How does battery degradation work in EVs?

One of the key concerns with prospective EV owners is that they will have to replace the battery after a period of time, such as 8-10 years, at significant cost, as by that time period the battery’s ability to hold a charge will be heavily compromised, thereby substantially reducing the battery range. 

The very nature of rechargeable lithium-ion batteries means that their ability to hold a charge gradually degrades over time, as they go through charge and discharge cycles. Practically, however, most car manufacturers warrant the batteries of their EVs to retain at least 70% of their original charge capacity after a period of eight years or 160,000km, and will generally replace or otherwise fix the battery if a customer’s car falls below this standard. 

Nevertheless, there are various actions that customers can take to help ensure that their battery continues to hold a higher level of charge.

What battery chemistry does your EV use?

You EV’s battery chemistry is crucial to determining how you should charge it, and to what level (commonly referred to as state of charge or SoC %).

Currently, electric vehicles typically use one of two battery chemistries. The majority of EVs available for sale today continue to use an NMC (nickel-manganese-cobalt) lithium-ion battery chemistry. However, some EVs such as entry-level, rear-wheel drive versions of the Tesla Model 3 and Y utilise an alternative LFP (lithium-iron phosphate) battery chemistries. Consult your vehicle’s manufacturer to determine the chemistry that your EV uses. 

Generally speaking, vehicles with an NMC battery chemistry have a ‘sweet spot’ with regard to the battery’s state of charge in order to minimise battery degradation. This range is usually from 20 per cent to 80 per cent, such that during everyday driving, you should ensure your battery isn’t charged above 80 percent of its total capacity, and it should also not drop below 20 per cent either.

Effectively, this means that in day-to-day use, it’s safe to assume that the usable range of an EV may be the equivalent of 60 per cent of its total battery capacity. Of course, charging to 100 per cent for the occasional road trip or other long distance journey (and letting the charge drop to below 20%) is perfectly reasonable, and shouldn’t have a substantial impact on battery longevity.

However, constantly charging an NMC equipped vehicle from 0-100% over an extended period of time may have a significant adverse impact on battery longevity, and could cause the vehicle’s battery to lose its original charge capacity more quickly.  

For vehicles equipped with LFP batteries, these restrictions may not apply to the same extent. Tesla, for example, suggests that regularly charging LFP-equipped Model 3 and Model Y cars to full will have zero impact on battery longevity.

Your best bet is to consult your EV’s owner’s manual and other documentation for guidance from the manufacturer on whether there are any suggested charging limits or restrictions during everyday driving to prolong battery life. 

The impact of DC fast-charging

Frequently using publicly available DC fast-charging stations to charge your EV is another factor that may also shorten battery longevity. 

DC fast charging results in high currents being created within the battery, which in turn increases the battery temperature, burdening it with additional strain that could degrade the longevity of the battery.

Instead, charging your vehicle overnight through slower AC charging (Level 1 and Level 2 charging, if available at your residence) should be the primary way to charge your electric vehicle. This also carries the added benefit of allowing you to leave home each morning with a full charge.

Nevertheless, some newer EVs come with an advanced form of battery pre-conditioning, that may further reduce any longer-term impacts associated with frequently using DC rapid charging stations.

If the driver uses the car’s in-built GPS system to navigate directly to a fast charging station, the car will automatically begin cooling or heating the battery as appropriate given the ambient temperature and other environmental conditions, to ensure it is at the optimal temperature by the time it arrives at the charger.

This may not only reduce stresses on the battery and thereby maximise its longevity, but could also reduce charging times as the battery temperature doesn’t have to be significantly raised or lowered after the car is plugged in. 

Nevertheless, perhaps the key takeaway here is that the safest bet is to ensure that charging your EV’s battery at a DC fast charging station is done sparingly, and used to quickly ‘top-up’ the battery, rather than being solely relied upon to charge your battery from empty to full on a regular basis, or it becoming the only way that your EV is charged. 

What do most manufacturers guarantee?

As hinted at above, most carmakers provide a dedicated battery warranty for your EV that is separate from the warranty covering the remainder of the vehicle’s components.

The current industry standard for EV battery warranties is eight years or 160,000 km, whichever comes first. In most cases, the manufacturer will guarantee that the battery will retain at least 70 per cent of its original charging capacity over this time period.

Of course, some manufacturers do vary slightly from this norm. BMW, for example, covers the batteries of its electric vehicles to eight years or 100,000km, while Nissan’s refers to a minimum retention of ‘9 bars out of 12’ (i.e. 75 per cent) for its Leaf. 

Meanwhile, Tesla exceeds the industry threshold with its Model 3 in Long Range and Performance guises, with each of these variants guaranteed to eight years or 192,000 km.

Perhaps a key development in this space are the EVs that have been developed on Toyota’s new dedicated electric vehicle platform, e-TNGA, namely the Toyota bZ4X, Subaru Solterra and the upcoming Lexus RZ.

For the bZ4X in particular (but likely to be applicable to other e-TNGA cars), Toyota has claimed that it’s aiming for the battery to retain 90 per cent of its original charge even after 10 years or 240,000 km of use, a substantial improvement on the battery warranties offered by most electric vehicles today.

MORE: How to maximise electric vehicle battery longevity?