In-depth

When Will Your EV Battery Need Replacing?

The battery is one of the most expensive parts of an EV. Will you need to replace it?

July 22, 2024       12 min read
ev battery charging

When we think of a battery, we tend to think of :

  1. The 12-volt lead-acid battery that sits under the bonnet of all our cars (the one that seems to fail us at the worst possible moment).
  2. The battery in our phone (that after a few years seems to run out of charge in mere hours).

An EV battery is far more sophisticated, featuring a high-tech management system and active thermal control.

How long will an EV battery last? Honest answer: We don’t (yet) know.

Almost all EV batteries are still in the vehicle. However, we have two great sources of info to help us predict:

  1. Telematics data from organisations that record data from thousands of EVs (as they are driven).
  2. Extensive lab research and testing of lithium-ion batteries.

When will the battery need replacing? Current data says: Probably never.

Will my battery need replacing?

There’s a major distinction between early electric cars (2011-2016) and newer cars (2016+).

Among those newer cars, US research shows only 1% have had battery replacements [1] (which were under warranty). Outside of this 1%, some vehicles have had recalls.

(It’s likely that for NZ, this percentage may be even lower – due to the different EV models that comprise our fleet.)

In New Zealand, those early cars are mostly Nissan Leafs – one of the first EVs to market. NZ imported thousands of these (second-hand) from Japan.

What’s up with the Leaf?

The Leaf is a special case and has created a misconception that all EV batteries are alike in NZ.

The Leaf is early technology, and the small-capacity battery has no form of active cooling. So, 10-12 years on, the Leaf has considerably less capacity than the original, leading some owners to replace the battery.

It’s worth noting, that the Leaf is a beloved car, and outside of the battery issues – claimed one of the top spots in Consumer NZ’s car reliability guide.

The battery is covered by warranty

All new EVs have a battery warranty (typically 7-8 years), which covers any battery failure.

What about after this, and the loss of battery capacity?

For context, many parts of our cars degrade.

In a combustion car, this includes sparkplugs, timing belts, alternators and starter motors, to name a few. Even engines can get reconditioned.

However, the battery is an expensive part of an EV, and its degradation limits the distance it can travel on a single charge.

How do lithium-ion batteries lose capacity?

Lithium-ion (Li-ion) batteries have different chemistries – indicating the elements used in the positive electrode (cathode).

An EV battery pack comprises modules, which in turn are made up of cells.

For example, NMC622 means 60% nickel, 20% manganese, and 20% cobalt. LFP means Lithium iron phosphate. Different chemistries behave differently and have pros and cons.

Lithium batteries lose capacity over time, depending on how they are used. For EVs, battery capacity dictates how far you can travel on a single charge.

However, the way the battery is maintained can significantly lengthen its lifespan.

There are two ways the battery loses its capacity.

Li-ion cells lose capacity during storage (NMC chemistry)

Extensive lab testing shows a battery loses capacity if stored at high temperatures (> 30 degrees) and in a highly charged state [2]. When the voltage is higher, the positive electrode is more reactive to the electrolyte in the cell.

How to prevent: NZ doesn’t often get that hot. But leaving your EV at 100% charged in a stifling hot garage day after day won’t help. If it needs to be left anywhere – leave it at ~50% charge.

Li-ion cells lose capacity during the charge-discharge cycle

What’s a cycle?

Battery lifetime is measured in cycles. One full cycle means charging the battery from 0% to 100%, then completely discharging back to zero again.

However, this can happen in bits! If your car was at 80% and you drove until 60%, you would have to do this five times before it counts as a full cycle. That’s because each trip and charge used only 20% of the battery (5 x 20% = 100%).

Cells lose capacity more quickly when charged and discharged over extended depth of discharge ranges.

What’s depth of discharge?

If you drove your car from 100% down to 10% — that’s a depth of discharge (DoD) of 90%

How to prevent: Lots of short drives, followed by charging are better than long drives with infrequent charging.

Long drives are fine. But charge when you can. EVs are meant to be driven!

Best practice: Plug your car in every night. Don’t drive it for a week, and then recharge it over the weekend.

How much does depth of discharge make a difference?

Lab test results of an NMC622 cell (22,000 hours of continuous charging and discharging):

  • Cells that did the majority of their cycling at 25% DoD lost only 8% of capacity.
  • Cells that were cycled at 100% DoD lost 27% of capacity.

Some Li-ion cells lose capacity when continually charged to 100%

As a battery becomes full, the voltage increases. Testing of various Li-ion cells (particularly those with a high nickel content, such as NMC 811) shows a volume change in the cell at higher voltages [3].

This results in capacity loss, which is why charging to 75-80% most of the time is good practice for NMC batteries. This is not the case for LFP batteries [4].

So, if you can, set a charging limit of 75-80%. Charge to 100% when you are going on a longer trip.

What is the real world data telling us?

Recurrent (US) have been tracking car usage data for years.

Their Tesla data shows that, after 5 years, a Model 3 has lost 8% of capacity; but this is not linear. As with other testing, there is an initial loss (about 6%) before stabilisation [1].

The Tesla 2023 impact report shows that after 320,000 km (200,000 miles), Model Y/3 lost 15% of its capacity. Model S/X have lost 12% [6].

Why the difference between Model Y/3 and S/X?

The Y and 3 typically have smaller batteries (therefore higher depth of discharge). It also points to the diversity of battery chemistries used in vehicles. E.g. some newer 3/Y cars have LFP batteries (this will lead to longer life). This makes the data a generalisation.

