My Energi battery had degraded to just over 4 kWh after about 90,000 miles (6 years). Finally I saw the tips from some battery gurus on the old Energi forum (raj in particular), including leaving a smidgen of charge in the battery at all times, avoiding hot-sun charging, etc. Now that I'm at 125,000 miles and 8 years, the rate of depletion seems to have slowed--I'm now just under 4 kWh. So that's a hopeful sign.