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    Choosing a battery for the most motor torque

    I have a very limited and basic understanding of electricity so please bear with me.

    I live in Canada where we have a 500w & 32km/hr onroad limit. It is obviously a challenge to keep a bike legal when the BBSxx motors have so much more available. What I notice with a 52v battery and BBS02 or HD is that on flats I hit the speed limit cutoff after barely shifting into second gear (46 front and 34 rear) In other words, more ability to go fast is not what I need. But when I hit a big hill, with 500w at 52v the bike slows down to a crawl because it doesn't have the torque it needs (I do realize I can improve this by using a smaller chainring, but I like to pedal along with the bike at higher speeds so I don't like the 42 or 36 lekkies on this bike in this situation).

    As I understand it, volts = speed and amps = torque

    If that is right, then the bike doesn't have the torque it needs because the only way to get 500w at 52v is to limit the amps to approx 10. So if I want to be legal with a 52v battery I end up with a bike that is faster than it needs to be on the flats and slower than it should be on the hills. Would I not be better using say a 24v battery and cranking the amps higher?

    Is this good reasoning or am I missing something?

    To sum up: for my mountainous area, I am much more interested in a bike that drives like a diesel train than a sport bike. How can I best achieve this in an onroad situation where I want to stay in the legal limit?
    Last edited by AndyZ; 1 week ago.

    #2
    I'll just take a guess here, a guess as you failed to tell us what SIZE battery (as in total AH capacity) you have. You may just be experiencing voltage sag, common with drawing lots of amps out of a smaller battery. Less common , and less of a factor, when drawing lots of amps out of a larger battery. You may have a choice to make:pack around a larger battery then you think you need, for range anyway, just to have less voltage sag. When I am running on my 6 AH MINI, I don't expect the same performance when romping on the throttle as when riding on my 17 AH battery, sad but true.

    Comment


    • AndyZ
      AndyZ commented
      Editing a comment
      Thanks for the reply CPG.

      I have a 17ah Samsung 35e battery. It is new and in great shape. The BBSHD performs very well when I program it to 30A, but at 30A I am averaging 1500 watts which is three times the legal limit. If I want to be legal (500 watts in Canada) then I have to cut the amperage down to 10 if I am using a 52v battery. At this amperage, the bike still goes faster than it needs to on the flats, but lacks the torque to get me up the big hills at any kind of speed. So what I am trying to figure out is if I can essentially half the battery voltage (from 52 to say 24 using another battery) and double the amperage (from 10 to a little over 20) which would still keep it legal but would perhaps give me more torque for hills.

      I have no idea if that's how it rolls though, or if I am speaking electrical nonsense.
      Last edited by AndyZ; 1 week ago.

    #3
    Hello Andy - I asked a similar question (torque/controllers/heat) over in Endless Sphere a few minutes ago, be interesting to see how this pans out too

    Comment


      #4
      The LVC on a BBSHD can be programmed down to 38v I believe. So 24v would probably not work very well if at all. 36v battery full charge is around 40v and you would be really cutting into your range compared to your 52v pack. It's all a give and take, if you are stuck with 500 watts your kind of limited.

      Amps will increase your torque.

      Watts Law says to figure amps:
      watts ÷ voltage=Amps.

      So if your fixed at 500 watts and a 36v battery, 500 watts ÷ 36 volts = 13.8 amps

      Fixed 500 watts and a 48v battery,
      500 watts ÷ 48 volts = 10.4 amps

      And you already have a 52v you said around 10amps in real world riding.

      Other possibilities is run some gears on the back, or get a Egg Rider with its 2 modes, you can have a legal mode and a "hill climbing" mode. Good luck.

      Comment


        #5
        Originally posted by AndyZ View Post
        ....

        As I understand it, volts = speed and amps = torque

        ...
        Motor voltage is proportional to the motor speed and motor current is proportional to motor torque

        The rub here (this got me in trouble for a while) is that when the controllers and displays refer to current they are referring to the battery current and not the motor current - my experience with motor control in industry when folks spoke of current it always referred to the motor current but in this world it's nearly always the battery current and that threw me.


        I ride where it's very steep, sometimes as much as 100% grade but I also ride on the flats and want to pedal at 20+mph so the solution for me was a very wide range cassette, 11-46t. With that I have the torque in the low gear to climb anything I got the juevos to climb and I can pedal at up to about 25mph. I've run 11-42t and that was the minimum I found acceptable and the 11-46t has some margin.

