# What voltage/wattage combination for power assist at > 25km/h ?

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khammo01
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What voltage/wattage combination for power assist at > 25km/h ?

Hello - this is my first post on this wonderful forum.

I live in Asia and I recently got a 24v 250w kit with a 26 inch rim (front hub, brushless). I paired it with a LiFePo4 battery (24v10Ah). The setup is working great. I get a decent power assist and a good balance between effort and ease. The 10Ah battery has sufficient range for my needs as well.

However, the top speed without pedaling is only about 22km/h. If I pick the wheel off the ground, my cycle meter shows the motor at a free spinning speed of 25km/h. This means that if I pedal faster than 25km/h, I get zero assist from the motor. Before I upgrade to ebike, I routinely pedaled as fast as 30km/h (with effort, of course).

My questions are:
1. For a 26inch wheel, what motor/battery combinations would allow me to get a top speed of 30km/h+?
I understand more watts=more speed, but is there any way to calculate this so I can know what to expect before I buy?

2. Alternately, is this an arbitrary limitation inside the controller? Is there a controller hack I can do to raise the maximum speed? Would a different 24v controller give me a faster top RPM?

3. Are there any front hub motors out there that work at 24v that will give me the RPMs I need to get power assist even up to 30km/h?

chas_stevenson
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Re: What voltage/wattage combination for power assist at > ...

I understand more watts=more speed

This statement is both true and false. Allow me to explain;
Watts is the combination of Volts and Amps.

• More Amps = more power or torque
• More Volts = more RPM or speed
• More Volts or Amps = more Watts, so you see more Watts could be more power or more speed or both.
• Adding Amps will do very little to make the motor turn faster at top speed, but it will allow better acceleration to top speed.
• Adding Volts will do very little to add acceleration, but it will add RPM to the motor therefore increasing the top speed.

For your other question there is no answer because there are so many variables. I would make an educated guess, based on my experience, and say if you change you 24-Volt battery pack to a 36-Volt battery pack you will achieve the speed you seek. Don't worry about the motor it can take the additional voltage. You controller may also be OK. Open the controller and check the voltage limits on the internal parts, marked on the parts. If they are in the 50-Volt or higher range, which I would expect, then the controller should work fine. BTW changing controllers will not give you a faster bike. They all put out the battery voltage to the motor and there is no more power to go any faster. Using a higher battery voltage is the best way to make a motor run faster.

Grandpa Chas S.

khammo01
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Re: What voltage/wattage combination for power assist at > ...

Thanks for the comments. I suspected the motor could take the extra voltage but am concerned about the controller? It has "24v" penned on to it and I can't take it apart because the heat sinks are glued to the inside of the casing.

I was considering a 24v400w wheel, but based on your explanation it sounds like the wheel would not spin any faster. Would I be better off getting a 36v10Ah battery, or keeping my battery and getting a 24v400w wheel? I found one on ebay which is about the same price as a 36v battery...

I'll be happy if I can get the top assist speed to 30km/h. Any faster than that and my gear ratio would mean my legs would be going too fast.

Update: I just did a quick but risky test adding a SLA 12v battery in series to the LiFePo4 battery pack. I got exactly the 30km/h speed I wanted. Excellent. I took a quick ride and didn't notice any unusual temperatures, although it's winter now.

The only odd behavior I noticed is my pedal assist system (PAS) stopped working. Hope I didn't fry that. (It's required in Japan.)

I'll probably get a 36v battery pack since this is confirmed to be working and since it will probably be easier to sell off a battery than a standalone wheel.

Thanks!

chas_stevenson
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Re: What voltage/wattage combination for power assist at > ...

khammo01,
A 24-volt 400-watt motor will not go much faster than what you have. As I said Watts is Amps and Volts. The formula is Volts X Amps = Watts. So if we do some math here is what we have;

 Volts Amps Watts RPM 24 10.4 250 ~ 200 24 16.6 400 ~ 202

As you can see the speed of these motors are almost the same. The only 2 things you get from the 400 Watt motor over the 250 Watt motor is better acceleration and less range. The 400 Watt motor will use more current therefore you will get less range.

As I said the best thing to do is change the battery pack to a 36-volt pack. As far as the controller goes it does not matter what it says on the outside of the case, what matters is what it says on the parts inside used to build it. From my experience, I would just put a 36-Volt battery pack on the bike and ride it. If the controller smokes, then replace it, if not, you saved a few dollars on a new controller.

Grandpa Chas S.

khammo01
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Re: What voltage/wattage combination for power assist at > ...

Yes, that makes sense now.

... I connected a 12v 10ah SLA battery in series with my 24v 10ah battery pack to create 36volts, and I took the bike out for a very long shakedown ride. Nothing smoked or blew up and performance was great until the SLA battery started to die off, at which point the top speeds and power started to slack off.

I was able to ride 25 kms non-stop on a single charge, with a combination of some short hills, riding with pedal assist and riding with full power. This is more range than I am ever going to need. (My daily ride will only be 5kms or so)

Based on this successful test, I have placed an order for a 36v 10ah battery pack. Thanks for your advice!

One thing I noticed was it was a little difficult to fine-tune the amount of power with the throttle when running at full voltage, and found myself going faster than I wanted. I suppose this is due to the cheap parts of my kit throttle.

chas_stevenson
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Re: What voltage/wattage combination for power assist at > ...

One thing I noticed was it was a little difficult to fine-tune the amount of power with the throttle when running at full voltage, and found myself going faster than I wanted. I suppose this is due to the cheap parts of my kit throttle.

This is normal for the type of controller you have. With a little practice you will get much better and be able to control your speed much better. I am glad to hear the experiment was successful and i think you can look forward to many miles of enjoyable riding.
Grandpa Chas S.

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