I was wondering how frequently. My 700 series doesn't mention it but my buddy just got a Rad and there's stuff in the manual about fully charging the battery to condition it or something. Only thing I read on mine is the battery longevity being better if you keep it between 30 and 80%This sounds like your battery cells are badly out of balance. Suggest you forget about charging to 90% for a while. Seriously. Instead, charge FULLY, until the battery charger shuts off. Go for one of your normal rides, then charge it again, FULLY, until the charger shuts off. Do that for several cycles. My bet is you're going to see a battery that's MUCH happier, behaving as you would expect.
When you tell us that you charged to 90%, and come back later and find it at 80%, what you are seeing is very likely the result of fully charged cells charging those elsewhere within the pack with less charge. This is reinforced by the fact you rode a couple of blocks only to find the charge at 68%. Noteworthy maybe, is that my personal bike will go about 17 miles before the first of 5 bars goes out. This battery is normally charged when it gets down to about 46 volts, and that generally will happen at about 35 miles. Low voltage cut off is set for 42 volts.
You need to know that the charger is balancing the pack during that last 10% of the charge cycle - to prevent exactly what you have going on, which is reduced capacity and unevenly charged cells.....
When (if?) you get this situation resolved, then maybe re-consider this charging to 90% concept. Just keep in mind that balance charging is needed frequently - to prevent exactly what you have going on right now...... -Al