Manual methods:

I just rode until the battery was dead (and then pedaled manually to get home) and that gave me my range. Works best in flat areas where there is not much variation to the power needs of the motor.

That and I know it takes 5 hours to charge a dead battery, so knowing how long it takes to recharge (some fraction of that 5 hours) tells me how much power I used. If it takes me 2.5 hours to recharge, I know I have used half of the batteries capacity.


Also...

You will get best lifespan of the battery if you don’t use it at full charge or near empty, and don’t use the full power of the battery (based on AH * C-rate).

To do this, charge to 90% of the battery capacity (requires a charger with this option) and don’t discharge below 30% of capacity. That and make sure your battery’s C rate is a lot higher than what you need. Those Headway batteries at 10C will be fine, but a battery rated at 1C is going to be stressed.
Obviously you can use the full capacity of the battery, but if you consistently run it dry, and often ask it to run at full current, you will shorten its life.

That probably applies more to Li-Ion batteries that don’t last as long as lifepo4, but the same applies to both.