It's a bit cool this morning, about 0 C, and the batteries had started out cold, instead of being brought out after storing indoors, neither of which is good for their performance. In endurance EV racing, people even preheat their batteries before a race. Initial pack voltage was 26.5. My goals this morning were to arrive on time, relaxed, and clean, and I'd chosen a slightly different route.

Total trip time was 27 minutes, driveway to bike rack, and while not entirely relaxed, I'd spent less time dicing with traffic. Unfortunately, that was near the end of the trip, and the batteries didn't seem to want to help much at that point. The motor cut out while gunning it up the last hill after arriving at work. The final voltage (after coming inside) is 25.1, a little higher than before, but colder batteries show higher voltages at the same SOC.

I previously wrote that the batteries can dominate the running cost of an EV. To that add, at least for a simple vehicle like my bike, they can also dominate the performance envelope. The voltage and power drops as the battery is discharged, and since simple electronics can't increase the voltage seen by the motor above that shown by the battery, the top speed and torque at any given speed deteriorate. The solution is to either put up with it (using deteriorating performance as a kind of battery gauge), design the vehicle around the worst case performance of the battery pack (i.e. oversize the battery pack), or find a way to let electronics buffer the way power is delivered. They already do that, usually using pulse width modulation to cut the voltage for the amount of power desired, but increasing the voltage is more difficult. I don't even know if it's possible to do efficiently. Finally, increasing the power beyond what the battery can do requires violating the first law of thermodynamics.

Addendum: still 0C on return, strong headwind. Good news is that the gearing was perhaps about right, the bad news is that the battery didn't make it all the way home. Was able to nurse it along at about half throttle, trying not to trigger the cut off. 24.6 V in the end.