An electric backpack

[swbluto]

Hello all. In the pursuit of making a compact electric scooter, I've found the idea of an electric backpack(to carry the batteries and controller) to be very intriguing. Number one is to minimize the weight of the vehicle which, for a bicycle, can mean the difference between bunny hopping and your face on the concrete but, for a scooter, just means my scooter is easier to maneuver and handle(like for carrying up stairs, or up high curbs and so on - a versatility that definitely has its share of rewards in an urban environment). A backpack also has the property(assuming you have the right backpack!) of shielding its contents from the weather, so it can add a certain amount of water resistance to the electronics(Kind of essential where I live). It also would have the benefit of keeping most of the expensive electronics secured to the user reducing the risk and severity of theft, assuming one keeps ones backpack better secured than the bike/scooter! It seems etard also shares this interest -

Thanks Gary [for the notion that motor phase wire length doesn't matter that much for RC controllers], that makes my idea of having all electronics safely inside of my backpack a reality, plus it will be more stealth with only a wire running to matt's drive. Plus, if I use a little fan to cool the controller, it could be powered by something like this.

So, here I devote this thread to making this a reality! So, how would one construct such a backpack? What special issues would have to be dealt with and how can they be dealt with?

One problem I'm seeing is creating a flexible, retractable coiling from the controller to the motor. I've tried a regular wire once but let's say it was kind of detaining and I'm all out to improve that part of the experience. Apparently there's "flex coil"? What's out there that could handle the 80 amps of a controller but yet remain flexible? What kind of place would you buy it from and where?

Also, another issue is possibly heating issues. Assuming the controller is inside the backpack, how would one cool it off? I'd imagine an ordinary backpack would serve to thermally insulate the controller.Would cooling it off actually be a worry of some kind? Maybe a small insulating box could have bags of ice put inside with the controller with the controller's wires leading out a hole near the top? Condensation might be an issue. :lol:

 

[TylerDurden]

80A is quite a bit... even coiled trailer cables don't carry that kind of current.

I'd DIY some lightweight auto jumper cable wire with a coiled bikelock cable. Wrap all with splitjacket or electrical tape.

 

[Miles]

http://cablescience.net/power-coils.php?osCsid=27777dbd428cde7c64dc215fa8ed0b99
You'll need to use multiple-ganged conductors.

My backpack:

 

[John in CR]

Since you need enough stretch in the cable harness to get off of the bike comfortably, how do you create absolute assurance that it can't stretch enough to catch in the rear tire during a bump or hard move. In my relatively short e-bike experience, I've had the tire grab 2 bungee cords which I thought to be impossible when I hooked them up. One of the incidents crushed the whole bike rack, albeit a flimsy one, right down to the tire and slightly damaged a battery, but had it been in backpack form, it would have cranked me backward and down making for a nasty accident.

My vote would be for a detachable saddlebag over the top tube and/or inside the triangle.

John

 

[swbluto]

Since you need enough stretch in the cable harness to get off of the bike comfortably, how do you create absolute assurance that it can't stretch enough to catch in the rear tire during a bump or hard move. In my relatively short e-bike experience, I've had the tire grab 2 bungee cords which I thought to be impossible when I hooked them up. One of the incidents crushed the whole bike rack, albeit a flimsy one, right down to the tire and slightly damaged a battery, but had it been in backpack form, it would have cranked me backward and down making for a nasty accident.

My vote would be for a detachable saddlebag over the top tube and/or inside the triangle.

John

I'd imagine the point of connection would be made closer to the user away from the tire(for safety reasons) so that the cord could be designed/chosen such that it wouldn't naturally run into the tire during operation. Of course, it could be stretched given enough force in the direction of the tire, but properly designed, it seems like it'd be extremely unlikely. Like, for a bike, I can imagine the cable hook-up could be placed along the inside of the triangle where it leads to the seat-post and that a small loose ring can be attached to the the frame/seat-post(depending where you extend it) that the cord would be easily threaded through so that in case of a disconnect, it wouldn't bounce into the rear tire. Since it'd be a loose ring, it also be easily to pull up and down from. "Ring" can be replaced with a tube, I'd imagine, or several rings to restrict its lateral movement but still allow flex and ease of connection/disconnection.

But my particular problem is with a small scooter that has some solid rims, so that doesn't really affect me that much. Yet.

