How many watts is my motor?

ClintBX

1 kW
Joined
Mar 6, 2014
Messages
419
Hi EPFMs,

I'm shopping around for a new battery for my 36v ebike. There's just one thing that confuses me.

I see that a lot ebike batteries are being sold with different watt specs. 250, 350, 500, etc

The ebike I have now, I bought it second hand and I didn't know to ask about the watt specs.

How do I find that out so I purchase the right battery?

Clinton
 
Any frame sticker showing conformity?
Motor stickers?
Photo?
 
Describe the wires coming out of the motor. Are there three fatter wires, and also five smaller wires? (that would be a brushless three phase motor with hall sensors, very common). If there are two fat wires, maybe it is a brushed motor.

Do you have a model number for the motor? If you don't have any details about the motor, can you measure the width and diameter? Could you post a pic?
 
I think its a brushless motor. I know there are 3 large wires and 5 smaller ones that go to the controller. I can't spot any labels or stickers on it. Which is why Im unsure.
 
I'll post a measurement when I can get my hands on a tape measure.

I have been comparing the appearance of different motor types and noticed that the 250 and the 350 are smaller in width and usually chrome. Mine is a bit wider and black.
 
Since it's brushless, the controller is what should actually be focused on if possible. The motor is kinda arbitrary/ secondary importance.

A 1000w battery will be great, because it's overkill and likely up to 2x what the controller draws. (25amps for 36v = 1000w)
From your description of the motor, the controller might be more like 500w than a 250/350w (~15a), so if you're not hard on the throttle all the time a typical 500w battery should suffice and could be much smaller and lighter than the 1000w one.
 
Watt (what) battery are you replacing?
Yer trips around town etc mostly flat? Any "killer" hills to climb?
Two ratings: Continuous wattages and PEAK wattages. Meaning how efficient is the motor (waste discarded as heat) and also the "C" rating for the battery itself. How fast it can discharge (and recharge) electricity. Depends on the chemistry involved. Ideally your motor and controller and the battery never get "hot".
Desired range per charge (with no pedaling or up hills or stops)? Then add/allow for extra weight ("cargo"), head winds, plenty of stop and gos, tired batteries, fast starts (showing off)... etc. Pavements smooth? ... or wanna ride muddy trails?
 
Yeah, if you can pull the numbers off the controller, they will be far more useful. You might have a motor stat started life being factory rated at 250w, but the vendor paired it with a 500w controller. That's often not a problem, since many motors are extremely conservatively rated, but it might cause a weaker battery to puke up it's waffles if it tried power your bike.

And the opposite can happen, too. a vendor worried about warranty work might under rate the bike, using a 500w motor but a 250w controller making a very robust 250w bike. In that case, the motor's original rating would be meaningless.

If you can find no markings, post pics. we might be able to identify it. And if not, the only danger with a battery's rating is that it might be too small. Too small and the battery eats it's self trying to serve the motor. Too large isn't a thing. As long as the voltage is in range of the controller, you could put a 36 volt, 20000000000000000000000000000000000000000 billion amp battery on it with a 72x10^29 watts and it would work just fine. It's the controller's job to limit what the motor takes.
 
The size of the brushless motor means nothing. Only the controller draws power from the battery. And the watt rating of the controller is also meaningless when selecting a battery. You have to know the max amp draw of the controller and provide a battery pack capable of an output of those amps. IOW's if your controller is 30A max, you need a battery pack capable of a 30A minimum output. The higher the batter as to put less strain on the battery.
 
There's no labels on the controller either.

The hub is about 20cm + diameter. (rough measurement)

I'll attach a photo.

This brings up a question that only just occurred to me

My last ebike was 250 or 350 watt front hub motor and I had a spare 36v 12ah battery since I last had that bike.

I recently added it to my current ebike and I was disappointed to only get a 45 minute ride out of it. I used to be able to go for an hour and 20 mins in the other bike. I assumed that the battery had degraded. But now I wonder.

Maybe it was only a 250 watt battery and my current motor is possibly higher and was eating it quicker. Could that be why?
 
That pic didn't attach for some reason. Let try that again..... I'm getting "the file is too big" message.
Attaching stuff in forums is always so fiddley

How do I get my image down to512kb?
 
Okay here'sa compress version of the image
 

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That is a 9 continent direct drive 500w rated motor, which will typically be mated with a 20-25 amps controller. That would pull as much as 700-800w with 36v, and 1000-1200w with 48v.

Likely it can eat your battery a bit faster, AND, likely it did degrade at least some. Your battery may be undersize for this setup, so try to use only half throttle starting up with pedaling, ( till going 10 mph) and limit cruise speed to the same speed you used to go with the other motor.

FWIW, that motor can handle up to 2000w pretty good, and some even push them to 3000w. So your motor is not going to limit you much.

Start figuring out where to get more battery, it's clear you want more range. a 20 ah should give you plenty. It's very likely the controller can work fine with 48v, if you want more power and speed. If it won't, then a new 48v controller can be fairly cheap.
 
So, its a 500 watt motor?

Are you saying that I could get a 48v battery and it would work fine?

If I get a 48v battery, could that put more stress on the motor? Im also concerned about longevity of each parts.
 
It'll work fine with any voltage battery as long as the controller will support that voltage. The motor gets it power from the controller, not the battery. You could use a 72V battery with a 72V controller if you wanted to. I run my 48V 1000W motor on an 88.8V pack charged to 100.8V. Just have to not overheat the motor. I also ran my old 500W motor on the same voltage.
 
Regardless if the current controller can actually handle it, I don't think you want more voltage if you are happy with your speed now. It will be a bigger more expensive battery, and if you're not going to ride any faster, the motor will be running alot more in it's inefficient lower end.

It would handle it fine in most conditions, but I just don't see any benefit unless you want more speed.
Go for just bigger AH for more range, and also more watts. I like lipo- multistars for this application, if you know what you're doing and don't mind extra care given.
 
Back to choosing that battery.. We still have no idea at all what your controller will allow. the motor's watt spec has nothing to do with what the controller might allow.

But in general,,, It's unlikely it's paired with more than a 30 amps controller, so look for "30 amps continuous" in the battery spec. Or more.

Chances are your speed and power are ok as it is. So you can stick with 36v. If you do live where there are very steep hills, then you might enjoy 48v, for more power up real mountains.
 
Thanks guys for the really useful info.

Yeah, I'm not exactly sure on the speed of my controller since there no labels on it. I'll attach a couple of pics to see if any of you can tell me. (if you can tell by just looking at it)

That 30 amps continuous spec. Are you sure that's not 30 and max?

I see a lot of 30 amps max discharge with 15 amps continuous on the market. (and some 40 max/20 continuous). Would that be to weak?
 

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The 20/40 might be a good match.
At most the controller is probably 20-25a peak (500-800w), so a 20a continuous bms/pack should be ok, imo. 15/30 might be pushing it.

You could test it and know for sure, but because most multimeters do only 10a thru-put, you'd need to buy an 8-10$ volt/ammeter w/ shunt.

http://www.ebay.com/itm/LED-Digital-Voltmeter-Ammeter-Amp-Volt-Meter-Current-Shunt-DC-100V-50A-Dual-/141455619876?hash=item20ef697724:g:8PMAAOSw-jhT~EZI
Here's the cheapest one I know of that would work, and any such wouldn't be a bad permanent addition to any bike. They're not hard to install.
 
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