What’s the difference with LFP batteries?

An increasing number of EVs are using LFP chemistry.

  • LFP (Lithium iron phosphate) cells have a longer cycle life than NMC (i.e. last even longer), and don’t suffer from the volume expansion that occurs with NMC cells at higher voltages (as the voltage stays lower in LFP) [4].
  • LFP cells are significantly less affected by depth of discharge and being left at 100% state of charge.
  • However, LFP cells have poorer performance at lower temperatures (i.e. potentially slower to charge).

What can I expect from my EV battery?

The mounting evidence shows that modern lithium batteries will last a very long time.

It seems extremely unlikely that a new EV bought today would ever need its battery replaced (except for a fault).

  • Newer, larger batteries (60 kWh or greater) have a smaller depth of discharge, which leads to less degradation.
  • Manufacturers have sophisticated technology to manage heat (such as charging curves and active cooling).

Example: if a battery with a range of 450 km had 1,000 cycles, that’s equivalent to < 450,000 km. Even if it lost 20% of capacity – it would still achieve a range of 360 km – after all those hundreds of thousands of kms.

What happens if the range gets lower and lower?

What we’ve seen with the Leaf in NZ is that owners upgrade to a newer car when the range no longer meets their needs.

Someone else is happy with a cheaper car that does less range.

We’ve also seen that when the range gets untenable (i.e. too low to be useful), the battery modules are sold as home storage, and a battery from a scrapped vehicle is put into the car.

For many new EVs, this process may never happen (the entire car may be at end-of-life before considering battery replacement).

How does battery health affect the EV’s value?

Evidence from the Nissan Leaf shows that battery health is correlated with the car’s resale value.

A Leaf with a battery State of Health (SoH) of 70% is worth less than a car with an SoH of 87%.

Right now, New Zealand lacks an independent nationwide battery certification process.

Many other countries have these systems in place (such as Altelium and Aviloo). This allows battery health certificates to be issued for any EV – providing certainty to buyers and insurers.

What is ‘usable’ battery?

Manufacturers list battery capacity as either gross (total) or net (usable).

This is confusing as some makers don’t state which is which.

  • Gross (total) – the complete capacity.
  • Net (usable) – how much capacity the vehicle is allowed to access.

Why the difference?

To maintain lithium-ion batteries in good condition, they should not be allowed to be empty (0% charge) or full (100% charge). Note that charging to 100% in a LFP is not considered to be an issue.

This buffer also allows for maintaining capacity despite degradation (i.e., the artificial buffer gets smaller as the battery degrades).

Total vs Usable Battery Capacity

How does this work? And why?

The vehicle maintains an artificial buffer (around 2-5% of total capacity) that prevents the driver from completely draining or fully charging.

Sophisticated electronics manage this ‘buffer’ called the Battery Management System (BMS).

Even though your dashboard might say 100% – there is still some room in the battery.

When your battery ‘seems’ empty, more electricity remains. Most EVs will go into ‘snail mode’ to prevent the complete draining of the battery. Snail mode severely restricts speed to avoid stranding.

What are the pros and cons of LFP and NMC?

ChemistryProsCons
NMCMore energy dense (longer range for same size battery).
More power
Charges faster.
More expensive
Less durable
Conflict materials (cobalt)
LFPLonger lifespan (slower degradation)
Lower cost
Safer (lower risk of thermal runaway)
No conflict materials (cobalt)
Less energy dense (shorter range for same size battery).
Less performant
Slower charging in cold temperatures

Sodium-ion vs Lithium-ion

Sodium-ion batteries are coming on the market (you can already buy AA cells).

It will take some time before production efficiencies lead to lower prices, but we can expect them in energy storage systems and entry-level EVs.

Sodium-ion Pros and Cons

  • Good power density (high charge and discharge rates)
  • Can be fully discharged without damaging cycle life (great for shipping)
  • Safe
  • Low volumetric energy density (cells are much larger for the same amount of energy.
  • Sodium is an abundant mineral.
  • Probable use: energy storage and low-range EVs.

References

  1. Recurrent Auto: How long do batteries last? (ref)
  2. Ecker, M., Nieto, N., Käbitz, S., Schmalstieg, J., Blanke, H., Warnecke, A., & Sauer, D. U. (2014). Calendar and cycle life study of Li (NiMnCo) O2-based 18650 lithium-ion batteries. Journal of Power Sources, 248, 839-851.
  3. Bree, G., Hao, H., Stoeva, Z., & Low, C. T. J. (2023). Monitoring state of charge and volume expansion in lithium-ion batteries: an approach using surface mounted thin-film graphene sensors. RSC advances, 13(10), 7045-7054.
  4. Preger, Y., Barkholtz, H. M., Fresquez, A., Campbell, D. L., Juba, B. W., Romàn-Kustas, J., … & Chalamala, B. (2020). Degradation of commercial lithium-ion cells as a function of chemistry and cycling conditions. Journal of The Electrochemical Society, 167(12), 120532.
  5. Extensive seminar from Dr Jeff Dahn – Professor in the Department of Physics & Atmospheric Science, Dalhousie University.
  6. Tesla Impact Report 2023 (ref).

This graph from Dr Dahn shows how depth of discharge affects capacity. The cell was charged and discharged non-stop.

The black line (16680 cycles) shows charging and discharging 25% at a time. Then every so often, the cell is charged to full – showing the actual capacity (the black dots) — and so on for each colour.

It demonstrates the incredible lifetime of Li-ion cells.

li ion cycles

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