Could be the way it was stored long term I suppose... they have recommendations on their site for best battery life.
http://www.torqeedo.com/us/en-us/service-centre/faq.htmlQuote:
What is the life expectancy of the battery?
The life expectancy of a lithium battery does not really depend on the charging cycles. It is not subject to memory effect, either, i.e. you can fully charge it after each trip irrespective of the charge status shown in the display.
Generally speaking, a capacity loss of approx. 4 % per year is to be expected. Capacity loss will, however, accelerate if the battery is permanently exposed to heat.
While there are no concerns about using the battery in high temperatures, we recommend that you take the battery out of the sun and store it in a cool place after use. Your battery must be returned to a Torqeedo Service Center for service 8 years after the date of manufacture.
Could the heat be an issue?