By Henrik Moller, Flip the Fleet

The traction battery is the heart of an electric vehicle. The amount of energy it can store and then deliver to the powertrain determines the EV’s range, efficiency and overall practicality. 

People mainly rate the size and power of a combustion vehicle by its cylinders’ CC rating.  With an EV it’s the size of the battery that matters most.

Saying that a Nissan Leaf has a 24kWh battery is a bit like giving the tank size of a combustion vehicle, but there’s a complication – an EV’s “tank size” shrinks as the battery ages. 

Just as you have to charge your cellphone more as it gets older, so too the EV battery’s energy-holding capacity gradually fades.

A group of New Zealand EV owners have clubbed together in an organisation called ‘Flip the Fleet’ to, among other things, measure the rate of capacity fade of their batteries.  Each month they report their battery’s ‘State of Health’ (SoH) – the percentage of the original energy-holding capacity it can still hold.   

The results up till recently showed that the 30kWh Leaf batteries were fading fast.  Thankfully this turned out to be a false alarm. A firmware error in the lithium-ion battery controller was under-reporting the amount of energy stored in the 30 kWh battery.

Once the instrumentation is patched, there is no statistical evidence for battery capacity fade to be different on average between the 24kWh and 30kWh variants.

The results so far are encouraging – the Nissan Leaf batteries are losing around 3 percent of their capacity every year.  That means they are lasting better on average than Nissan’s expectation of 20 percent loss after 5 years (the red dot in the graph). 

We now need more time to go by to measure capacity fade in New Zealand conditions – the current capacity is largely determined by the way the batteries fared in Japan and UK before they got here.

It’s fair to consider the Nissan Leaf battery fade rate as a worst-case scenario for New Zealand’s EVs because the model is an early example of rapidly improving battery chemistry and technology.  The manufacturers are scrambling to make them last longer.  Also, the Nissan Leafs are one of the few EVs that do not have an active “thermal management” of the batteries. 

As batteries are charged, they get hot.  A combination of hot temperatures and high state of charge hastens battery degradation.  The newer EV models nearly all have a system of blowing air over the battery pack, or passing a cooling liquid past them. 

The very best system is in the Tesla that has a liquid cooling system which both cools the battery when needed, and warms it up in cold winter conditions. The battery is thereby maintained at the optimum temperature for energy efficiency and battery life.  So far Tesla batteries have only dropped about 1 percent of capacity each year, a third of the rate seen in Nissan Leafs.      

Although the picture so far is reasonably encouraging, it’s important to realise that at some stage an old battery will tip over – they fade fast when the electrodes become denatured.  It’s impossible to say when this final end-of-life phase will kick-in, so we cannot predict how long the current gradual decline will last for. That’s often the rub with a new technology like EVs – the only way to know for sure if the battery will last for 15 years is to use one for 15 years!  

In the meantime, we must help New Zealand mechanics gain the skills, equipment and supplies to service and refurbish batteries. “Hot swapping” damaged cells and rebalancing the battery pack can keep the car’s range up for longer. Old ones might be reused – there is already a strong demand for cells from crashed Leafs to make home-built eBikes and replace weak cells in other cars. They may end up as a “power wall” to store energy for household use.  Eventually they must be recycled and disposed of safely. 

Most of all we need to secure a reliable supply of new and affordable cells to replace the worn-out originals in the EVs themselves.  EVs require very little repair and maintenance, so the EV’s body and other parts will far outlive the original battery, if not several batteries. 

It would be an awful waste if we don’t gear up to be able to replace the batteries when needed.  Eventually mechanics will occasionally swap batteries rather like they eventually replace the transmission on a gracefully aging combustion vehicle – it will be just one of the big jobs that are part of keeping the car rolling.  

So how long will the EV batteries last?  The bottom line is that we don’t know for sure, but the initial results are encouraging for the earliest models. The new thermally managed batteries are likely to do even better. And there are exciting times ahead for mechanics to apply new battery repair skills to keep our nation’s wheels turning.

• Henrik Moller is a retired sustainability scientist from Dunedin and co-founder of Flip The Fleet. More information on EVs is available on discussion and resources pages of www.flipthefleet.org. Queries can be emailed to wecan@flipthefleet.org.

Figure caption: The graph includes all 2,600 state of health measurements submitted to Flip the Fleet so far for 24kWh (upper) and 30kWh (lower) Nissan Leafs.  Nissan’s estimate for decline is 80 percent SoH after five years and is shown by a red dot. The data for 30kWh Leafs has been retrospectively corrected by the battery management firmware upgrade released by Nissan in New Zealand in August. 

Photo caption:  The 24kWh Nissan Leaf battery pack consists of 48 modules, each with 4 battery cells, so there are 192 cells altogether.  The pack helps stabilise of the car because it weighs 218kg and sits below the seats and rear foot space.