 

[swbluto]

http://cablescience.net/power-coils.php?osCsid=27777dbd428cde7c64dc215fa8ed0b99
You'll need to use multiple-ganged conductors.

My backpack:

Nice ... but, holy crap! Using their amp ratings, it'd look like an 80 amp set-up would cost 140 dollars, at least! Is this how expensive flex power coils are usually or is this a specialized/expensive company?

I think I'll look into coiling some speaker wire around a rod. If that doesn't work as well as I hope, I'll try some home-depot stiff wire.

Edit: Ok, I tried out the speaker wire and it didn't really hold its "springy" shape - it just slowly unraveled itself back into a line. The stiff home depot wire kept its spring-like shape, but it was a pretty stiff spring! I definitely wouldn't call it "flexible" but, I was only messing with ~ 4 inches of spring so maybe 4 feet is a little more flexible?

 

[Miles]

Do you really need an 80 amp continuously rated cable?

Mine is an old-style electric kettle lead - cost £4-50

 

[swbluto]

So, from that, I just need to find several junk appliances that have coiled power wires and DIY! Oh, yeah!

Except, I don't really know where to find an appliance graveyard. Eh...

I think I might look into attaching some 10 gauge speaker wire to a cheap coiled bicycle cable(like from the dollar store) or possibly some lightweight auto-jumper cables - except, I wouldn't imagine the high-current ones would be lightweight, would they?

 

[TylerDurden]

Cheepie jumper cables are often 8ga. Not very heavy. (you can even find 10ga. )

More expensive for 6,4,2 and heavier.

 

[j3tch1u]

i use a hydrating backpack (camelbak knock-off from costco) to carry my 36v10ah LiFe pack. fill it up with ice before a ride and there's your cooling and beverage delivery system. watch the condensation though.

 

[etard]

I have not put much thought into this, but I do have some ideas....

1. Design some sort of wheel that retracts the wire coiled around it. Set up in such manner that when fully standing there is still about 4-6 inches of travel, and when seated it retracts into a stopper that will not allow the tension of the coil to pull on the rider ( I think this would feel constraining). The flex cord would seem ideal, as long as it doesn't rob power from the system or get caught in the wheel. Maybe you could mount a shaft externally with a spring that extends or contracts along the shaft depending on position (not very stealth).

2. Cooling. Of course the batteries will be in Lipo safety sacks. I plan on having all circuit boards and ESC, etc... in the backpack so keeping these cool could be an issue. I like someone's idea of using a camelbak to cool things, I don't think it would be too much effort to keep electonics shielded from condensation. An extra measure could be a supplemental 12 volt RC battery that runs a fan to get some airflow going, but this might sacrifice weather insulation. However, you could hookup a cigarrette lighter plug to power phones, ipods, or even lights.

 

[etard]

How about something like this?

You could mount on the inside or outside of the backpack. I think you would have to mod it so that it would always retract instead of how they sometimes stay extended.

 

[recumpence]

A vacuum cleaner cord rewind mechanism would be perfect for retract.

Matt

 

[marks]

What about a magnetic type connector like some Macs have?
http://en.wikipedia.org/wiki/MagSafe

 

[Bobocop]

I'm assuming your going to make some kind of box inside the backpack to keep the batteries still (keep from moving around due to the material of the bag)

If so, why not use a shoulder bag, something like this.



Then you can build a box to fit inside for your batteries and controller and clip it (the bag) to your rear rack (if you have one) with 4 hold downs.
something like these.



One piece of the hold down would bolt/screw through the bag to the box inside, the other half would bolt to the rack (one in each corner.
This way you can clamp the bag down and remove it (easily) anytime you wanted to,
you won't have to ride around with the weight on your back and you won't have to worry about retractable wires.
Of course you still need a connection, but it only needs to be a couple of inches long coming out of the bag.

 

[overclock999]

you know you can form a coiled wire youself. depending on the insulation you can wrap you wire of choice around a metal pipe to form a coil, then heat it with a heat gun let it cool, hey presto your insulation should keep the wire coiled

 

[swbluto]

Nice tip!

I eventually learned that the flexibility of a cable was determined by the copper's strandedness and then by its insulation material, so I just got some speaker wires (really thin, high-count strands) and used that. It's pretty flexible so it doesn't have that "rigid" feeling that electrical house wire has. I tripled up on the 14(or 16?) gauge wire per phase, and it seems to be working so far. The wire is inherently wind cooled, so it doesn't get that hot.