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i have been thinking about this for a while and seen the questions and negative responses on various forums...
but here goes.

i know the range of an ebike is more determined by the battery's amp hours then by the voltage so... let's say you have a 48v motor and a 48v battery, let's say you don't want the full throttle e motorcycle but you just want a small boost and mostly rely on your legs (think eco or low power mode) can you mod your 48v battery into a 24v battery (buy a 24v bms, put it on there) and get more range at lower output?

if yes, are there things to look out for?
if no, why would it not work?

ps let's say you have a 1000w motor and you don't mind running it at a really low wattage like for example somewhere between 50w or 200w as long as you get a little boost
(wattage 1000w/48v=21amp, 50w/48v=1amp, 200/48=4, 50/24=2, 200/24=8
splitting the battery 21ah48v->mod->42ah24v)
 

Hoggdoc

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If you lower the voltage and the motor is calling for the same power level the amps will increase. Generally speaking a modern ebike brushless motor relies on voltage to determine the maximum rpm it will run based on the windings in the motor.

In brushless motors this is called KV rating which determines the revs per volt applied to the motor. For instance 48 volts fed to a motor with a KV rating of 100 would be able to spin 4800 rpm (48v x 100KV).

What you are suggesting would require a redesign of the physical build of the battery pack in terms of the number of cell configured in series and number of the series packs configured in parallel. Then you would need a motor controller designed for that voltage and amps you want to deliver to the brushless motor. A 48 volt motor should be able to handle the reduced voltage ok.

Basically I get what you are trying to do, but seriously think you can achieve your desires by using low setting on pedal assist or very little throttle input when needed.
 
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If you lower the voltage and the motor is calling for the same power level the amps will increase. Generally speaking a modern ebike brushless motor relies on voltage to determine the maximum rpm it will run based on the windings in the motor.

In brushless motors this is called KV rating which determines the revs per volt applied to the motor. For instance 48 volts fed to a motor with a KV rating of 100 would be able to spin 4800 rpm (48v x 100KV).

What you are suggesting would require a redesign of the physical build of the battery pack in terms of the number of cell configured in series and number of the series packs configured in parallel. Then you would need a motor controller designed for that voltage and amps you want to deliver to the brushless motor. A 48 volt motor should be able to handle the reduced voltage ok.

Basically I get what you are trying to do, but seriously think you can achieve your desires by using low setting on pedal assist or very little throttle input when needed.
thank you, that's very enlightening.

if I understand you right,
-the motor would be fine with lower voltages, but the controller (brains) wouldn't understand what to do to the point you'd need a different controller
-I hadn't heard about kv before, so... kv is the number of revolutions per minute per volt right? a 100kv at 1v would be 100rpm and at 40v it would 4000rpm if I'm not mistaken

yes it would require a redesign of the battery pack (or just buying a lower voltage battery pack (6s or 7s for 24v)) and I'd be willing to mod it in that way

I was wondering about this because I looked at older second-hand ebikes and I noticed many run at 24v on NiMH batteries or if you go to even older ebikes 12v? on lead-acid batteries. Because the laws here require a moped license for any ebike that can go over 25kmh and even with a license you can't go over 45kmh (without getting it classified as a motorcycle), I was wondering if pedal assist up to 15 or 25kmh would be achievable with 24v given that 48v motors can reach speeds up to 50kmh. the reason I'm interested in it is more for a multi-day trek or for a 6-12hour ride without a massive battery
 

Hoggdoc

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thank you, that's very enlightening.

if I understand you right,
-the motor would be fine with lower voltages, but the controller (brains) wouldn't understand what to do to the point you'd need a different controller
-I hadn't heard about kv before, so... kv is the number of revolutions per minute per volt right? a 100kv at 1v would be 100rpm and at 40v it would 4000rpm if I'm not mistaken

yes it would require a redesign of the battery pack (or just buying a lower voltage battery pack (6s or 7s for 24v)) and I'd be willing to mod it in that way

I was wondering about this because I looked at older second-hand ebikes and I noticed many run at 24v on NiMH batteries or if you go to even older ebikes 12v? on lead-acid batteries. Because the laws here require a moped license for any ebike that can go over 25kmh and even with a license you can't go over 45kmh (without getting it classified as a motorcycle), I was wondering if pedal assist up to 15 or 25kmh would be achievable with 24v given that 48v motors can reach speeds up to 50kmh. the reason I'm interested in it is more for a multi-day trek or for a 6-12hour ride without a massive battery
As far as the controller it’s really a motor speed controller that varies the voltage to the motor and amperage which intern the motor supplies the RPM and torque.

Do you understand the KV calculation correctly. I learned all this from flying electric powered RC planes. Takes a little while to wrap your head around it for sure.

I’m not sure if you can find a 24 V battery pack these days. Batteries and all other eBay components are in short supply because Of issues within China. There’s a resurgence of the Covid virus In the region does a lot of manufacturing for the components that go into making E bikes.

Even if they had a supply of these items there’s a back up at the ports in China because of the lack of containers. We all know that China ships out a lot more than they should be in and nobody’s willing to put empty containers on a ship to take them back to China. Therefore the lack of containers.

I’m sure there would be no problem reaching the 25 km/h with a 24 V system it’s just how long it will take to get there. It’s kind of like a small four-cylinder engine compared to a large V8 they will both get to the same place eventually.

You might wanna consider going to Hobby King on the web and look at some other hobby batteries pick something that has the milliamp hours that you’re looking for and voltage or You can always connect batteries in series to increase voltage or in parallel to increase their storage capacity to i.e. milliamp hours.

I don’t know if that helps hope so good luck with your project.
 
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Just get off the throttle a bit.

Otherwise, you would need someone knowledgeable to do the work, a spot welder, new BMS, and new controller.

The first option is a lot simpler.
 

CrossRoads

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This may be above my pay-grade but I'm very under-convinced that after all that work your low-voltage setup would be any more efficient, i.e., extended range, than simply a very low PAS setting or a lightly touched throttle.

Generally speaking higher voltage is more efficient. That's why high power consuming electric appliances use higher voltage. I guess, specifically, that means you get less energy burned in heating the wires leading to the device for a given amount of power delivered to the device at a higher voltage. Wires on an ebike are shorter than household wire, but it might be significant at 12 volts. I don't know but would suspect the increased resistive losses from lower voltage may apply to energy lost in the controller and perhaps motor windings as well.

If i were going to go about hacking something i would keep your 48V battery as is and look into hacking a PAS sensor that would modulate down to lower power than, say the "stock" PAS #1 setting. That seems like it could be simply a matter of googling for the right PAS product, or modifying a throttle.

There is also of course just turning your system off for the flats and descents, and on for the climbs.
 

CrossRoads

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I have a friend working on a sailboat that has 12V wiring for all the instruments and so he already has a big 12V battery bank installed next to the alternator at the motor near the stern. Now he needs to wire up a 1000 watt windlass (winch for the anchor) on the bow (40 feet away), and has to spend I think $600 for 0-guage wire to cover that distance with minimal voltage line loss. I told him if that were a typical 120V AC residential wiring job, the romex would be 14-guage for a 1000 watt device and cost a few bucks.
 
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