JDH: CB750 Conversion - The World's LIGHTEST Batteries!

jdh2550_1's picture

Battery and Motor Layouts

The following pictures show my trial layouts using 55Ah batteries and an ADC motor (6.75" diameter, 11 inch length). The box for the batteries includes the terminal height and is also oversized by 1/4 inch (the internal measurment is the external measurement of the battery).

My design is now at 84V - which requires seven 12 volt batteries. When I researched my options I found out that a 96V capable speed controller was going to cost me about $1200. I found that Kelly Controllers do an 84V model for $389 - I need an extra DC-DC convertor in their circuit (the control voltage needs to be 24V).

Now, if I could manufacture real batteries which were this light I'd be a VERY rich man and I could use a Zilla (which is closer to $2000).

Layout One
This is my favorite layout. It packages 5 batteries within the frame rails in a simple to implement box. Two other batteries would go as saddlebags/paniers - I will probably make larger paniers so that I can mount 7 seperate 12 volt battery chargers on the bike (not sure about this yet). I feel comfortable that the weight distribution is appropriate (the batteries are 38.5lbs each and the gas motor was over 180lbs (I broke my bathroom scale trying to weigh the motor!). So the majority of the weight is where it "belongs" - in the engine bay and as close to center and as low down as possible. Also, no changes to any of the foot pegs or rear brake lever is needed - so ergonomics aren't affected.
The downside of this design is that I can't run a chain directly from the motor (mounted where the battery and oil tank used to be) to the rear sprocket. The swing arm gets in the way - and there's no way I want to modify that. So, I'd need to use a jack shaft - and I'd need to figure out where to mount it.
Layout Two
Next up, layout number two, places the ADC motor so that the output shaft is approximately where the output drive of the gas motor was. Above it are the two side by side batteries which were on the bottom before. The fifth battery is where the ADC motor was in layout number one.
This still keeps the weight centered, and the ADC motor is 65lbs so the weight of shifting two batteries higher vs. bringing the motor lower pretty much cancels out. The battery placement is more tricky now - but not too bad.
However, this design is "tight" - until I get the real batteries I'm not confident I can make this all fit. In the photos the fifth battery is resting on the rear tire - however, in real life it will mount an inch or so higher. That gives some clearance but not enough. To make this design work I need to shift all the batteries forward slightly - to do this I'd need to grind away the "back side" of a bracket up by the front of the gas tank. I'm pretty sure that won't affect strength - but I'm still not sure I'd end up with enough clearance for the rear wheel (I can also put the rear wheel towards the back of the chain adjustment slot (and sacrifice adjustability).
Another area where this design is "tight" is around the right foot peg and brake pedal. I will need to bend the brake pedal out - I might be able to leave the foot peg alone - or I might not. I don't think either of these are a deal breaker.
Layout Three
Layout three addresses the concerns of not having rear wheel clearance for the fifth battery in layout two. I've mounted one battery on top of the ADC and between the frame rails and then I've mounted two batteries outside the frame rails next to the front stack of two batteries. As I look at these photos I realize that I should move those outside batteries lower to get the weight lower down. I'm also a little concerned about putting too much weight too far foward on the bike - however, it's still within the profile of where the original gas motor was.
Obviously this design sitll suffers from the "tightness" around the right foot area.

Front Profile

This last picture shows the front profile of the bike. It's for layout #1. Note that the ADC sticks out "a little bit but not too much" (hey, I never said I was an automotive engineer - this is as accurate as I get at this stage!).

So there you have it - three contenders. Any thoughts on which are best? Or a completely different set up? Feel free to leave comments or send me an email or PM (both can be done through links on this site). At the moment I'm thinking layout number three - but we'll see.

Finally, I'm trying to decide whether to look more at batteries to see if I can find something with higher Ah that I can still get to fit. I should probably do some more checking on the specs of my chosen Universal Battery UB12550 to see if they're up to the job...

