Battery power density for the electric bicycle

Power Density is the amount of electric charge by weight. Different types of batteries have different power density. Automotive Battery Power density is projected to increase 3 fold by using lithium ion batteries, yielding power availability of 1 HP for 5.4 hours (prior to loss) for one full charge. Zinc battery promoters project1.5 x lithium ion battery power density within several years. For the electric bicycle power density is even more important than for the electric car because battery weight is a greater percentage of the total electric bicycle weight, than of the total electric car weight. Consequently increases in power density will impact the electric bicycle significantly more. Either the range increases, the speed increases, or the weight decreases.

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Theoretical range of the electric bicycle

The theoretical range of the electric bicycle is a function of the following equation:

Human output + electric output = total output
Total output x efficiency = total work applied
Total work applied x total load x rolling resistances value x road efficiency x wind resistance factor x weather factor = total distance traveled
Weather factor = (wind factor [wind speed and direction] + rain factor [rain presence and intensity])
Road efficiency = (elevation change value x % of total road length value x road surface value)

Wind resistance increases as a function of speed, decreases as a function of aerodynamics.

Rolling resistance is a constant factor at most speeds, determined by the tire shape, material, and inflation psi, and load carried.

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System output for the electric bicycle

I have considered combined systems (pedal charge generation to all electric output; electric assistance to pedal output) and separate systems (dual electric and pedal powered rear drive; pedal powered front wheel/electric powered rear; electric powered front wheel/pedal powered rear).

Currently I favor direct pedal output to keep the electric bicycle still mobile in case of electric system failure. The electric system is more likely to fail according to reports I see in the electric vehicle lists. Many electric components are marketed with theoretical values rather than real life values.

I considered the following: Pedal charge generation to all electric output. The advantages: smooth power control, and frictionless charge regeneration capabilities; constant pedal cadence. Disadvantages included: efficiency loss from pedal power conversion to battery charge then to power output: componentry cost and availability on failure; cost and difficulty of system development; single system reliance.

I stopped further consideration of that system for the electric bicycle for the following reasons: Experience in recumbent manufacturing demonstrated that a system may be theoretically feasible, but actual construction will demonstrate unanticipated problems. While this is expected, effective pre-existing solutions save money and time, and do not derail the project towards the development of an ancillary component of the project. Pre-existing bicycle componentry is more readily available and hopefully has been subjected to real life testing. Going to a bike shop is much quicker and cheaper than going a machinist then welder, or electrical engineer then electrical manufacturer, particularly when on the road, and not near specialists I know!

Electric assistance to pedal output:

The advantages of this system include: using a developed and road tested bicycle drive-train, with modifications to incorporate the electric assistance. It may be possible to include an electric motor by a tandem bicycle crankset crossover; previously developed componentry increases replacement availability. Disadvantages include: currently built bicycle componentry was not developed for this load at sustained high speeds; highly specialized bicycle componentry decreases replacement availability.

Separate Systems:

After my share of bicycle and vehicle breakdowns I can appreciate having a backup system. Waiting for someone to tow you in or to bring out a part they do not understand can be a trying or even harrowing experience, just as leaving the vehicle behind to get the part yourself can be disconcerting. I'd rather go slow than not go.

Considering the weight of this bicycle, dual output offers the advantage of limited mobility factor if bicycle componentry fails. Also considering the weight of this bicycle, electric output may be desired if the human machine fails!

Separate electric and pedal powered rear:
This could be done with a friction system, such as the Zap electric bicycle. I discuss the disadvantages of the friction system below. Separate systems could also done by attaching a single cog inboard of the disc brake. The electric motor turns that cog. A disadvantage to fixed cog design is the chain moves whenever the wheel turns, resulting in an energy loss when the electric system is off. There is also the possibility of rear wheel lockup if that chain fails. It may be possible to use a reverse thread left side BMX freewheel (released in 1999), with an adaptor designed for the electric bicycle rear disc brake hub. One advantages of this design is decreased stress through the bicycle drive system, reducing the risk of failure there. A disadvantage is by attaching at the disk brake rotor, it may increase stress on the rotor or hub, potentially result in a disk brake failure.

A disadvantage of the dual rear wheel drive system over separate front and rear drive systems is that all the drive force is still channeled through the rear wheel, it might be advantageous to distribute the drive force over the front and rear wheels.

Separate electric and pedal powered wheels: The first question under this design is whether to power electric front wheel and pedal rear wheel, or electric rear and pedal front. Bicycle componentry is designed for rear pedal power, so it makes sense to go electric front and pedal rear, and to design only the front electric system. With that decision, further options emerge:

Is the electric drive accomplished by a Hub Motor, Friction Drive, Toothed belt drive, or Chain drive. Here are issues I have considered arriving at my current design

Motor hub assembly: Following extensive discussions with engineers I abandoned current consideration of this system because:

I have not found a pancake type permanent magnet low RPM motor that meets these requirements: operates efficiently at low RPMs; adds no drag when not on; adds no significant rotating weight; puts out adequate power; can be adapted to use as a wheel hub. I rode an electric bicycle with a power assisted hub from Germany, but the power boost was insignificant. A further issue with this system is disc brake adaptability.

Friction drive front wheel:

Without a complete analysis I have stopped further consideration of this system for the following reasons. Friction drive system loss is approximately 15%, this is in addition to motor efficiency loss. At significant speeds the system could add tire heat, increasing the likelihood of a blowout. Motor placement could cause a handling problem. One minimal advantage is that charge regeneration system is direct (see Charge Regeneration Application discussion).

Toothed belt drive front wheel: This system allows the motor to be frame mounted while the drive cog is fork mounted. The belt flex allows the wheel to turn. Loss is currently unknown.

Chain drive front wheel: This system eliminates the 15% loss and the tire heat problems associated with friction drive but it may still have the handling problem described above. This system could use a BMX bicycle rear cassette disc brake hub (just now available, 3/14/ 2001).

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Propulsion for the electric bicycle

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Charge Regeneration for the electric bicycle

I want the electric bicycle to have regeneration slowing for long downhill rides. Although I prefer frictionless drive, a friction regeneration system may be easier to implement, and more cost effective initially. Since the amount of regen is not significant, an additional loss here is also not significant. Maximum regeneration conditions exist primarily in two situations, excess downhill speeds and excess braking. Excess braking suggests city or suburban travel where booster charges (lunch before hitting the road) may be available.

A simple friction drive system can be completely engaged and disengaged mechanically. It is on or off, and when off has no energy drain.

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Sponsorship is Welcome! Actualizing this electric bicycle will not be cheap. I am particularly interested in obtaining assistance from manufacturers who are able to donate existing high tech components (disc braking, electric motor, electric charging, etc). If you are interested in donating research energy, materials or otherwise, please contact me.