Volkswagen’s current electric e-Golf churns out 115 horsepower with 199 ft/lbs of torque. Variable battery regeneration gives options for deceleration vs. recharging. At full regen, one hardly needs the brakes. Once again, Volkswagen has worked hard to top the field. The e-Golf easily leads the other electric car offerings in its class for trim, driveability and comfort. The distance per charge is just over the competition, and that leads to a serious discussion. Battery longevity will no doubt become the leading issue with electric cars. Everyone is aware that batteries all lose their capacity over time, but few understand all the variables.
Battery life is defined as ending when a battery reaches about 2/3 of its capacity, sometimes referred to as Cycle Life. How quickly it reaches this limit has a number of factors. Depth of Discharge cycles. Operation and storage in hot or cold environments. High peak charge voltages. Fast rates of charge or discharge. Amount of time left discharged.
Most hybrid cars are still chugging around after 10 years, although a few have needed battery and/or power control/charging modules replaced. That’s because the gas engine is constantly recharging the battery, not allowing it to drop to a low level of charge, or stay discharged for very long. Cell phones on the other hand are commonly left to discharge near empty, and sometimes left at low states of charge for long periods. It is common for cell phones to be at 70% or less of their original capacity by 2 years old, and not usable before 3 years old. Electric car batteries will also degrade over time, and different consumers will have different results. The real question will be when is said battery unusable for the needs of said consumer. The consensus more or less matches the VW warranty of 8 years or 100,000 miles against 70% capacity. So that is 70% of the original stated range with no hills, at 68 degrees Fahrenheit, no climate control, and no electrical consumers turned on.
Battery Capacity and Cycle Life are related, but not the same. Battery Capacity is the amount of available energy before the battery reaches about 3 volts per cell. Cycle life refers to how many Charge / Discharge cycles one gets before the battery reaches 70% of its rated capacity. The indicated driving range on the dash of an EV is only loosely related to both these factors. Generally when an EV is new, the battery is only allowed to charge to 80% capacity and discharged to 30% capacity. As the car ages, this range is extended to allow minimal reduction in driving distances. It’s not just a trick, this also extends battery life.
Higher depth of discharge (DoD) reduces cycle life. A lithium battery that consistently goes from full charge of around 4.2 volts to full discharge at about 3 volts will only have 300 to 500 cycles before reaching 70% capacity. The same battery discharging only 10% will get well over 4000 cycles before reaching the same 70% capacity. Loosely translated, a vehicle that is consistently driven until most of the available miles are used up will need a battery much sooner than one which is driven shorter distances between charges. As just a feasible approximation, the best battery life is achieved when the usage before charging is about half the indicated driving range when the car is new, and about 1/3 the indicated driving range when the car is several years old.
Batteries lose cycle life when operated in hot weather. Cycle life drops by 20% at 86 F, and by 40% at 104 F. Electric car batteries just won’t last as long in desert conditions. Self discharge rates also increase with temperature. Lastly, in a hot climate, the battery should not be charged at a high rate or up to fully charged. Either condition will reduce cycle life considerably, and may damage the battery.
Batteries lose capacity in cold weather. The optimum temperature for a battery is around or slightly below 68 degrees Fahrenheit. A Flooded Battery (lead-acid) like your car’s starting battery loses about 10% capacity for every 10 degrees below 50 F. The six different Lithium Ion batteries vary in capacity and ability to deliver power with colder temperatures. Most are down to under 50% capacity by 32 F. Add to the fact that the vehicle’s cabin heat must be generated from the battery, and an electric car loses range pretty radically as the weather gets colder. Furthermore, battery charging is inhibited at lower temperatures. Some EV’s have electric blankets to warm the battery during charging, and of course the power for the blanket is coming from the grid. Charging a lithium battery at freezing temperatures may very well damage it.
High peak charge voltages quickly reduce battery cycle life, but render more capacity. The optimal voltage for maximum cycle life is 3.92 volts, but the capacity at that state of charge is only 58%. Most consumer devices are set to keep the charge up at 4.2 volts so the device is always at a high capacity and ready for use. Industrial batteries and electric cars tend to use lower peak charge voltages to extend their useful cycle life.
The rate of charging is important. Charging from full discharge to full charge in less than one hour can damage a battery, but even if damage does not perceptibly occur, the cycle life is cut by up to half. Most modern chargers are “smart” as are the batteries. The charger knows the state of charge, the temperature and the condition on the cells and charges accordingly. The charger can fast or ultra-fast charge up to 70% then a slower saturation charge finishes the charging. Ultra-fast charging always stresses the battery. Fast charging will damage a battery that is cold, hot or too old and weak.
An in depth discussion of rate of discharge of EV batteries is complicated. The variables are complex and depend on the pilot more than the equipment. Suffice it to say that driving very briskly or over lots of hills will diminish battery cycle life and capacity. Without including information of the types of lithium ion batteries, basically the larger the battery in kilowatt/hours (kWh), the more sustained high power usage it will allow with less diminishing of the cycle life and capacity. Without naming brands, the more expensive electric cars also have much larger batteries, and are likely to render a far longer usable lifespan.
The effect of leaving an EV discharged for long periods is not well understood. The biggest variable is storage temperature. The hotter the storage environment, the less capacity that can be restored by a large factor. The second biggest factor is the state of charge of the battery to be stored. At San Francisco temperatures of 40 F to 70 F, a battery stored for a year at 40% charge will still have almost all its capacity. The same battery stored in the same environment at 100% charge will recover just over 80% of its capacity. A battery stored at 104 F for a year at 40% charge will recover 85% capacity. The same storage conditions at 100% charge will recover 65% capacity, which is to say that the battery is marginally usable. In the unusual circumstance that a battery is stored near or at fully discharged, the rules completely change. Leaving an EV for months in a completely discharged state will definitely lower the capacity. After a year of storage while completely discharged, the battery may only yield a dozen miles of driving or be completely unusable. Additionally, the self discharge characteristic might lower the voltage below 1.5 volts per cell, which effectively renders the battery unrecoverable.
It would seem that the optimal battery charger would be something like a psychic charger that knew exactly when a consumer was going to take the device or car off the charger and use it. Fortunately, Volkswagen provides a programmed charge application for the e-Golf. It charges the battery to 40% capacity and holds it there, then tops off the charge just before the consumer needs to drive. Unfortunately, VW charges for this service by the month, a marginal enticement for proper battery care.
It should be noted that the graph describing capacity loss is not linear at all. Once a battery degrades to below 70% capacity, the curve accelerates and the battery can drop to 50% capacity within months, often to completely unusable within a year. For the technically minded, try this link to BatteryUniversity.com for a discussion about lithium batteries and how to keep them working longer.
It would seem that the optimum way to own and drive an electric car might be to lease one. Bear in mind that the cost to replace the battery in an electric car is not prohibitive, but it is a certainty. And if a car worth $36,000 new is worth $18,000 when it needs the $6000 battery, then what. If it is only worth $10,000? With gas and diesel powered cars, only a fraction require the huge expense of an engine during their service life. But every battery will fail eventually. Our guess is that a 10 year old electric car may not have the same sort of value as a petroleum powered cousin. That will depend of course on the perceived value generated by the public.
Over 1 million electric cars have been sold so far this century as of 2016. Aside from the early pioneers, most of which failed about a century ago, the current market of electric vehicles reflects more of an upward trend towards saving the planet than saving one’s wallet. And that may just be a laudable sacrifice that maybe more folks should make.