How Usage and Management Affect Longevity
Car makers are extending the driving range of the electric vehicle to resemble a gasoline-powered car. This requires larger batteries that grow exponentially with the distance driven. Figure 1 illustrates the estimated driving ranges with different battery systems and hydrogen as a function of size
Doubling battery size does not extend the driving range linearly and the vehicle becomes inefficient with increasing weight. Li-ion performs better than lead acid in energy density, but no battery meets hydrogen with a fuel cell, or fossil fuel feeding the traditional internal combustion engine (not shown). Extending the driving range with a larger tank is almost negligible compared to oversizing a battery. There is a threshold as to battery size and weight in a vehicle; going beyond a critical point has a negative return and efficiency suffers because of increased vehicle weight.
The EV battery also ages and the capacity fades, but the EV manufacturer must guarantee the battery for eight years. This is done by oversizing the battery. When the battery is new, only about half of the available energy is utilized. This is done by charging the pack to only 80% instead of a full charge, and discharging to 30% when the available driving range is spent. As the battery fades, more of the battery storage is demanded. The driving range stays constant but unknown to the driver, the battery is gradually charged to a higher level and discharged deeper to compensate for the fade.
Once the battery capacity has dropped to 80%, the oversize protection is consumed and the battery maintenance system (BMS) applies a full charge and discharge. This exposes the EV battery to a similar stress level of a mobile phone and the driver begins noticing reduced driving range. Battery replacement may become necessary but the cost will be steep and higher than a combustion engine.
The EV begins to impersonate a mobile phone in terms of obsolescence when the battery fades. This may be the time when the buyer is flooded with faster and flashier models; something the smartphone user is all too familiar with, but price will be the shocker. It’s still too early to tell how long an EV battery will last. Some say the battery will outlive the car and find secondary application in energy storage systems.
Driving habits and temperature also affects aging, a characteristic that came to light when EV batteries operating in a warm climate faded prematurely. It was learned that keeping a battery at elevated temperature and high state-of-charge causes more stress than aggressive driving. In other words, keeping a fully charged Li-ion at 30°C (86°F) and above hastens the aging process more than driving at a moderate temperature. Many EV batteries include liquid cooling to reduce heat-related battery fade.
Dynamic Stress Tests (DST) on Li-ion
The Li-ion batteries in the above table perform well but the largest capacity loss occurs with the pack that is charged to 100% and discharged to 25% (black stars). Cycling between 85% and 25% (green) provides longer service life than charging to 100% and discharging to 50% (dark blue).
The lowest capacity loss occurs when charging Li-ion to 75% and discharging to 65%. This, however, takes oversizing to the extreme and the battery is underutilized. Such practice is applied in satellites to achieve high cycle life and less for terrestrial applications as it increases cost, size and weight beyond a reasonable point of return. The dynamic stress test does not include a battery that is charged to 100% and discharged to zero, as is the case with a mobile phone. A full cycle provides the best battery utilization but reduces longevity.
Batteries tested in a laboratory do not always replicate true life conditions, and the results tend to be better than experienced in field use. In a lab environment, batteries are cycled over a period of a few months, often at controlled temperature and with an ideal charge and discharge regime. Random usage in real life adds the exposure to heat, vibration and harsh charging practices.
You must be logged in to post a comment.