[Kalle]

Hello,
Me and a friend are planing to convert a old kawasaki KDX Dirtbike to a EV.
This will be ouer first EV projeckt so we are biginers in this feald.
The only thing we have at this point is the bike.
We will try to ceep it as cheap as we can (somwher in the 2000-3000 dollar range?)
We have speculated a bit on what we would need and what parts to use but since we are N00bs we might be way of.

For the motor we have been looking at a 5 or 10 kw 48v BLDC from http://www.goldenmotor.com that would be 765 dollars without controler.
We cear not to much about top-speed, but we need rapid acceleration (would be fun to ceep up with ouer petrol friends on the dirt track)

For the batteries we have our eyes on ebaying A123 AMP20 cells ( http://info.a123systems.com/Portals/133376/docs/data sheets/a123 datasheet_amp20m1hd-a.pdf )
they are 20 Ah 3,2V cells that can be found @ around 25 dollar/cell.
We were thinking of using 60 of them in packs of 15 (to get 48v) that would give us 2880 Wh (or 60 Ah @ 48V)

Any thoughts? Can this be done or are we way of?



PS, Apologies for any broken english, it is not my first language

 

[Yabert]

To try to help you it will be useful to know what kind of performance you expect. Your bike is a 125? 250? 400?
Here an example: http://www.diyelectriccar.com/forums/showthread.php?t=48239

 

[Kalle]

Thanks for the link, awsome build!

The goal with my projeckt is to make a fun "play bike" to drive around in the forest of eastern france. i will not be using it for comuting to work due to leagal dificultese. Sinc i live in basel (a citty on the borders of switzerland, france and germany) the bike is ceept in france and i am working in switzerland so comuting meens crosing borders, and registration of hommade suff is tricky.

the idea is bacicly to build a decent play bike for 40-60 minuts of fun, and in the process lern as much know how possible for more serius future builds (thinking of making somthing from scratch in the long term future)

 

[Kalle]

the petrol one i have is a small 125 cc with a dry weight of 104 kg

 

[Kalle]

here are the first scetches of my idea. i was planing for 48v thinking it would give the same pover aslong as the kw of the engine is the same. is this true or do i have a misconseption here? is their other reasons to go for higer voltage than to lower the amps needed to the motor?

 

[Yabert]

Ok! If you need roughly same acceleration than with your ICE bike (11 Kw and 14Nm peak) with direct drive electric system, the peak torque of your electric motor will be way more important than his power (Hp or Kw).

The 14 Nm of torque from ICE can seem small compare to the torque capacity of an electric motor, but this torque is grow up by the transmission ratio.
In direct drive application, an electric motor will need to have a torque 5 to 10 times higher to permit the same acceleration of a ICE bike. You can also play with sprocket ratio to change the torque at wheel.

Here is a link showing many elecrtric bike (motorcycle at half page): http://www.chargedevs.com/Buy-an-EV

Au fait, je dois dire que c'est une belle modélisation 3D...

 

[Kalle]

haha thanks, googel sketchup for the win


i got the performance curves for the www.goldenmotor.com 72 volt and 48v HPM-10KW motors,they claim that the 72V one to give around 30 N/m but i must say they look rather confusing.

if i need 5 - 10 times the tourq 30 will be no were near that what i need.
what motor specs do you have on your bike? it seem to have a decent acceleration.

48V http://www.goldenmotor.com/eCar/HPM10KW (48V) Curve.pdf

72V http://www.goldenmotor.com/eCar/HPM10KW (72V) Curve.pdf

i do not know if i read theese corectly but
both of them seems to whant more Volts than specified to produce full power.
and the math do not ad up. at 50 volts and 300 amps they say the 48v motor will give you 20000w, that is simply not true, if it has a 100% efectivity you will only get 15000.
on the 72v one you will get 24000w at 300 amp 80v when they say 20000.


also if they need 80v to performe i need to re-think my battery layout.

 

[Yabert]

Sorry, my explanation wasn't clear. Torque at wheel is what you need to calculate.

Example:
My gas DRZ need three speed to accelerate from 0 to 60 mph because his max torque is 26 lbs-ft.
26 lb-ft x 14:1 (in second gear) = 364 lbs-ft at wheel!

My electric DRZ only need single speed to do the same because his max torque was around 90-100 lbs-ft at 700 Amps.
90 lbs-ft x 4.1:1 (electric ratio) = 369 lbs-ft at wheel!

And don't forget, the electric torque is constant from 0 rpm and the gas torque is only peak (around 6000 rpm for DRZ).

Hope that help.

 

[frodus]

2-300ftlbs at the wheel from a stop is a good number. I had had a series wound motor at 400A and ~80ftlbs and 5:1 ratio, it calcs out to about 400ftlbs. This was also a MUCH larger motor than the goldenmotor, and a much heavier bike.

