What is the discharge rate of an EV battery if not used for a week or two?
The overall estimate is 2-3% per month for lithium-ion (Li-ion) batteries. It is more for Nickel-metal-hydride (NiMH) but this technology has pretty much been superseded by Li-ion. Nissan, for example, say that in most circumstances leaving a Leaf unplugged for a week or so, with a moderate amount of charge, will lose a small amount of range.
Do batteries run flat?
No! However, they will discharge to the point where they will not operate the car. Sophisticated battery management systems actually mean the lithium-ion batteries are never fully charged and never fully discharged. In some cases they only allow the state of charge (SoC) to be between 20% and 80%, because this is a much better way to maintain the battery state of health (SoH). The car will show this as fully charged and fully discharged.
What is the life of an EV battery?
This is a difficult question to answer because it depends on many factors – the question would be equally difficult were it relating to the life of an engine in a petrol or diesel vehicle.
There are many Toyota Prius cars from over 20 years ago that are still running on the original battery. Many Teslas have done 200,000 miles or more and are happily running on 15 year old batteries. My own EV is 5 years old and I have not noticed any fall off in capacity. Research suggests that battery state of health (capacity) drops by about 2% per year.
Generally, it is accepted that when battery capacity drops below 80%, it is time to do something. However, this does not mean that the battery needs to be replaced, and you can just keep using it. It is already commonplace for battery packs to be repaired; new cells or modules replaced for example. Clearly this needs special training and skills but this is already happening.
Electric car batteries have improved considerably in recent years and advancements in battery technology and manufacturing will continue. Modern EVs have sophisticated battery temperature control systems to prolong life, even when fast charging is used (which generates heat). There are things EV owners can do to help prevent loss of battery capacity. For example, more charging slowly at home rather than fast charging. High temperature is the enemy of li-ion batteries. Some say that it is best to never fully charge or fully discharge a battery, so some owners try to keep the battery above 45% and below 85%. I’m not a great believer in this as the battery management system mostly does the job for us anyway – but it will certainly do no harm.
Things vary significantly between manufacturers but if pushed for a figure, I see no reason why the vast majority of batteries would not last for 15 years and be able to cover over 100,000 miles. Over this time, they need little or no attention, unlike an internal combustion engine that would have required lots of work. Remember, even at this point, the batteries can almost always be repaired rather than replaced. If not, they still have a second use value for home storage systems.