Finding the E-Bike’s limits

A common question asked in these parts when I am describing the power and speed capabilities of the electric bike is “Well, can it scale Marin?”. Marin is an extremely steep street, notoriously and constantly steep- it goes from bottom to the top of the same hill I climbed last time, just in less than half the distance. I usually replied that I thought probably not- and now I can confirm- no Marin for this e-bike.

I didn’t actually take it up Marin today, but took it up a similarly challenging street, Centennial Way, on the South side of the UCB campus. Just above the Botanical Gardens the bike suddenly lost power. No connectors had come undone and the Cycle Analyst reported an idle energy usage of 3w, instead of the usual 6w. I figured the controller had either blown up or shut down, a suspicion reinforced by the extremely high temperature of the controller and motor case. The electric motor drive is a direct-drive single speed affair which affords good acceleration and a comfortable 20mph top speed on the flat, as well as good hill-climbing assist up to a limit. Climbing the big hill on the less steep street Spruce I was able to maintain a speed above 13 or 15mph and the motor did fine. On Centennial, with lower speeds and higher torque and current the controller overheated.

I also tried climbing solely on electric power- by this time the motor and controller were already hot and the batteries a bit soft,  unsurprisingly the bike slowed to a stop without pedal assist. It pulled about 650 watts at 22v at stall and was not able to move again without pedaling.

Well, it's sort of regen

I had descended a smaller hill earlier in the ride and taken the bike to higher speed than ever before- I held the throttle wide open and pedaled down the hill, hitting about 30mph. I noticed at the bottom of the hill that the Cycle Analyst had recorded a negative current flow during the decent- the controller is not set up for regen braking, but it continues commutating even when the motor EMF voltage is greater than the battery voltage- so an open throttle, presumably causing a full duty cycle PWM on this dumb controller means current flowing back to the battery. I was hoping to test this a bit better while descending the big hill but the controller didn’t cool enough to re-engage until I was already out of the hills, half-way back home. I would have expected opening the throttle at greater than 30 mph would result in some noticable drag from the motor charging- I was wondering what the “terminal velocity” would be on a steep hill with this sort of pseudo-regen. Maybe it’s just as well that I didn’t get the chance on account of my own safety on this crappy old 75lb bike with shitty brakes.

I spent most of the ride down the hill wondering if the controller would come back online and thinking about how I didn’t really mind an opportunity to justify picking up an controller anyways- it’d be pretty sweet to try out some real regen, or maybe some higher voltage hot-rodding. Now that it’s back working again I’ll have to go back to making up some good reason…

Since the sun has been shining the last couple weeks I’ve been charging the bike on pure solar electricity- when there’s plenty to go around I don’t feel bad burning it up through an inverter and a remarkably inefficient Soneil charger. I hear there are some hobby airplane chargers that will charge a 24v lead pack from a 12v source- that’d be ideal. Of course I’m also thinking seriously about dropping some money on LiFePO4 cells sometime soon. We’ll see 🙂

Posted on May 8, 2010 at 10:47 pm by Henry · Permalink
In: Electric Poly-V Bike