500w Controller for 250w motor?

Hi,

I have a basic conversion kit (originally bought 2016) that consists of a controller rated at 36V/7A for the small 250w hub motor I have in my front wheel. I looked around and googled it of course and it is likely a bafang 8fun. So start of this year I got a new battery and found it runs a lot better and picks up a lot faster. The new battery (also 36v) has a continuous output of 15A, the old one says 10A max. discharge on it. I assume the old battery was fairly worn, because it would be at 2/3 LEDs showing charge even when "full". The new one lights up properly as full.

Now my questions;

1.Does the controller limit my power or is the rating on the housing only a recommendation? So is it actually running on what the battery can deliver or am I now just back on the "full potential", being 250w at 7A? I saw that the battery charge LEDs only go off voltage so it doesn't show how much power it actually uses. Not sure what that means, because I don't think it's even drawing the 15A it could, given that the controller says 7A on it; if that's "fixed" I should be at 250w, is that correct?

2.Now since the new battery could handle a lot more output, would it be safe to run the motor with a 500w controller? I found one rated at 36v/500w/14A. The motor says 250w on it. If I use said controller would the motor a)still limit and run on 250w b)get damaged or would it actually have more power/faster pick up (being reliable and safe of course)? I read a lot about people overvolting motors and most only talk about top speed but I would still be on 36v with double the wattage, and I pretty much only care about acceleration on this.

Most I found about going 500w on a small motor use 48v and I would like to stay on the 36v and increase the power just by replacing the controller.

Im really curios as to what more knowledgeable people would advise going for.

Thanks in advance

dudgggg said:

1.Does the controller limit my power or is the rating on the housing only a recommendation?

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The controller should be limiting to it's current limit rating.

If it is a 7A controller, then you should only see about 7A battery current draw continously, though you may see momentary peaks on starting up the motor of up to twice that, that quickly drop to the 7A or so.

The controller's limit is there to protect the motor from overheating, and to prevent overdrawing current on the battery.

I saw that the battery charge LEDs only go off voltage so it doesn't show how much power it actually uses. Not sure what that means, because I don't think it's even drawing the 15A it could, given that the controller says 7A on it; if that's "fixed" I should be at 250w, is that correct?

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You can install a wattmeter in series with the battery-to-controller connection to measure the actual power, current, and voltage usage. Bunch of threads about differnet kinds, if you're interested.

2.Now since the new battery could handle a lot more output, would it be safe to run the motor with a 500w controller?

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If the battery is 15A *continous* capability at 36v, then it will be able to safely handle 500w. But if that's a *peak* current ability, you'd be pushing the battery hard at the higher power and it will have a shorter life than it could have.

I found one rated at 36v/500w/14A. The motor says 250w on it. If I use said controller would the motor a)still limit and run on 250w b)get damaged or would it actually have more power/faster pick up

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The motor can't limit anything. The controller does all of that.

If you put enough of a load on a 250w motor, and put an unlimited controller and battery on it, the motor will draw as much power as it needs to support that load, until the motor melts or catches on fire.

If your riding doesn't put more than 250w of load on the motor, it wont' matter that the controller can put out more than that, becuse the mtoor wont' ask for it, except on startups from a stop (or hill climbing, etc).

If your riding does put say, the full 500w of load on the motor, then the motor will use that much power if the controller and battery can provide it.

If the motor is a little 250w geared hubmotor, it may or may not be able to shed all the extra waste heat from the higher power level. If it's a DD hubmotor, it may be more able to shed the heat, but if it's really small it might still get really hot if at the doulbed power level for a while.

Small bursts of higher power are not usually a problem, except if it exceeds the torque that a geared hub's clutch or gears can handle. So as long as those are ok, you could get a lot better acceleration out of the motor.

If you go to http://ebikes.ca/simulator , you can play around with a 250w system like yours, then change to a higher current controller, and see what happens under the conditions you would be using it under. It's no guarantee that the same thing will happen with your exact components, but it's usually close.

(being reliable and safe of course)?

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If you want reliable, I'd leave it the way it's working. Exceeding designed / stated limits on a system removes the certainty you already have of it's reliablity. It might work fine...forever, or it might only work for a while, or it might only last a day. It depends on a lot of things, which you mostly wont' have information about, so you won't find out till something goes wrong whether it would or not.

Wow, thank you so much for that detailed response!

amberwolf said:

The controller should be limiting to it's current limit rating.

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OK. So that means I'm now actually on my 250w that this was made for, before that it must have been even less.

If the battery is 15A *continous* capability at 36v, then it will be able to safely handle 500w. But if that's a *peak* current ability, you'd be pushing the battery hard at the higher power and it will have a shorter life than it could have.

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It definitely says max. cont. discharge = 15a. The one I had before just says max. 10a, no info if that's continuous or peak. But I will only need the 15a as peak, given the fact that I'm only going for acceleration; this probably won't hold the motor at peak 15a for very long because I'm a city driver at rather low speed with a lot of stops. To get going I just need said short bursts of that power, we're talking literally a few seconds here.

If your riding doesn't put more than 250w of load on the motor, it wont' matter that the controller can put out more than that, becuse the mtoor wont' ask for it, except on startups from a stop (or hill climbing, etc).

If your riding does put say, the full 500w of load on the motor, then the motor will use that much power if the controller and battery can provide it.

