We've not had many reports of battery life issues with the i3. BMW has previously said that its batteries are designed to last as long as the vehicle, quoting a lifespan of 15 years and beyond. It also guarantees the i3 batteries will have a minimum of 70% capacity after 8 years or 100,000 miles. I wouldn't be too concerned about the battery life, particularly as you'll save money in other areas such as fuel and servicing.