Usable range predictions for moped conversion?

[northernmike]

Hi folks.

If you've been following my moped conversion at all, sorry it's taking so long!

I've recently become worried about my batteries though. I was planning to use a bunch of 12V 7AH sla I happen to have on hand - 4 in series, specifically - to run my HXT 80-100-B. I'm concerned I won't get enough current, or enough range.

The bike is said to weigh 90lbs with the I.C.E., I'm expecting it to come in at about 85 without the batteries. I'm thinking to gear it such that at 130kv / 48v it does about 50kph.

What do you guys think my practical range will be with these batteries? Should I forget it and go to 12AH units?

 

[gogo]

You'll get from 25-50 Wh/mi. I'd only count on 150 Wh from your batteries. So, somewhere around 3 miles to 5 miles?

 

[Russell]

Yikes, I'm no expert but a quick look at the bicycle power calculator says you'd need something like 1200W to the wheel for that speed. Figuring a probably optimistic 75% motor efficiency kicks that up to 1600W from the battery or some 33A at 48V. If your controller can handle that then the batteries could probably deliver the current (with lots of voltage sag) for 5 minutes maybe a bit longer. Become more realistic with the speed, say 32Kph and the power required drops a ton to perhaps only 10A allowing you to go maybe 20-25 minutes on the 48V 7AH pack.

This is all just based on theorectical data of course but it's probably pretty close.

http://www.kreuzotter.de/english/espeed.htm

http://www.bb-battery.com/productpages/BP/BP7-12.pdf

I'd say go for the 12AH AND cut the speed!

-R

 

[northernmike]

Actually - this is all looking pretty acceptable.

My commute is only 4.6km, and pretty flat.

Karen, with the same motor in her Vespa Boxer is making 25 wh/km on 36V, and the HXT from my understanding likes 48V better.

32kph is probably a more realistic speed, especially for legal reasons.

Now I'm wondering - should I gear for a top end of 50kph and be able to cruise at 3/4 PWM throttle at 32kph? Or is that just too inefficient?

 

[northernmike]

Russel, how did you use that cycle calculator?

 

[Russell]

Russel, how did you use that cycle calculator?

Mike,

I selected "roadster" for your bike (the worst case, aerodynamically speaking) then input 105 pounds for the bike's weight and selected "robust wide touring tire". After that I played with the power number to get around 30MPH. That was about 1200W. That is the estimated power to the road. To figure the power required from your battery pack divide that by the efficiency of your drive system. I used 75% to get the 1600W. I think this is probably worst case for a level road with no wind. 1600W divided by a nominal 48V is around 33A. Then I Iooked at the discharge curve for a generic 12V/7AH battery to see how long it would last at that rate. If you drop the speed to 20MPH the power required from the battery drops to about 10A and once again looking at the battery curves gives around 20-25 minutes. With 12AH batteries and keeping at 20MPH you could probably go at least 45 minutes or 15 miles. I think better in miles so you can convert to kilometers.

Just to check out my assumptions I looked up battery powered scooters and found the 100 pound, 20MPH X-treme XB-500 had 4-12AH batteries and a quoted range of "Up to 20 - 25 miles per charge". Since manufacturers generally overstate such things I figure the 15 miles with the 12AH batteries estimation is a sound one. If you go to a better battery such as the BB Battery EB series (or equivalent) you'll get a bit better range than a general purpose SLA.

As I said this is theorectical but should be in the ballpark.


-R

 

[northernmike]

Hmmn.

I wonder how the cadence field factors in to that equation.

Also, my bike was capable of 30+ mph with the I.C.E. - in stock form - which is rated at 1 hp. I think that says as much about the rating as it does about our calculations, and again as much about how hard it is to convert accurately between watts and BHP.

5 miles will be an OK starting point. I just hope the efficiency isn't horrible - I seem to remember Miles' Moulton conversion running something like 15 wh/mile?!? That's a nice light frame, though, compared to my moby.

But, I'd better eat my breakfast and get to work.