        I don't think there's a better way to skin the cat in order to be able to have bot hill climbing and level cruising speed with a BBS mid-drive than a wide-ratio cassette and the appropriate chain ring. If you try to climb hills in too high a gear ratio so the pedal cadence is way low, you really risk burning up motor and/or controller and to do that. If you were to try and respect the 500W limit by creative programming that allowed for large battery currents as long as the motor speed were low enough to not exceed the 500W (and likewise limited the battery current to lower levels as the motor speed/voltage increased) you'd be operating outside the "safe operating area" at low speeds requiring lots of torque.


        How many gears do you presently have?
        Last edited by AZguy; 1 week ago.

        Comment


          #6
          Thanks for the responses everyone. To keep this short I will generally reply to everyone in one post. I see that I should just stick with what I have and focus on gearing (or do nothing).

          I currently have two bbshd bikes. One offroad FS downhill bike with an egg for when it is used on the road. The other is a townie for onroad only which is the bike in question here.

          ​​​​​​I have the stock 11-34 Townie gearing with a 46t lekkie. This setup works well for me on the flats up to our legal limit and it gets me up any of our hills without a problem other than it just isn't as powerful as when I run it without the 500w restriction, which is obvious. I was trying to see if there were a way to get more grunt on the hills by sacrificing top speed (from a volts/amps) perspective. I see that is not really possible (thanks for educating me on that guys). Due to the way he Townie is used, I probably won't go to the trouble of getting a few larger gears on the back, but it is good to know that that is my only practical option for a future bike which may need it.

          Comment


            #7
            Here's another question: how are bike motors rated for legality purposes? Which speaks to your issue AZguy of how amperage was being measured.

            What I mean is, some cars HP ratings are at the motor and some at the wheel. When I set my controller to approx 10A and see my power cut off at aprox 500w on my screen, is that a fair way to rate my motor or am I measuring it pre-losses so to speak when other ebike manufacturers are measuring it after-losses. I hope my explanation makes sense.

            I haven't ever ridden a factory e-bike, but some of them are over $12k. I can't imagine a $12k 350w ebike having less power than my 500w bbshd.

            I actually want this Townie to be legal, and I am not trying to skirt the law, but I am trying to understand what a fair way to measure watts is.

            Comment


              #8
              Every municipality has it's own way of defining and regulating and it's all over the road map. I know the AZ laws very well and that's all I can speak to. A couple of highlights are:

              The 750W and 20mph speed limits in order to be categorized as a class 1 or 2 electric bike can be limited by electronics and firmware regardless of the motor capability.

              Beginning in 2019 AZ will require all manufactured electric bikes to have the motor labeled with power and speed. This doesn't appear to apply to RYO bikes - the statues don't even touch that part from adding the "manufactured" as a prefix to electric bike.


              On the first point it wouldn't make economic, weight, etc. sense to put a larger motor than what the electronics limit it too so that seems sensible

              On the second it doesn't really apply to RYO bikes and I have very low likelihood of encountering LEO's of any sort and even then few are likely to even care about the bike. I've been considering just laser printing a small unobtrusive decal "750W / 20mph" and putting it somewhere pretty obvious on the motor though. Why not?

              In AZ if you are class 1 or 2 (2 has a throttle, 1 doesn't) then you are an "electric bike" by the statutes and in AZ electric bike = pedal bike 100% and the funny thing is a regular bike has pedestrian access privileges so sidewalks and every other place a pedestrian can go is open. Local municipalities or managers can add their own layers of further restrictions but here in AZ, at least today, there's not a whole lot of that. Admittedly, AZ is near the one side of the spectrum of permissibility... Other places are just the opposite with 250W limits and no pedestrian parity - or they may just be illegal altogether!

              Comment


              • AndyZ
                AndyZ commented
                Editing a comment
                Our law basically says it has to have pedals and be 500w or less and capable of 32km/h or less to be classed as an electric assist bike. Throttle is OK. As such it is privileged to the regular bike rules (wear a helmet and stay off the sidewalks). If it exceeds that power or speed it needs to be insured, but a bike cannot be insured here, so it is just illigal period.

                It would be amazing if they would limit it by speed only and not power (like any other vehicle), because 500w in a flat area isn't 500w in a mountainous area, but 32km/hr is always the same speed.
                Last edited by AndyZ; 1 week ago.

              • AZguy
                AZguy commented
                Editing a comment
                If you are concerned about encounters with LEO why not just put a decal on it yourself? You can get laser printable slip-on decals (soak in water and slip-on). I would do it in a way that looks subtle but it's something.

              #9
              Originally posted by AndyZ View Post
              Here's another question: how are bike motors rated for legality purposes? Which speaks to your issue AZguy of how amperage was being measured.

