Not long ago, Xiaomi gave a relatively official response to the issue of tram life:SU7 can be used for 15 years or 300,000 kilometers under normal use.
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As soon as these words came out, the whole network exploded, and everyone expressed disbelief.
In the past, regardless of whether it was ternary lithium or lithium iron phosphate, our impression was that batteries would need to be replaced after 8 years.
The battery cell warranty of most car companies is 8 years. After all, car companies are not stupid. A battery costs more than 100,000 yuan and half of the car is built in it. Who dares to just replace it?
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Later, BYD, Geely, GAC, etc. began to offer lifetime warranties for battery cells, all of which were betting on the replacement cycle of car owners.because
According to JDPower,Chinese private car owners replace a car every 4.5 years on average, and the average driving mileage is less than 100,000 kilometers. If you can drive it for more than 10 years, you will be one of the 1% of car owners in the country.
Then, before changing the battery cell, you will most likely have changed the car.
But the problem is, Xiaomi says its lifespan is 15 years, so why is your warranty still 8 years/160,000 kilometers?
Because, after all, this matter is final, but there are still too many variables in 15 years.
According to academic circles, a battery is considered “end of life” when its capacity declines to 60-80%.
So when we buy a car, the warranty also states that the battery will be replaced when the battery degrades to 70%.
There are actually many reasons for a battery to age. Not only will it be consumed during use, but it will also naturally wear out even if you leave it alone.
So, don’t think that if you have two boxes of electricity a year, you can pass one car down to three generations.
The battery life that everyone often talks about actually refers to the consumption and recharging situation, that is, the battery cycle. There is only one indicator to measure it, and that is the number of cycles.
This means that under normal circumstances, how many cycles does it take for a battery to be fully charged and fully discharged to drain it dry? So why does circulation cause attenuation?
Because the battery is charging and discharging, lithium ions are essentially traveling between the positive and negative poles.
However, some lithium ions are gone as time goes by.
For example,As the environment changes, some lithium ions choose to lie down and are reduced to metallic lithium (lithium dendrites) at the negative electrode. As a result, there are fewer lithium ions actually working in the electrolyte, and more electricity can be charged. If less, this process is called “lithium precipitation”.
And as time goes by, the electrode material will also react with the electrolyte to form an SEI film (solid electrolyte interface film). This thing cannot block the entry and exit of lithium ions. It is not a bad thing in the first place. It can also inhibit the decomposition of the electrolyte and increase the lifespan. .
However, the formation process of the SEI film will also consume lithium ions. If the growth is not controlled, it will also destroy the electrode material and cause battery aging.
Therefore, how long a power battery in a car can be used is related to the microscopic chemical reaction of the battery.
This reaction is mainly affected by two things: whether the battery can work on its own, and then it depends on how you use it.
Let’s talk about batteries first. The most critical thing here is the cathode material, because this is the home of lithium ions. If the home is gone, how can lithium ions still work?
The crystal structure of lithium iron phosphate is more stable than ternary lithium and is not easily destroyed. Therefore, lithium iron phosphate has a longer life and the battery has more cycles.
Generally speaking, the number of cycles of ternary lithium batteries is 500-1500 times, while that of lithium iron phosphate is about 2000 times.
However, if you buy a tram, it can basically be fully charged and discharged more than 1,000 times. If your battery life is 600km, that is 600,000 kilometers. If you drive 30,000 kilometers a year, it can be driven for 20 years. If you include losses, this Isn’t it consistent with Xiaomi’s statement?
However, the bigger problem lies in this loss, which depends on how you use the battery. This is related to the lithium precipitation and SEI film discussed before.
We can understand that as time goes by, the battery will eventually end its life, but some additional factors will speed up this process, such as temperature, overcharge and overdischarge, and charge and discharge rates. Temperature has the strongest impact on battery life here.
The battery is suitable for an environment of 15 – 35 ℃, and is optimal at 25 ℃.
If the temperature is too high, it will accelerate the growth of the SEI film. If the temperature is too low, it will cause the precipitation of lithium dendrites. In short, the battery life will be shortened.
Take lithium iron phosphate as an example. Every time the ambient temperature increases by 1°C, the number of cycles will be reduced by about 100 times. If the battery is kept working at 40°C, the lifespan may quickly decrease to 1 year.
However, we cannot control this process. It depends on how well the electronic control of the car company is done. What we can do is try to prevent the tram from being exposed to the sun outdoors.
But considering the price of the parking space at my house… I would still choose to park under the tree.
The second is the problem of overcharge and overdischarge.
Overcharging will accelerate the precipitation of lithium dendrites, which will not only shorten the life, but may also cause spontaneous combustion or explosion; over-discharging will also cause damage to the negative electrode structure and accelerate battery aging.
Just like our mobile phone, every time it is turned off, or even turned off and forced to turn on, it is very easy to damage the battery.
Fortunately, trams have a battery locking mechanism, which generally won’t let you fully charge or run out of electricity. For example, in the early years, Roewe reported a 20% battery lock.
This standard actually varies from car to car. Generally, when the battery is low, the SOC (remaining battery capacity) is difficult to estimate. It is normal to maintain 10%, and it will maintain about 3% before it is fully charged.
In addition, overcharging will cause greater damage than overdischarging. In addition to being prone to spontaneous combustion, experiments have shown that when the battery SOC is 75-100%, the battery decay rate is twice as fast as that of 0-25%.
so……It's better to run out of battery than to charge it fully. If you don't use the car for a long time, it's best to charge half of the battery and leave it alone.
The last thing is the charge and discharge rate, which is what we often call 4C and 5C.
Theoretically speaking, if we often use ultra-fast charging, it will indeed increase the current and temperature during charging, thereby increasing the formation of negative SEI film and reducing battery life.
But the key depends on how big the impact is.
According to foreign research,After using fast charging, the full charge time of lithium iron phosphate was reduced to 20 minutes, and after 4500 cycles, the capacity was only lost by 17%, which is basically the same as normal charging loss, so we don’t have to worry about this.
Speaking of which, everyone knows what the problem is.
If it is well maintained, the power battery can last 15-20 years according to its own cycle number, but this depends not only on the car owner, but also on the battery management system (BMS).
Whether the 15 years mentioned by Xiaomi is reasonable or not mainly depends on the battery manufacturer.
BYD's Blade Batteries and CATL's Shenxing Batteries are both lithium iron phosphate batteries, which are inherently gifted with longevity. However, in order for ternary lithium to achieve longevity, it will test the battery management technology of the battery factory.
For example, Ningde's Kirin battery adds a layer of water-cooling plate like a louver between two battery cells. It seems to consume the space of the battery pack, but in fact it saves the country and controls the temperature.
In this way, in addition to improving energy density, more than half of the problem of battery life can be solved.
So in theory, based on a cycle life of 1,000 times, there is no problem in buying a tram and driving it for 15 years.
In fact, as car owners, no matter how many years the warranty lasts, the possibility of driving a tram until the battery is scrapped is very low.
But we still hope that the longer the battery life, the better, because only by reducing battery maintenance costs can vehicles truly maintain their value, and by then, new energy can truly save money.