25-08-2017, 09:32 PM
Is Regenerative Braking Useful on an Electric Bicycle
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The first or second question everyone asks about an electric bicycle is “Does it have regenerative
braking?”. It seems that the idea has seeped deep into the popular consciousness and is now associated
with all electric vehicles. Some bicycles do indeed have it, but the question this paper asks is, “is it
worth the effort and expense to put regenerative braking on an electric bicycle?”.
Many people will say “of course, I want it all!”. But, in the real world, you never get it “all”. Your
bicycle doesn't have a rocket engine on it, for instance, even though it's perfectly technically feasible to
do that. What you “hopefully” get, is the list of items that get you the most for your dollar. In a perfect
world, e-bike designers would go down that hypothetical list in order of “most bang for your buck”
until they had put on all of the features that you can afford to pay for. So, how far up that list should
regenerative braking be?
One way to approach this would be to arbitrarily decide on some range increase that would make
regenerative braking a worthwhile investment. So, I will do that by saying that I don't believe a
regenerative braking system is worth its transistors unless you get at least a 10% range boost from it.
Now, 10% is pretty modest, so that's not too high a standard.
So, what does it take to get 10% more range? Well, it takes 10% more battery power. In other words
your regenerative braking system has to be able to recover 10% of whatever the capacity of your
battery is, over the time it normally takes you to discharge the battery completely.
I'll use our 16 Amp-hour, 37 Volt lithium polymer battery system as our example. It has a total usable
energy of about 500 Watt-hours. A typical energy use for one our our EMD units on a bicycle is about
15 Wh per mile. So the range with 500 Watt-hours “in the tank” would be 33 miles. So, what would it
take for a regen system to recover 10% of that, or 50 Watt-hours, thereby extending our range by 3.3
miles?
The obvious source for recovering energy would be stopping at stop signs or traffic lights. If we
assume a total weight of bike plus rider of 220 lbs (100 kg), moving at about 16 mph (25 km/h), it's a
simple physics 101 problem to calculate the energy available to be recovered by slowing the bike to
zero mph. It works out to about 2400 Joules.
A Joule is a tiny unit. 3600 Joules make one Watt-hour. So 2400 Joules is .67 Watt-hours. So how
many stops would it take from 16 mph to recover 50 Watt-hours at .67 Watt-hours per stop? The
answer is 75.
Of course, we can't recover 100% of the kinetic energy because all real systems are less than 100%
efficient. A reasonable efficiency would be more like 75%. If we factor that in, we can only recover .67
times .75, or .50 Watt-hours per stop. Now were talking 100 stops to recover 10% of the energy in the
battery.
If we divide 100 stops into 33 miles, that's an average of 1742 feet between stops for the entire 33
miles. A typical city block is about 500 feet, so that corresponds to a full stop every 3 city blocks. In a
congested urban area that might happen. More typically stops will be further apart. Also, under such
conditions bicyclists often don't stop completely at intersections, but rather roll through at low speed.