Longer, faster, more!

[KirstinS]

Hi all

My commute will change from a very hilly 8 miles round trip to around 25 on flat although along coast often with strong headwinds

I have 250w bafang , 14a controller, 9ah Panasonic celled bottle battery (all from 8fun uk in kit form)

Very happy but the battery is very weak in that voltage sag /speed loss is apparent after just a mile or so. I did not appreciate that the ah of a pack hugly influences sag. Previously I thought it was just chemistry and c rating. Live and learn !

My requirements are - need to be able to comfortably do commute with room for minor detours. I would very much like to do a higher assisted speed (19 to 21 ideally). More torque would be icing on cake

For distance I will buy a bms 15 or 20ah LiCoMn. Job done

I do not think it possible to mod controller for more speed, is it ? I know I can shunt mod for torque

So I guess it's new controller time ?

I understand the motor can easily handle speed

Does this sound about right ?

To be honest if it is going to cost much more than a new battery I won't bother

Thoughts appreciated !

Thanks
Kirstin

 

[Old_Dave]

So I guess it's new controller time ?

In the main... more speed comes from more volts, whereas more torque comes from allowing the motor to get more amps.

So step one in your quest is the battery, step two is modding the controller

 

[Jeremy]

First some fundamentals relating to efficiency, which may help you choose.

You can lose a lot of power within the battery when you start pulling high current from it, unless it has a low internal resistance. The greater the battery internal resistance (the resistance that appears to be in series with the battery internally) the greater the power loss and the greater the voltage sag.

For example, the battery pack you have at the moment has some fairly high internal resistance cells. If they are the Panasonic ones I'm thinking of they are rated at 0.085ohms each by Panasonic, and there are three in parallel in each cell group for a nominal 9Ah, so a 36V nominal pack (10S) will have a total internal resistance when new of around 0.283 ohms. At 14A this battery would sag by around 3.96V and waste around 55W as heat within the battery, which effectively reduces the useful range of the bike.

If you used a 10Ah battery pack made from cheap LiCoO2 cells (RC type packs) then the internal resistance would be around 0.01 to 0.02 ohms for a 10S, 2P arrangement. At the same 14A the voltage sag would only be around 0.14V to 0.28V, and the wasted power would drop down to between 0.2 and 0.4W, a very big difference in overall efficiency.

Speed would increase roughly in proportion to the reduction in voltage sag, so assuming the battery is really around 37V nominal (3.7V per cell group , 10 cell groups in series) then you would get a percentage speed increase of around 10 to 12%, assuming that the controller current limit isn't holding the speed back due to the power needed.

The general rule, which I suspect you already know, is that current determines motor torque, and hence hill climbing ability, and voltage determines motor speed. There is a fair bit of interaction at the upper end, because if the current limit is too low then the motor may not be able to deliver the power needed to reach the maximum speed determined by the battery voltage.

There is a very good simulator on the Grin Cyclery site: ebikes.ca Hub Motor and Ebike Simulator and if you know which motor you have you can play around with the battery voltage and controller current limit, and adjust the throttle and steepness of any hills, and see the effect these changes have on performance.

If it looks a bit daunting at first, I could easily work through some options for you and see what they give.

 

[amigafan2003]

A really easy way to up the speed AND the torque on a 36v ebike is to substitute the battery for two 6s lipo packs and wire them in series (44.4v). 95% of all 36v controllers will accept a 12s lipo pack.

Just be aware that you'll lose the LVC and do some research into looking after and charging lipo packs - they don't suit everyone.

 

[Deleted member 4366]

Get one of those 20aH shrink tube batteries from BMSBattery. Both Saneagle and myself have them on our bikes and pull up to 35 amps from them, and even in these cold temperatures and high discharge rates, they'll give at least 20aH. Their main advantage however, is their relatively light weight. If you look at Saneagle's Kraken build, you'll see a very cheap and simple way to install the battery and controller. You want the battery as low as possible and as close to the seat-post as you can get it for optimum handling.