              What I mean is, some cars HP ratings are at the motor and some at the wheel. When I set my controller to approx 10A and see my power cut off at aprox 500w on my screen, is that a fair way to rate my motor or am I measuring it pre-losses so to speak when other ebike manufacturers are measuring it after-losses. I hope my explanation makes sense.

              I haven't ever ridden a factory e-bike, but some of them are over $12k. I can't imagine a $12k 350w ebike having less power than my 500w bbshd.

              I actually want this Townie to be legal, and I am not trying to skirt the law, but I am trying to understand what a fair way to measure watts is.
              You can measure your BBSHD hp at the crank or wheel. The only place you are going to measure your current is between your batt and motor. Did I miss something. Mabe I didn't understand the question.

              Comment


              • AndyZ
                AndyZ commented
                Editing a comment
                Sorry I didn't mean to imply that. I was just using cars as an example where there are different ways to measure the power output. I was wondering if there are different ways ebike manufacturers rate their motors. For example, perhaps one measures 500w of input moving between the battery and motor, and another measures the equivalent of 500w of output at the crank or at the wheel using a dyno.

                Not to get stuck on technical details because I really have no clue what I am talking about, but just observing that people seem really impressed with some of the newer 250 & 350w factory ebikes, and that seems surprising if they are measuring power the same way we are because my bbshd at 350w isn't that impressive.
                Last edited by AndyZ; 1 week ago.

              • AZguy
                AZguy commented
                Editing a comment
                Those 250-350w mid-drives that are eMTB's almost certainly have much smaller chain rings than you have - your gearing isn't setup for that - and I'd be that even the commuter ebikes with the small mid-drives are still geared pretty low

              • AndyZ
                AndyZ commented
                Editing a comment
                Yeah maybe that's it. I haven't actually paid any attention to their gearing but will take more notice from now on.

              #10
              For the sake of being super easy the manufacturer's just measure battery current and voltage and limit based on what the battery supplies. Frankly they just look at current and don't account for voltage change.

              tIf they wanted to "goose the system" they could certainly measure motor voltage and current and then limit based on power supplied to the motor but that would take more non-trivial electronics. They could even put a torque sensor on the output and limit based on applied power but at what cost and for how much benefit?

              It doesn't really matter. It's not like they are going to put them on a dyno. Lawyers and legislatures aren't engineers and the laws are pretty squishy. An engineer would have specified a tolerance LOL...

              Comment


                #11
                Originally posted by AZguy View Post
                An engineer would have specified a tolerance LOL...
                ...lawyers don't have tolerance (ha...ha...ha....)

                Comment


                  #12
                  Check out this power chart I found for some of the factory motors. All except the Brose are way above their rated power of 250-350w and would seem to be illigal for the European markets in which they are predominantly sold, yet they aren't illigal for some reason due to the way they are rated.

                  That and these guys are running the motors at 36v. Maybe they are doing that so they can have higher amperage higher torque motors and still come in under the legal limit (which they clearly aren't in practice, but maybe in theory).

                  ​​​​​​ Click image for larger version  Name:	POWERS.jpg Views:	1 Size:	82.8 KB ID:	81238
                  Last edited by AndyZ; 5 days ago.

                  Comment


                    #13
                    In AZ it doesn't matter if you have a 10kw motor as long as the electrics limit it to 750W / 20mph

                    Different euro markets have different max power too - many are 500W and from engineer's standpoint they all make that given ~15% tolerance =]

                    For different markets with lower power they can just limit them electronically

                    As has been brought up it also matters how they measure it... If it's battery power then efficiency losses will eat up that 15% anyway

                    Comment


                      #14
                      That chart is interesting. I have no clue unless it's rated the 250-300 continual and 500 max bursts. Have no idea what they are doing. Very interesting, keep posting if you find anything else. I'd like to know.
                      I'm in WA and laws are like AZguy said, 750, max speed 20, ride sidewalks ok. My BBSHD is stamped 750w on the bottom, that's convenient. As long as your not a real bone head and act stupid no one cares. I have a egg rider also so in the future if they start clamping down I have the 2 modes.
                      I love B.C., have done lots of fishing and camping up there, looks like i won't be taking my e-bike with me anytime soon. Caught a 30# King out of Tofino, that's my biggest Salmon so far.

                      Comment


                        #15
                        Wow I guess Tofino has more than big waves (big fish too!). The only thing I ever got good at catching were crabs and rock cod. I'm glad there are people like you out there with some skill and patience because I sure love salmon. A friend's friend caught a bunch in the Capilano this year and had them cold smoked. We ate our last piece about two weeks ago. It was (soo) good while it lasted!

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