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72 Volt System 


Why use a 72 volt controller?  Raising the voltage from 48 volts to 72 volts roughly doubles the power output and increases the speed of the motor by 50%.  It allows the use of slower higher torque motors and/or smaller wheels to improve low speed acceleration, hill climbing and cargo hauling power while retaining the ability to reach a higher top speed.

The 72 volt controller is a modification of the 48 volt controller design with a new circuit board and upgraded components rated for a maximum of 100 volts.  The low voltage cutoff is set to 60 volts which is appropriate for use with a 20 cell Thundersky li-ion pack.  These cells are 3.0v min, 3.6v nominal and 4.35v max.  The 72 volt pack voltage will range from 60 to 87 volts depending on the state of charge.

The controller works well with a pack of 6 sla 12 volt batteries in series which give a maximum voltage of 79.2 volts. Regularly discharging all the way down to the 60 volt cutoff will be hard on sla batteries and give a short pack life.

It should work well with Nicad and Nimh batteries which are typically offered in 24 and 36 volt packs.  Use three 24 volt packs or two 36 volt packs in series to make a 72 volt pack.

To tailor this controller for use with different battery packs or if you want to raise the voltage cutoff to guarantee a longer pack life you will need to replace the resistor that sets the low voltage cutoff.  Instructions for this will be posted soon.

Motor recommendations for a 72 volt 20 amp controller

wheel size

 

inches

erto

motor

26

559

4011

20

406

409

16

305

408

These combinations give a top speed of roughly 30 mph (50 kph).  With these choices both instant start and peddle first systems will work well.  The smaller wheels give lower gearing allowing the use of the lower turn count winding motors which are faster, more powerful but lower peak torque.  The smaller wheel configurations will give stronger acceleration and better hill climbing but slightly less range. If you have very steep hills or if your gross weight (you, bike, batteries & cargo) is very heavy you may prefer a higher numbered motor than suggested.  If you are a very strong rider you might select a lower numbered motor for higher top speed.  Other wheel sizes will work too but realize that smaller wheels give better acceleration with less current strain on the batteries, motor and controller while larger wheels will give a higher top speed at higher current draws.  Even tire sizes will have a modest effect on your final gearing.  At speeds over 30 mph power requirements soar. If you do not peddle vigorously your range will suffer dramatically with faster motors or bigger wheels.

For a comfortable peddling cadence at 30 mph either use the free hub and cassette system with a cassette with an 11 tooth smallest sprocket or if using a motor that mounts a freewheel get a Shimano Mega7 11-34 freewheel.  For smaller wheels a large front chain ring is desirable.  A 53 tooth ring works well for a 20 inch wheel.  A 60 tooth or larger ring is better for a 16 inch wheel.

The 72 volt controller, like its lower voltage cousins, can sustain a 20 amp load but it does not limit current to 20 amps.  Hard acceleration, especially on hills or with a heavy load, and especially with motors faster than the above recommendations can draw more than 20 amps.  To protect the controller from possible damage you should install a 20 amp circuit breaker or fuse between the controller and the batteries.

Todd Allens 72 Volt bike

 

To increase performance this bike was modified to use a rear drive motor in a 20 inch wheel.

 

It is a standard 26 inch mountain bike with a bolt on Xtracycle FreeLoader kit rear end.  The front clamp to the stays behind the bottom bracket was changed to raise the nose 3 inches.  This lowered the rear to keep the bike at the same height when mounting a 20 inch rear wheel instead of the standard 26 inch wheel.  A new rear brake mount was welded on to hook up the rear brakes.

 

The bottom bracket was replaced with a wider unit to give clearance with the chainstay for the 53 tooth chain ring replacing the the stock 42 tooth ring.

 

Treadless high pressure slick tires were selected for best on road efficiency. The rear is a heavy duty 1.75 inch to minimize flats which are a little more cumbersome to repair and it keeps the ride plush on the smaller wheel.  A narrow 1.25 inch front was selected for speed.

 

An 8 lb 36 volt 10 ah li-ion pack is carried inside the panniers on each side of the rear rack.  The Xtracycle is rated for 200 lbs. This bike can carry a lot of cargo or even a passenger.  It could also carry a cheaper 72 volt sla battery pack or more li-ion batteries for even greater range.

 

This bike is currently running a 408 instant start system but will be changed to a 409 peddle first system for improved acceleration while loaded and improved range and battery life.  As currently set up acceleration and hill climbing performance are very good when not loaded but needs peddling with the motor for smooth starts when heavily loaded.  With a 170 lb rider it will go 30 miles without peddling at 30 mph when the charge is fresh dropping down to 23 mph as the charge is depleted.  50 mile trips have been done at a 25 mph with peddling at roughly a 150 watt contribution level..

 

Laws and regulations very widely from country to country and even between states and provinces with in a country and  enforcement policies may vary from city to city.  By modifying your bike to use this higher powered controller you may be in violation of your local laws and possibly subject to enforcement.  You are responsible so be responsible.

 


For more information or to purchase a 72 volt controller in the US contact Todd Allen.

speedebikes@gmail.com