If you decide to get one, you can then add a bit of solder to the shunt in your controller to increase the current to about 19 or 20 amps, which will make the motor much more perky and able to hold its speed in a headwind. I've never looked inside a 8Fun controller. It might be potted, in which case you're knackered. If it is potted, you can get a KU65 from BMSbattery that will work with your panel. They're fairly cheap- especially if you get it at the same time as the battery.

Another option is to change the battery to 12S lipos, which will increase your speed and power, but you have to do a bit of reading first on how to use them because they're not quite as straight-forward as other batteries. 12S lipos and KU65 with soldered shunt for 17 amps will make your bike fly.

 

[Old_Dave]

At hobbyking there is a great price on a 4 cell battery, 3 in series and another 3 added in parallel = 6 required

http://www.hobbyking.com/hobbyking/store/__26801__Turnigy_5000mAh_4S1P_14_8v_20C_hardcase_pack_UK_Warehouse_.html

Only downside is with the faff of charging

6 packs (10A) = £95.52


Sent from my iPhone using Tapatalk

 

[RobF]

Get one of those 20aH shrink tube batteries from BMSBattery. Both Saneagle and myself have them on our bikes and pull up to 35 amps from them, and even in these cold temperatures and high discharge rates, they'll give at least 20aH. Their main advantage however, is their relatively light weight. If you look at Saneagle's Kraken build, you'll see a very cheap and simple way to install the battery and controller. You want the battery as low as possible and as close to the seat-post as you can get it for optimum handling.

If you decide to get one, you can then add a bit of solder to the shunt in your controller to increase the current to about 19 or 20 amps, which will make the motor much more perky and able to hold its speed in a headwind. I've never looked inside a 8Fun controller. It might be potted, in which case you're knackered. If it is potted, you can get a KU65 from BMSbattery that will work with your panel. They're fairly cheap- especially if you get it at the same time as the battery.

Another option is to change the battery to 12S lipos, which will increase your speed and power, but you have to do a bit of reading first on how to use them because they're not quite as straight-forward as other batteries. 12S lipos and KU65 with soldered shunt for 17 amps will make your bike fly.

I love these threads, they keep me bemused for days as I fail to understand one technical post after another.

(With a nod to d8veh's post number 45 here: http://www.pedelecs.co.uk/forum/electric-bicycles/13397-3k-spend-what-would-you-recommend-5.html )

 

[barrycoll]

your so right Robf.....I live in hope that just reading this stuff, one day I will awake to a bright new dawn where amps/volts and watts will light that bulb above my head...

or alternatively, accept the conclusions arrived at by the latest A to B mag, e.g "that blokes on websites that revel in watts, volts etc, should spend more time with their loved ones"

Xmas next week........

cheers barrycoll

 

[amigafan2003]

your so right Robf.....I live in hope that just reading this stuff, one day I will awake to a bright new dawn where amps/volts and watts will light that bulb above my head...

or alternatively, accept the conclusions arrived at by the latest A to B mag, e.g "that blokes on websites that revel in watts, volts etc, should spend more time with their loved ones"

Xmas next week........

cheers barrycoll

My wifes a trained electrical engineer (although she's actually a teacher as a profession) so there is no hope for me!

 

[KirstinS]

Really useful stuff - thanks very much

I thought it was possible to increase speed without upping voltage (appreciate you can a bit with a better battery - ta Jeremy ) but seems you cannot ( I had the mark 1 torq bikes in mind that I read about - believe thu did 21 on 36v -somehow ?)

Lipo is out for me - too much effort for my lazy self and wouldn't want my kids on bike. Maybe silly but there we are

Just the battery and maybe s small shunt

 

[rpurchon]

i have the same 8 fun kit.or did have, just got the motor left.
sold the battery and controler on ebay.
.now running on 48v 12s lipos and with 700c wheel, bike will do 21 mph on the flat,
16 up hill with pedal asist.
20mph with original controler on flat @48v
5000mah will laast apx 12 miles.+ battery only weighs 1 kg total bike weight 17kg.
dont bother soldering the shunt .original controler didnt like it.
i got a e crazyman programmable controler,i found altering amps didnt really alter the on the flat speed.
and wasnt much better up hill either.the motor just hasnt got the balls.it just drained the battery quicker.
trying to get a bpm or cst motor next,but they havent got the slow rpm one in stock.
richard

