700li - Voltage Drop after 2000mi, 3000mi

Has anyone else put 2000 or 3000+ miles on their 700li? I have just over 3000 miles on mine in 16 months, and my full charge voltage is right around 52.7. After 4 miles of riding I am at 51.5. And my total range is 18 miles. I installed a nice 2 wire digital voltmeter for monitoring the battery health, and a biycle speedometer/odometer. My top speed is about 20 miles on the flat. (bike loaded at 225 lbs). Brand new the bike would charge to over 53 volts. Is it possible I have a bad cell? I am thinking of taking the battery apart and measuring each cell individually at full charge, and then full dishcarge. Can you replace just a single cell? And are the LIFEPO4 really a large improvement over my LIMN04?
Thanks!!