 

[dogman dan]

Actually you want the 18 ah sla's . The reason why is that if you discharge small batteries near 100% each time they will not last long. 50% discharge is the number you want with sla's. Very soon, you will not be making it all the way. Take a theoretical minimum. then double it, then figure you need 25% more or so for a windy day. If 7 ah is enough, then to be nice to the battery you nee 14 ah, and then another 4 because this is the real world, and the tire may get low, or the wind in your face, whatever. If you have 7 ah batteries on hand, use em if you can, but do budget for and build for, much larger ones in the future.

 

[Russell]

Hmmn.

I wonder how the cadence field factors in to that equation.

Also, my bike was capable of 30+ mph with the I.C.E. - in stock form - which is rated at 1 hp. I think that says as much about the rating as it does about our calculations, and again as much about how hard it is to convert accurately between watts and BHP.

5 miles will be an OK starting point. I just hope the efficiency isn't horrible - I seem to remember Miles' Moulton conversion running something like 15 wh/mile?!? That's a nice light frame, though, compared to my moby.

But, I'd better eat my breakfast and get to work.

Mike,


I don't see how 1hp (746W) could move your scooter to 30MPH; I believe it would have to be about 2hp.

I think you have to look at the size and weight of the SLA batteries you want to use and decide how you are going to mount them then go with the largest feasible size. The 7AH really is out of the question unless you want to keep your speed UNDER 20MPH and go short distances. If you already have the batteries on hand I suppose you might as well experiment with them but they definitely won't be satisfactory, they will of course be the lightest SLA solution (~22lbs). If for some reason you want or need the smallest dimensions then you could use 10AH SLA’s and keep the same width as the 7AH and not suffer a huge weight penalty (~29-30lbs). When you step up to the 12AH SLA's you also have to contend with a physically larger battery and weights from 35 to 38 lbs for four. Go to the 18AH as Dogman suggests and they get to be pretty darn heavy for a relatively lightweight scooter like yours so mounting them low becomes a major concern. Now you see why everyone wants Lithium!

This website is a pretty good reference:

http://www.electricrider.com/

They sell high power cyrstalyte hub motor kits but the information on speed, power, range and batteries is still applicable.



-R

 

[dogman dan]

True enough, the motor may be too small for so much battery, so you need a bigger motor that needs a bigger battery that needs a bigger motor that needs a bigger battery. Lithium is the answer. Seriously though, double what ever your calculations say you need, it's how it is in the real world.

 

[northernmike]

Karen is making almost the exact same commute I plan to, using 150wh / trip. Same motor, similar bike - she is running 36V, I will be running 48V.

http://www.gpsy.com/ev/2008/10/8-cents-a-day.html

 

[Russell]

Karen is making almost the exact same commute I plan to, using 150wh / trip. Same motor, similar bike - she is running 36V, I will be running 48V.

http://www.gpsy.com/ev/2008/10/8-cents-a-day.html

It sounds like she is using a full charge to go the 3.1 miles then charging for 6 hours so she can make the return trip.

Is that right, 150WH ONE-WAY?

 

[northernmike]

It would seem to be so. 150wh one way.

From another post in the blog:

A great test run today using my three Rhino 12 Volt 18 Ah SLA batteries. Here's my data table according to my Cycle Analyst:

Distance: 8.089 km
Ah: 6.730
Watt-Hours: 228.49
Wh/k: 28.3
Amax: 69.55 A
MaxS: 31.6 km/h
AvgS: 24.6 km/h
Time: 19 min 39s
Odometer: 1563 km

 

[Russell]

Mike,

Even there she is using 45WH per mile and that's with an average speed of just 15.4 MPH. Sounds VERY inefficient to me!

-R

 

[northernmike]

Could be the CVT wasting energy - 1/2 of a 2 pulley system on this bike has been replaced with a fixed pulley... resulting in, I think, some significant drivetrain losses...

Interesting blog though!

I think I can use this motor better, optimizing gearing and eliminating the CVT and clutch...