If you aim for say 2-300ftlbs, and have a 6:1, that's 30-50ftlbs required by the motor (peak).

30Nm is ~22ftlbs but note that they do not list locked rotor torque, so you may get much more torque at 0RPM than you think. I'd email them and ask what the torque is at 0RPM..... they only list numbers between 4000 and 5000RPM

 

[Kalle]

I emaild goldenmotor an asked for the torqe at 0 rpm,
It is 85Nm ~ 62 ft lb
The motors top rpm is acording to them only 3900 rpm (seems low to me?)

If i where to use 5:1 ratio i would gett:
425 Nm ~ 310 ft lb at the hub of the wheel.
At max rpm i will den have a top speed of 70km/h ~ 44mph.


Lets see, if i have 425 Nm at the hub and my wheel radius is 0,25 m (1/4)
425 • 4 = 1700N = 173 Kg
It will be like somone puch you along with a force of 173 kg.
If i now know the weight of the bike + driver i can calculate the estimated initial acceleration

Do anyone know a good motor with about the same performans but a higer top rpm?
If not i will probably settel for this one.

 

[Kalle]

I have spent the last couple of weeks looking at a bunch of diferent batteries, i think i will end up going for 15 thundersky 90ah batteries.
It will give me 48v 90ah ( they are 3C continus and 10C peak )

I allso weight all parts i will remove from the bike:

Stuff off
Motor 21,4 kg
Luft burk 2,2
Avgas rör 2,4
Bensin tank 2,7
Kylare 0,8
Batteri 1,4
Små pryttlar 0,1
Avgas krök 4,1
Olje burk 2,5

Tot - 37,6 kg


Stuff on:
15st LiFePO4 batterys 34,5 kg
El Motor 17 kg
Controler 2,9 kg
BMS Ca 1 kg
Cables and suport Ca 5 kg

Tot +60,4


Total EV weight gain +22,8 kg
Total overall weight 127,8 kg

 

[Kalle]

90 Ah 48v will give me 4320 Wh (but i guess i can not use all of it since the cells are not alowed to dichcharge all the way to 0? )
I am hoping i can get a range of around 50 km with theese batteries.

If they will fitt...

 

[Kalle]

They do fit
I will probably move 4 batteries from the back to the front (wher the cooler used to be) to get the center of gravity closer to wher the original bike had it.

 

[Yabert]

Generally speaking, 16S (51.2v) is considering the equivalent of four 12v battery (48v) because at full charge, 16 x 3.6v = 4 x 14.4v.
More voltage = more power!

 

[frodus]

plus, finding a charger to do 16s will be much easier than finding one for 15s.

 

[Kalle]

Aah! 16 it will be then, thanks.

I have tried to read up on bms systems, as far as i understand they do only monitor the bateries and let u no when somthing has gon wrong, they do not activly balance the cells.
Is this understanding correct?

 

[frodus]

Depends on what you get. Some only balance and don't realld do any feedback. Some monitor for faults (overvoltage, undervoltage, overcurrent, etc). Some balance and monitor.

 

[Kalle]

i found this one that i think works good for me,
http://www.ev-power.com.au/-Thundersky-Battery-Balancing-System-.html
BCU-NEV-16C - 16 Cell 48V
they seemd to be made for the tundersky batteries and looks fairly simpel, and they come with a "fuel gauge" that is nice.
They do say they balance the cells, im not sure how that work but i guess they discharge all the cells to the one with the lowest voltage?
it is allso mentiond that it does "talk" to the charger (some moddels anyway) during charging, not shure if neded but i guess to protect against over charging.

 

[frodus]

That looks like a decent system.


Those are top balancing BMS boards. The charger charges the pack up to whatever voltage, if one cell gets over the threshold (whatever it's set at, lets say 3.5V) it starts shunting current trhough a resistor, around the cell. This allows the other cells to catch up. If too many are shunting, the charger shuts down. During discharge, if a cell goes too low, you can either have a warning light, or shut down a contactor or controller.

You ALWAYS want a BMS/monitor to be able to shut down in case of undervoltage (while discharging), or overvoltage (while charging) of an individual cell. That's what protects your pack. I don't care about shunting either way, that's up for debate.... but I absolutely recommend a way to shut down the charger or the controller in either case.

AFAIK, there's no readily available bottom balancing BMS's.

 

[cpct]

We built a kart with a GoldenMotor 5kW 48v bldc and their matching controller. The controller is really bad. Documentation is almost nonexisting and the thing went into fault mode randomly. We've never tested the motor with another controller but I'd recommend against it after what we've been through.
In the end we used an ME 3001