The EM 5.7's battery is rated at 25 Ah. So a discharge rate of 25 amps is called the 1C rate. My 10-amp test was 0.4C (10/25) and the 20-amp test was 0.8C (20/25). Generally, the more slowly energy is withdrawn from a battery, the more it will provide.
Starting with a fully-charged battery and a 10-amp draw required the load to initially consume about 500 watts. The power dissipation diminishes as the battery's voltage drops. I could have drawn up to about 15A and stayed within the 750W capability of a single active load.
Using both active loads in parallel, I could draw 12.5A with each for a 25-amp (1C) test, but I don't see much point in doing that now.
The actual test tables are available as a downloadable spreadsheet, but the main takeaways are:
The 5.7's battery provides, at best, only about 80% of its rated capacity. Maybe that should not come as a surprise – it was probably manufactured in 2013. However, it also does not have a lot of cycles on it (as I discovered after learning how to communicate with the BMS).
The LED state-of-charge indicator appears more linear at a constant discharge rate than it does in actual use on the bike. (Although the end comes pretty abruptly in both cases.) Also, the data looks slightly more linear than it is because I did not indicate the exact time an LED transitioned. This makes sense as I learned after opening the battery -the SoC indicator is simply a voltmeter having no interaction with the BMS.
A fully-charged battery produces about 53 volts. This is a cell voltage of about 4.08 volts (53/13 = 4.08).
A fully-discharged battery produces about 40 volts. This is a cell voltage of about 3.08 volts (40/13 = 3.08).
When the BMS determines the battery is completely discharged, it will no longer allow it to provide any power and the bike stops dead. This is a safety feature as discharging a cell below a certain point (which varies by cell chemistry and from manufacturer to manufacturer) can cause irreparable damage.