CAPACITY DILEMMA :S

[carl2.0]

I'm finally in the build stage for my electric bike. I'm working with a Mars ME0708 motor and an AXE 4834 controller, a Suzuki gxsr 400 donor frame n due to money Im forced to work with lead acids...
My issue is :
I want a mileage of 25 -30 at a nominal speed around 37 mph (60 kmph) what Ah is best to do this ?

Based on the "product trolling Ive been doing alot of bikes use around 30-40 Ah to reach this mileage. and my equations I was using to calculate it seem to be too ideal .They suggest that at at a nominal speed of 60kmph the current draw from the motor would be around 28A and using a 32Ah battery pack is would ideally get over 40 miles so clearly my equation isn't accounting for ALOT of variables. Based on experiences or evaluation can anyone suggest to me the proper Ah batteries to use ?


For anyone who is interested here are the calculations done

At nominal speed or 60 kmph -----> 16.67 m/s
Using the equation:
ρa -air density
Af –frontal area
Cd –frictional coeff
V -Velocity in m/s
P = 0.5 x ρa x Af x Cd x vmax3
= 0.5 x 1.2 x 0.5 x 16.67 =1389.72 W

Giving a current draw of 1389.72/48V = 28.9 A
Using equation:
CAh – Amp hour rating
I – current
T – hrs
CAh = IxT
Using a 32Ah battery pack
32/28.9 = T = 1.1 hrs
= 1.1 x 60 mins = 66.4mins
At 1a run time of 1.1 hrs
Distance = v x T
= 60kmph x 1.1 = 66.0 km = 41 miles

[Malcolm]

The best I ever managed with my 60V 27Ah lead acid pack was around 70 Wh/mile, giving a maximum range of 15 miles. My bike weighed around 140 kg then. Based on that and your lower voltage of 48V, I reckon you'll need 60 Ah absolute minimum, and preferably 80+ Ah to avoid killing your pack quickly if you intend to do 25 miles regularly.

[carl2.0]

wow damn... thats alot of weight, starting to reallllly wish I had LiFePo money now

[carl2.0]

wow damn... thats alot of weight, starting to reallllly wish I had LiFePo money now

[carl2.0]

What motor did you run at 60V ??

[neptronix]

Save your money and get LiPo; you won't be disappointed in the results.

[Malcolm]

What motor did you run at 60V ??

It's an original Etek motor, with Alltrax 7245 controller. Sorry to disappoint you about the range, but better now than when you've shelled out on batteries. My own pack only lasted around 1000 miles before they started suffering from the repeated deep cycles, and they were good quality Enersys SLA.

[el_walto]

buying lipo will save you money in the long run as they will last more cycles.

This might get you where you want with built in BMS for a low price: http://www.pingbattery.com/servlet/the- ... PO4/Detail

[carl2.0]

Well the bike is actually being designed for my final year mechanical Engineering project... You would think that LifePo batteries would not be an issues with the university but after the frame motor and controller my budget got cut down. PLUS they will argue they wont have much use for the batteries afterward (most students who do electric drives have never gone beyond lead acids :S)....

Im startin to really hate my SLA limitation as I add up the pound tho...a 32 AH pack at 48v is a crazy 94 lbs !!!

[maydaverave]

You could try these.

viewtopic.php?f=14&t=23029

[neptronix]

Use a smaller lipo pack. Seriously, 5ah of 37v for me is under 2lbs & costs $80 + shipping.

It's real cost competitive over the long run. Go for less capacity at first and add more later.

[Samba]

You calculated the motive power needed at the wheel, you need more electric power than that to cover losses in the drivetrain and electronics.
I don't see the number for drag coefficient in your calculation, so thats for a 1.0 coefficient. A pretty good guess but it has a big impact if its much different.
Theres a tiny bit more power for rolling resistance - mainly tires resistance to the constant deforming. Pretty small.

The battery capacity needs fudge to be a workable design for that range. You never drain the batteries 100% - so you add some excess, often you don't charge them to 100% either (the last few % are slowww) so add a bit more, they will loose some with time and charge cycles - so add more. If, for instance, you were a carmaker that needs to warranty a vehicle's range for 2 years/4000 miles you'd need to oversize the battery like that.

You are draining them at 1.1C (that will get lower once you add the fudge capacities) No problem for lithium or nickle batteries, but Lead batteries spec their AH at a 1/20C discharge - 20 hour rate. When you drain them in 1 hour you get a lot less power than their ratings. So if its lead add a big hunk of fudge - like 35%. If you look at the detailed specs there may be a chart showing the discharge at different rates and you can estimate that closer.

So a lithium battery about 50AH, 2.4KWH, $2-3K, 25-35 KG, 10-18 Liters.
or
a lead battery of about 70AH, 3.4KWH, $700, 110 KG, 40-50 liters.

Maybe you build it with a 1KWH Lead battery, 30KG, 14 liter, $200 battery for 10 mile range, and show the math that you can fit a 2.4KWH lithium battery, $2-3K in the same compartment for a 35 mile range?

[carl2.0]

Wow very useful info... Ill have to make some changes to my calculation then... The bike is for a proof of concept n a bit of feasibility testin (testing the feasibility of the electric drive in the Caribbean) so it doesnt need to make 25-30 miles but id atleast like to get 20.. dont think my budget will allow for a 70 ah pack, its really hard n expensive to find anything above 15ah here soo Ill prolly have to import. Im also concerned with how that weight would affect my accel and handling which Im rankin higher than range right now soo I prolly wont go over 50 ah... by chance do you have a full equation for working out the power, or have the drag coeff ??