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Friday, January 8, 2010

Electric Bicycle Lessons Learned

This is the battery pack that I built for my electric bike. It fits into a standard trunk bag behind the seat.

I get a lot of emails from people who want to do the same thing (hook Dewalt 36 volt batteries up in a parallel-series configuration for 72 volts). I put together (but never finished) a website to let people know what they're getting into and to talk them out of it if they're not particularly handy. IMHO, if you have to ask, you probably should not attempt to build your own.

Hopefully, the new Panasonic batteries will be affordable when they arrive next year and people won't have to build their own.

The rules for electric bicycles vary a great deal around the world. In Washington State and California an electric bike is limited to 1000 watts and must be not be capable of going more than 20 mph on level ground without pedaling. In engineering terms, the bike has to have some kind of electronic speed limiting device (most likely based on a speedometer input to the electronic controller) that limits power to the motor once the bike starts to exceed 20 mph.

Some of the most restrictive rules of all, as you might guess, can be found in the European Union, where they are limited to 250 watts, 15 mph, 88 pounds, and to ice the cake, the power has to cut out whenever the rider stops pedaling. Good God. That might explain their high costs and limited popularity. Like the old saying goes, "You can't fight climate change with anal retentiveness." And yes, I just made that up.

In my experience, 1000 watts is a very reasonable number because you need more power on hills and a thousand watts is not enough to break a 20 mph speed limit on level ground without a lot of added effort by the rider. The added restriction that the bike must have an electronic speed limiter is not only unnecessary, but easily bypassed with a few modifications to the system.

A less expensive, and easier to enforce rule would restrict the size of the bike's front gearing (relative to wheel diameter) which would limit the rider's ability to exceed 20 mph by the simple fact that he can only pedal so fast.

Electric bikes are ubiquitous in China, which recently ruled that they must have a license and insurance. Some suspect the move was initiated by scooter manufacturers in a attempt to throttle their main competition.

This law would have doubled the cost of owning an electric bike and was not very popular, to say the least, among the literally tens of millions of electric bike owners.

Much to its credit, the government just reversed itself. Here in America, corporate lobbyists unofficially rule the country alongside our elected officials. China is just starting to get a taste of the power of corporate interests and I suspect this reversal was not so much to appease the cyclists, but to appease the lead industry lobbyists who supply the primitive but cheap batteries for these tens of millions of electric bikes.

In China, many electric bike manufacturers have already abused the definition of an electric bike. Human nature being what it is, you can bet that it will eventually happen here as well. Note the words "electric bicycle" printed on these electric scooters in China.

The bottom line: safety is all that matters. And safety is all about force of impact. Force is mass times deceleration. When you hit a wall (or pedestrian) with a bike, your speed upon impact determines your deceleration. This along with how fat you are, determines the force of impact.

We are not going to ban large people from riding bikes. So, safety all comes down to speed. Picking a safe speed is what it all boils down to. Again, my experience has shown 20 mph to be just right. A bike moving at 20 mph can get from point A to point B in most cities just as fast as a car because bikes can usually bypass long lines of stop and go traffic. It is hard to exceed 20 mph with 1000 watts and low gearing thanks to the exponential nature of wind resistance, and if you are stupid enough to be moving that fast when you hit a pedestrian, it is not likely to cause serious injury.

In theory, using Washington State's rules, you are legal if your machine has "functioning" pedals on it, the motor draws no more than 1000 watts, and will supply less and less power as you exceed 20 mph. There is no requirement to pedal at all and I have to admit, I see nothing unsafe about that.
At some point we will start to see machines that look very much like mopeds with riders not bothering to use the pedals.

Forcing electric bikes that can't keep up with car traffic on to the streets is an unsafe thing to do.

As is happening in China, and thanks to our human nature, conflict is inevitable as the number of people seeking refuge form SUVs and pickup trucks increases congestion on our limited trails. Attempts will be made by one group to shove another group out into the streets.

Car drivers don't like bicyclists on their streets, pedestrians don't like them on their sidewalk. It would be wise for bicyclists of all stripes to cooperate to expand bike lanes and trails, which are going to come at the expense of car parking and lanes. So far, that seems to be the case. Most bicycle organizations (although not all bicyclists) have welcomed electric bikes as allies instead of competitors. It also helps that here in Seattle, our new mayor rides an electric bike.

In a nutshell, it really does not matter much what you call the two wheeled vehicle. As long as the design limits its speed to about 20 mph, one vehicle is about as safe as the other, whether you pedal it or not. If you don't limit its speed by design, and give it a name to differentiate if from scooters and motorcycles, you will get electric versions of scooters sharing the trails with bikes and pedestrians.

You don't want that because those machines are also capable of rapid accelerations. A quick twist of the throttle, intentional or not, can easily injure a pedestrian or cyclist in close proximity.

Scooters are differentiated from motorcycles primarily by the fact that their wheels can't be larger than ten inches in diameter. This provides a physical limit to top speed. Imagine trying to spin the wheels on a skateboard fast enough to do 60 mph.

However, smaller wheels also allow faster accelerations from slow speeds or a dead stop. As I mentioned earlier, acceleration is the only variable we have real control over to limit the force of collisions unless we are willing to limit the weight of bicyclists. This is why scooters capable of rapid acceleration should not share trails even if they stay under the speed limit.

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  1. Hi there, the most frustrating part of building an e-bike now is the battery. We have battery pack that work perfectly with e-bike kits. No need to build your own pack. It is 48V 15Ah LiFePO4.
    check out

  2. I tried to commute from Issaquah to Seattle last week and I got 7 miles before my battery light went yellow. It took two hours. I got back because it was mostly downhill from Eastgate where I abandoned my attempt and turned back towards home. It ended up being a round trip of 17 miles.

  3. Russ,

    We should start a company that makes a 1000 watt bike for Seattle hills. My SLA battery pack sucks! And I only have 600 watts. We should at least start a Seattle Electric Bike home brew club!

  4. Tony, 7 miles is about right with SLA battery and going up and down hills. Basically SLA batteries NEVER give enough juice for the amount of weight you must cary around. LiMn or LiFePO4 Lithium is expensive, but makes a world of difference. Range of 35 miles is not hard to achieve. For the hills you might need to use Geared hub motor.

  5. I'm sure that if you look around you will now find dozens of lithium packs on the market. Here is a sampling:

    The problem is reliability and support. When something goes wrong, you are typically out of luck with a battery purchased off the internet. These are mostly small players with low volumes, high prices.

    What we need are bicycle stores selling reliable products from major manufacturers, like Panasonic or Sanyo.

  6. Russ, Yep, these are the problems... And we are trying to address these issues, not work against/rip the e-biker. We have 2 year warranty from manufactures and return policy if you are not satisfied. The price is reasonable considering the effort to import these items.

  7. Batteries are the most challenging component of the E-Bike because of cost and quality. They all pretty much come out of China, which makes logistics a challenge for anybody trying to sell them at a reasonable price. After looking at and trying a variety of different packs to sell, I found a 36 volt LifePO4 that provided the best dollar value and lived up to performance specs (a problem with many I tested). Not going to give the power of a 48 volt model, but will still go 20mph for 20 miles on flat ground with a 500-750 watt motor (ggod torque for hills), and be relatively easy on the pocket book:

  8. Did you build the custom battery out of necessity, or just trying to find a more efficient way. Thanks for sharing your experience with us, though. My brother might find this helpful, since he takes an electric bike to work. Thiago |

  9. You can (could) get these batteries at any Dewalt repair center or buy them at any number of places on the internet. They charge very quickly compared to anything else available.


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