Stop Press
The Universal Battery UB12750 (75Ah AGM) is the same height but 1.2 inches longer and 1.2 inches wider. I can fit them in the same basic layout as layout three above with the battery "on top of the motor" extending further back into the battery box compartment.


Deafscooter's picture

Deafscooter Read your Bike it must wrong design layout ( dangerous it can loose chain )...

Deafscooter re layout the your Bike to correct the layout on motor. batteries

here is simples picutre . you guess..... Motor attach swing arm stay chain length...

Craig Uyeda


jdh2550_1's picture

Thanks for the suggestion. If I were to do the top layout then I'd use a jack shaft (not mounted to the swing arm). The chain from the motor would run straight down to a shaft with two sprockets. It would then run from this shaft to the rear sprocket. That would allay some of the fears of a loose chain.

In your second suggestion - unfortunately my batteries aren't that small. I'm going to use UB12750 which are group 24 75Ah batteries. In the interests of simplicity I was going to mount the motor on the frame rather than the swing arm. I'm going to use an ADC motor which ways around 60lbs - mounting that to the swing arm would be difficult and would also be a lot of unsprung weight to add. Also, the front sprocket will be very close to where the original front sprocket was and so there shouldn't be an issue of varying chain length (i.e. the original bike didn't have a swing arm mount with constant chain length).

Yep, planning on mounting the controller under the tank (going to cut the bottom out of it) and use the battery box for other stuff.

In your diagram you've crossed out my rear battery - why was that? Bear in mind that the chain will be behind that battery so it won't interfere.

Thanks again!

John H. Founder of Current Motor Company - opinions on this site belong to me; not to my employer
Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.

Jeffkay's picture

Hi, I have a Yamaha 750 conversion that I have been messing with for a year. First I put in 4 group 24 flooded batts I get cheap. I have an Etek. Now, the reason I am replying to you is that I seem to remember Ann Arbor as somewhat hilly. Where I live, there are many steepish hills. The point is, the weight of those 75ah batts is going to be a lot of mass to move and accelerate. Also, since you plan more batteries than 4, the weight makes the bike a bit hard to handle when moving around the house, etc. So, I think the 55ah idea may be good and you will get about the same range in my opinion. A guy in my area just made a 900cc bike conversion with an Etek and four 75ah AGMs. I rode the bike and it was nice but will not get to your high max speed --more like 45mph max. I have destroyed several Eteks and think your idea of using a standard motor (even though heavier and harder to mount) is a good idea for reliability and peace of mind...
Jeff K. Burbank, CA "Deep Cycle" and "Bike to the Future"

jdh2550_1's picture

Hi Jeff,

Good point - however I have the 75Ah's on order now. I'll see how they go. If they prove too heavy I can first reduce the number and then maybe switch to 55Ah's instead. I'm concerned that 55Ah's won't give me the range I want.

Thanks for the ideas though.

John H. Founder of Current Motor Company - opinions on this site belong to me; not to my employer
Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.


I am new to the electric motorcycle world, but I have about 100 miles on a scratch built electric scooter/minibike. I used group U1 32ah batteries. My project "mad max" will reach a top speed of 48 mph with an Etek motor and has a range of 45 minutes applying normal throttle cruising & about 25 minutes WOT. I would agree with Jeff & tell you that battery weight is a huge factor when it comes to performance, specifically top speed. I realized that higher amp hour batteries were less valuable and my time was better spent optimizing operational efficiency (loose weight: me & the bike, lowering rolling resistance, & optimizing controller settings to manage power usage. I would also agree with Jeff & tell you that your graphs are impressive, but I cannot get the math to match real world performance yet! Frustrating, but a challenge! My gearing, tire size, rider weight, machine weight, & motor horsepower would indiacte a top speed of 55MPH! Good Luck!

Chad defender of all things electric!

jdh2550_1's picture

Thanks for the info. However, the batteries are arriving tomorrow - so I'm going to be trying out these 75Ah batteries no matter what. If they make it slower than a dog then I can downsize.

It will be interesting to see how close my math matches reality...

John H. Founder of Current Motor Company - opinions on this site belong to me; not to my employer
Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.

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