Upgrading 750w 48v

[tombenn444]

I have a customer who wants get his bike to go faster, it's a 48v 750w rear hub 20" city bike looking thing. If I'm calculating this correctly it's putting out about 15A. Im thinking about using a 30A 1000w controller maybe upgrading the BMS if I have to. He doesn't want to go as far as replacing the motor and he only is really asking to go about 30mph. It's his daily commuter and I've explained the risks of overheating but I personally don't think it'll be that bad as long as he only drives it to work. Can anyone recommend a good set or do I have the right idea?

 

[amberwolf]

If the battery is only meant to put out 15A, then upgrading the BMS is not a good idea--a BMS is sized to protect the cells against damage, so if it's at 15A it should mean that the cells are only capable of that. Using them at higher rates could damage them, which could lead to a fire.

As for "calculating" current, you'd need to actually measure the current to know what's happening. Or measure hte voltage and the wattage at the same moment of highest load, and use that to calculate the current.

You don't specify how you calculated it or what information was used to do so or under what conditions that informaiton was gathered, so we can't tell you fi tha't rigfht or not.

Besides, if he wants faster, increasing current doesn't do that--that increases torque. Increasing voltage increases speed. The increased speed may also require higher power to do this job, and higher current may be a part of that, but higher voltage is the first stage.

So you'd need a new battery that has sufficient voltage to get the speed you need with the wheel and mtoor you have. It's directly proportional, so whatever the max speed you get now, take the new speed and divide it by the old speed. Use that number and multiply your present voltage by that, whcih gives you the minimum new voltage required.

The battery also has to be able to supply the current required to get the total power needed to go the faster speed under the riding conditions you have.

Additionally, the controller (and display, etc) have to be designed to handle that higher voltage; if they're not you'll need new ones.

 

[tombenn444]

That's right it is voltage that increases top speed. I was thinking that the setup he has now is speed limited and with a new controller he would be able to increase that limiter or remove it entirely. Well if I were to put a 1000 watt controller on it do u think I could overvolt the system as long as it doesn't exceed the ratings of the caps

 

[E-HP]

Tell the customer to enjoy the bike as-is, and save up for something better than a city bike looking thing.

Here's an example of the effect of doubling the current; no increase in speed:

Motor Simulator - Tools

Our ebike motor simulator allows you to easily simulate the different performance characteristics of different ebike setups - with a wide selection of hub motors modeled, and the ability to add custom batteries and controllers and set a wide variety of vehicle parameters you'll be able to see...

ebikes.ca

 

[tombenn444]

I understand, and I totally agree. Short of a bigger motor not much can be done for his top speed, but just a handfulI of mph more

 

[lnanek]

> Short of a bigger motor not much can be done for his top speed

There usually isn't an issue in the motor running at a higher voltage, e.g. reusing the 48V motor at 52V is quite common. It's just a bunch of big copper windings that act as electromagnets. I wouldn't go slow uphill with it due to overheating, though.

I think people were just trying to point out that every motor has a kV rating, which is the RPM per volt, which determines your unloaded top speed. So a higher wattage controller at the same voltage (more amps current, not more volts) doesn't help with that (unless you get a controller with field weakening which can eke out a few percent more top speed for a lot more amps).

If you replace battery, controller, and display and leave motor alone you could definitely get a higher top speed.

 

[tombenn444]

I spoke with him last night and he's actually looking for more of a dramatic increase in speed. Here is the bike

 

[docw009]

That's a dumb looking bike for going past 30 mph. I hope it's a steel frame. Silverfish battery? YOu can get them in higher voltages, but a different case may have some advantages.

You must have a 52V battery in your parts bin. Just splice it into the harness and go for a test ride. That will give him a 14/13 gain in speed. Then he can judge if it's worth buying a battery and controller.

I suspect that if you install a 1000W DD motor in the back, you might get the speed. Smaller 20" wheel might hamper it, People install those cheapo Voilamarts on beater 26" bikes and claim to go over 30 mph.

I think it's a bad idea on that bike and you should have him sign a liability waiver if you do it,

 

[E-HP]

I spoke with him last night and he's actually looking for more of a dramatic increase in speed. Here is the bike

Not worth upgrading. If the battery could handle a 20A controller, and you find a motor that has a kV rating close to 13 or so, he can get close, but that assumes 100W of pedaling, which he can't do with the big rear cog and not enough chainring and a 20" wheel. Any controller upgrade would have to be mounted on the frame, since the small oven the stock one is housed in will just cook a controller that's bigger than the one he has.

 

[Chalo]

If he wants a fast bike, tell him to get a fast bike next time. Any bike that's slow with pedals only, will be slow with a low powered electric motor like that. Terrible bikes don't somehow become good when you motorize them.

 

[99t4]

I spoke with him last night and he's actually looking for more of a dramatic increase in speed.

What is his definition of "dramatic increase in speed?" 100% faster? 200% faster? More?

 

[amberwolf]

I spoke with him last night and he's actually looking for more of a dramatic increase in speed. Here is the bike

Sounds like time for a motorcycle.

Even for the originally stated 30mph, you'd need a new battery, controller/display, probably new motor, almost certainly new brakes, possibly tires, etc.

For "dramatically" more than 30mph, I don't think it'd be safe with that bike regardless of upgrades. (well, I don't think 30mph would be safe on it either, but...)