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Old 09-24-13, 02:25 PM   #1
nohorjo
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Exclamation help, home-made ebike - not enough power

Hi im new to this site.

I built an ebike from a specialized hardrock 2000. Its chain driven and has a 250watt motor (MY8029), with 2 x 12v 7ah lead acid batteries in series.

attatched to the motor is a 5 speed free-wheel sprocket. its smallest gear is connected to the largest of the actual rear wheel.
the torque is really low it doesnt feel like i get much of a push. the motor on its own cant even push at 4mph.
im thinking of moving the motor to the middle of the frame and connecting it to the chainset so i have a higher gear ratio so hopefully more torque.

Im really disappointed at the motors performance. when the wheel is lifted it reaches speeds of ~600rpm. it ony gives me +2mph to my normal cycling speed.

Thanks everyone for your help!

extra info:
ive dropped the sealed lead acid batteries a few times so im wondering if that might be causing a problem. i did order new ones that should arrive soon.
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Old 09-24-13, 04:12 PM   #2
SpecialX
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Quote:
Originally Posted by nohorjo View Post
Hi im new to this site.

I built an ebike from a specialized hardrock 2000. Its chain driven and has a 250watt motor (MY8029), with 2 x 12v 7ah lead acid batteries in series.

attatched to the motor is a 5 speed free-wheel sprocket. its smallest gear is connected to the largest of the actual rear wheel.
the torque is really low it doesnt feel like i get much of a push. the motor on its own cant even push at 4mph.
im thinking of moving the motor to the middle of the frame and connecting it to the chainset so i have a higher gear ratio so hopefully more torque.

Im really disappointed at the motors performance. when the wheel is lifted it reaches speeds of ~600rpm. it ony gives me +2mph to my normal cycling speed.

Thanks everyone for your help!

extra info:
ive dropped the sealed lead acid batteries a few times so im wondering if that might be causing a problem. i did order new ones that should arrive soon.
What you describe is exactly what's gonna happen....
A 24v 250w motor is gonna try to pull 10a from the batteries..
The are 7ah batteries and will only be able to supply 10a with no resistance.
The second you start adding a load to it, your gonna drop the available voltage that the batteries will be able to supply.
You're pulling 1.5C (10a) from a battery rated to provide C/20(.35a).

The voltage to that motor is gonna go from 25.2v (actual) to probably 18.0v or below while you're trying to run it on the bicycle, so you're gonna get very little torque.

i.e. The "harbor freight type" motors are either On or Off. There is no variable speed on them. So they are gonna try to pull full amperage which those size batteries won't be able to supply...
The ONLY way you're gonna get decent help from that motor is by using 2-12v 20AH batteries.
I know you were hoping that the simple (and relatively light) setup would help tremendously, but you won't get the desired results from such small batteries.

Last edited by SpecialX; 09-24-13 at 04:24 PM.
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Old 09-24-13, 08:06 PM   #3
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Yup...your batteries suck.
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Old 09-25-13, 12:11 AM   #4
nohorjo
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Thanks for the speedy reply. My motors rating is 14A. I'm getting 2 more batteries which I'll connect in parallel so I'd get 24v 14ah. Would that be enough or will I need more batteries?
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Old 09-26-13, 06:23 PM   #5
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Thanks for the speedy reply. My motors rating is 14A. I'm getting 2 more batteries which I'll connect in parallel so I'd get 24v 14ah. Would that be enough or will I need more batteries?
If the "old batteries" are Used AND have seen a lot of usage, then adding an additional set of batteries wouldn't be prudent. As the old adage goes.. Don't mix used and new batteries.
Either get a set of new 14AH or 20AH.
(The 14AH batteries will net you a range of approx. 10 miles and the 20AH will net you about 15 miles)
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Old 09-29-13, 07:35 AM   #6
cerewa
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Originally Posted by SpecialX View Post
If the "old batteries" are Used AND have seen a lot of usage, then adding an additional set of batteries wouldn't be prudent. As the old adage goes.. Don't mix used and new batteries.
Either get a set of new 14AH or 20AH.
(The 14AH batteries will net you a range of approx. 10 miles and the 20AH will net you about 15 miles)
Sounds like the problem is not the batteries but the gear ratio. Your motor is built to spin at a certain speed when running at 24v. With the gearing you used that works out to about 600 rpm at the wheel with no load. However, the ideal for a 250watt ebike is more like 200 to 250 rpm at the wheel under no load. Basically you don't have a good gear ratio and if you set up another stage of gear reduction you will. ideally, use a 1:3 gear reduction or similar.
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Old 09-29-13, 04:34 PM   #7
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Originally Posted by cerewa View Post
Sounds like the problem is not the batteries but the gear ratio. Your motor is built to spin at a certain speed when running at 24v. With the gearing you used that works out to about 600 rpm at the wheel with no load. However, the ideal for a 250watt ebike is more like 200 to 250 rpm at the wheel under no load. Basically you don't have a good gear ratio and if you set up another stage of gear reduction you will. ideally, use a 1:3 gear reduction or similar.
Gear ratio or not.. The motor is going to pull 14a, no matter what. Won't get any distance with 7ah batteries...
24v 7ah SLA batteries, at best, will give maybe 4.2AH.
24v x 4.2ah = 100wh.

Motor wants 24v x 14a =336w

100wh / 336w = .297 x 60min = approx 18 min run time.
18m run at 16mph = 4.7 miles

Given 24v 14AH (8.4ah usable) = 201.6wh
201.6wh / 336w = .6 hr x 16mph = 9.6 miles

24v 20ah = (12ah usable) = 288wh
288wh / 336w = .85hr x 16mph = 13.6 miles

Last edited by SpecialX; 09-29-13 at 05:38 PM.
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