If the motor is a little 250w geared hubmotor, it may or may not be able to shed all the extra waste heat from the higher power level. If it's a DD hubmotor, it may be more able to shed the heat, but if it's really small it might still get really hot if at the doulbed power level for a while.

Small bursts of higher power are not usually a problem, except if it exceeds the torque that a geared hub's clutch or gears can handle. So as long as those are ok, you could get a lot better acceleration out of the motor.

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Great info. Thats basically a go-ahead to me, because that should mean in normal driving where I pedal quite a lot it certainly won't need 500w to keep me assisted. Going down the road the 250w feels like sufficient power for me, so I assume it would actually run mostly unchanged in any situation other than getting the bike going from a stop.

My concerns were mainly the amps. The first thing I heard was that it's the amps that will melt your hardware and mess things up, higher voltage won't unless it's outrageous. So I assumed that going 500w with increased amps could cause problems as opposed to doing the same thing but with a higher voltage to achieve the 500w. BUT thats where I might be wrong, the same motor will have the same 500w, regardless if that's from 48v/11A or 36V and 14A. Correct me if I'm wrong but that also has to mean they will behave the same way in practice, it's just that one draws higher voltage as needed and the other one goes for the amps.

I'm willing to take the risk because small motors like this are affordable for me. I found one online for a little over a hundred bucks. I could get the 60 bucks controller, fry the motor, regret, buy a new one on aliexpress and still save hundreds over going for an 48v battery, 48v controller, 48v battery indicator throttle and two new charge bricks for at home and at work.

When I get home on my Pc I will try that simulator and see if increasing the power on same voltage presents any issue. Thanks again!

dudgggg said:

amberwolf said:

The controller should be limiting to it's current limit rating.

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OK. So that means I'm now actually on my 250w that this was made for, before that it must have been even less.

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That would mean that you have changed the controller to a higher rating than before? If the controller is the same, then the output rating of the system is the same, and so is the output capability.

If the battery was unable to supply enough current due to age, it would just sag in voltage a lot, which might reduce total power (and top speed) because watts is volts times amps, so when voltage drops so does watts. When voltage got low enough to drop below LVC, the BMS would shut off all power to protect the cells. If the battery was unable to supply enough current due to current limit in the BMS, the BMS would just shutdown all power for that reason. So it wouldn't limit power, it would just turn the whole bike off.

The first thing I heard was that it's the amps that will melt your hardware and mess things up, higher voltage won't unless it's outrageous.
<snip>
So I assumed that going 500w with increased amps could cause problems as opposed to doing the same thing but with a higher voltage to achieve the 500w.

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Depends on the situation and the specific hardware. Voltage isnt' a direct concern for the motor itself, but it does change the speed it will spin at, and higher voltage for the same resistance causes higher current flow, and higher current flow causes higher torque and heat.

I wouldn't run a 500w hub motor up front. 350 watts is my upfront limit so the bike at least performs like a front wheel heavy all wheel drive bike long as you pedal heavily.

boytitan said:

I wouldn't run a 500w hub motor up front. 350 watts is my upfront limit so the bike at least performs like a front wheel heavy all wheel drive bike long as you pedal heavily.

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I use a 1500W front hub motor and 48V/35A controller on both my e-bikes. They work fine, for me. I think how much power you can use in the front is a function of total weight, weight distribution, wheelbase, throttle discipline, and tuning. I don’t bother with controller tuning or with throttle discipline either, so it comes down to weight distribution (normal on one, front-heavy on the other), wheelbase (longish but normal in either case), and total weight (quite high in either case, 400 pounds plus).

Even if you scale for a lightweight person on a lightweight e-bike, 500W is no problem at all for a front hub motor.

Pedaling heavily, especially from a stop, shifts weight rearward and causes front skidding. Better to keep your feet turning at high RPM, to contribute plenty of power without significantly unweighting the front wheel.

Balmorhea said:

boytitan said:

I wouldn't run a 500w hub motor up front. 350 watts is my upfront limit so the bike at least performs like a front wheel heavy all wheel drive bike long as you pedal heavily.

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I use a 1500W front hub motor and 48V/35A controller on both my e-bikes. They work fine, for me. I think how much power you can use in the front is a function of total weight, weight distribution, wheelbase, throttle discipline, and tuning. I don’t bother with controller tuning or with throttle discipline either, so it comes down to weight distribution (normal on one, front-heavy on the other), wheelbase (longish but normal in either case), and total weight (quite high in either case, 400 pounds plus).

Even if you scale for a lightweight person on a lightweight e-bike, 500W is no problem at all for a front hub motor.

Pedaling heavily, especially from a stop, shifts weight rearward and causes front skidding. Better to keep your feet turning at high RPM, to contribute plenty of power without significantly unweighting the front wheel.

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And a rear wheel hub performs immensely better. First e bike motor was front wheel drive. Only came in handy in snow everywhere else it was worse.

boytitan said:

And a rear wheel hub performs immensely better. First e bike motor was front wheel drive. Only came in handy in snow everywhere else it was worse.

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Whatever you say. I've had three front and two rear drive e-bikes, and I can't say either one was categorically better at up to 1500W power levels. But front drive bikes hold up a whole lot better while stacking plenty of pedal power and plenty of motor power in the same bike.

These aren't motorcycles. Different principles apply.