To charge, or not to charge

Anybody over the age of 30 remembers the first NiCad rechargeable batteries from the 1990’s.  These batteries needed to be depleted completely before recharging or they would “remember” how much you used and only recharge that amount in the future.  NiMH batteries got rid of the “memory” issue, but did little to improve charge capacity or battery life.  My first hand-held device had a Li-Ion battery and came with instructions to let the battery drain to 25% to 75% before recharging.  I followed that advice to a large degree and managed to get three years of useful charge from the battery before it started to dwindle.  I did not follow that advice for my laptops and those batteries all failed to hold the originally promised charge beyond two years.  With my first two cell phones, I was diligent about appropriate battery charging and managed to get more than four years of nearly full charge capability from each before the batteries started to degrade.

Five ticks remaining on the I-MiEV charge gauge (5/16)

So, before I even bought my electric car back in 2012, I knew how to better care for the battery.  Wait for the charge to drop below 75% full before recharging and avoid complete discharges when possible.  My commute usually draws the battery down to about 45% remaining, so this represents an ideal point at which to recharge the car.  But, when I know I will be working from home the next day, should I wait to charge the car or charge it right away?  The ideal choice for extended battery life (according to articles I have read) is to complete charging within an hour or two of when you expect to drive again.  The thinking here is that maintaining a full charge for an extended period strains the battery, ultimately weakening it.  But, the practical choice is to have the car ready to drive in case of an unexpected need the next day (which seldom happens for me).  So, I choose to charge at night and have the car sit fully charged the next day.

The real trouble arises when I use the car the next day to run a short errand and I use less than 25% of the battery capacity.  (Typically 85% to 90%of the charge will remain.)  Then I have to drive to work on the following day.  I admit that I like my comfort zone when it comes to the car’s range.  I don’t enjoy learning just how far I can push the car without running out.  While I can get to work and back and still have at least 25% of the charge remaining, it reigns-in my after work activities, limiting me to a short trip to a local store.  On the other hand, recharging with only 10% to 15% drawn off the battery will accelerate its decay.  When I know I need to do some extra driving I will recharge before driving to work.  When I have no plans, I drive on the reduced charge and hope for the best.  Sometimes, I have the option to charge once I get to work, which allows me to run those after-work errands.  To date, my efforts have paid off as I have lost less than 5% of the original driving range after 41 months of ownership.  (Mitsubishi warrants that 80% of the charge capacity will remain useful for ten years.)