 

[Deleted member 4366]

Now that I've got my Giant NRS with CST motor properly sorted, I think it's the ideal bike for me, so I'm getting rid of some of my other motors and bikes. I've got a 350w rear CST and two front 350w BPMs. The CST does about 18mph even though it was sold as 270rpm. I think that the BPMs are code 15 201 rpm and code 13. maybe both code 15. All built into rims which have been centralised. Give me a pm if you want one.

 

[KirstinS]

Hi again

About to order my battery and notice that I could get a 323rpm 350w front motor at the same time

I think this would give me around 23mph real world top speed in a 700c rim (based on a chart i found at ES)

Any glaring errors in my thinking please ?!

Many thanks
Kirstin

 

[Deleted member 4366]

Hi again

About to order my battery and notice that I could get a 323rpm 350w front motor at the same time

I think this would give me around 23mph real world top speed in a 700c rim (based on a chart i found at ES)

Any glaring errors in my thinking please ?!

Many thanks
Kirstin

That calculation doesn't seem right. Here's how I calculate, which has allways been very accurate when you compare the prediction to actual speed:
A 201 rpm motor has a no-load speed of 15mph at nominal voltage (say 36v) in a 26" wheel
so a 323 rpm motor does 15 x 323/201 mph = 24.1 mph
Actual voltage is about 40v average, so actual no load speed is 24.1 x 40/36 = 26.8mph
For a 29" wheel, you have to multiply by 29/26 so speed is now 26.8 x 29/26 = 29.9 mph
That's no-load speed. In the real world you lose between 2 and 3 mph - say 2.5mph
Therefore your actual speed will be about 27.4mph, which will be a bit fast for the 350watt motor unless you're relatively light. I have a 500w motor and tit feels about right at 22mph max, though I weigh 100kg.

 

[KirstinS]

Ahh thank you , not sure what the actual calculations were behind the chart.

Hmm, that is a bit quick then although I'm only 73kg

I'll have a think, might get it anyway as its only 119 USD and try an build a light fast bike that is hopeless at hills !

 

[Deleted member 4366]

I tried twin 328rpm Q100 motors in my Giant FS, and it did about 28 mph, but the motors spent a lot of time at/near maximum current, so the controllers got very hot and the battery went down rather fast. I have twin 201 rpm ones in my Rocky Mountain and they're more efficient than a single BPM. At 73kg, things won't be so bad for you, but you need to be very gentle on the throttle. You might want to think about a cruise control so that you're not constantly wiggling it backwards and forwards. I guess your normal speed will be 15 to 20mph, so you'll only need about half throttle.

 

[hech]

I tried twin 328rpm Q100 motors in my Giant FS, and it did about 28 mph, but the motors spent a lot of time at/near maximum current, so the controllers got very hot and the battery went down rather fast. I have twin 201 rpm ones in my Rocky Mountain and they're more efficient than a single BPM. At 73kg, things won't be so bad for you, but you need to be very gentle on the throttle. You might want to think about a cruise control so that you're not constantly wiggling it backwards and forwards. I guess your normal speed will be 15 to 20mph, so you'll only need about half throttle.

So how do you explain that d8veh eh?
From your twin motor build thread you concluded this was an efficient set-up (I recall I think?).
But if the controllers are getting hot you must be getting 15amps plus from each which is 600watts per from the 36v20ah battery. At 1200watts you are theoretically breaking several speed limits not to mention the resultant sonic boom.
regards,
hech

 

[Deleted member 4366]

Sorry, I must have missed this one Hech.
My previouus conclusion was based on 201 rpm motors, The same conclusion probably applies to the 328 rpm ones, but it's now overwhelmed by an additional conclusion: That high speed motors are very inefficient if you run them at low/normal speeds. That's why I abandoned them and changed to a single CST instead, but the bike with twin high speed motors was fun while it lasted, but much too fast for normal riding.