Recently, Xiaomi has stirred up some controversy again. A user discovered a major bug in the Xiaomi SU7 battery while charging.
He drained the battery of the base model of Xiaomi SU7, then charged it from 0 to full. Surprisingly, it took 82.79 degrees of electricity to charge, while the actual battery capacity of his SU7 was only 73.6 degrees, indicating an overestimation of about 12.5%.
Under the same conditions, the difference in range between the NIO EC6 and Tesla Model 3 is only 0.7% and 4.0%.
As the saying goes, no contrast, no harm.
But why is this happening?
It's like having a 70-liter fuel tank, filling it up with 79 liters. Is it that my tank is overstated, or did you spill some when refueling?
Could it be that the extra electricity was all consumed by the Xiaomi SU7? The new players in the car industry, indeed...
This topic immediately trended, with netizens speculating that Xiaomi might be hiding some battery capacity.
The reason being that NIO may be trying to conceal its energy consumption by secretly adding a bit of extra battery cells beyond the rated capacity, making us feel like the battery drains slowly while driving.
But I don't think Xiaomi's "hidden battery" thing is very likely.
Why? Let's take a look at the national standard GB/T31484. It clearly states from the beginning that the discharge capacity of electric vehicle power batteries cannot exceed 110% of the rated capacity.
Although here "capacity" refers to "charge," it needs to be multiplied by the rated voltage to obtain what we call energy capacity, but this does not affect the conclusion.
Even though car manufacturers typically increase the capacity by around 2Ah to ensure better range, the actual battery capacity won't differ significantly as a result.
More importantly, car manufacturers do not really need to hide large amounts of battery capacity. After all, who wouldn't want to boast about having a bigger battery these days? Every extra unit of battery means an extra cost, and it would be a shame if users didn't perceive that.
As for energy consumption? There are numerous ways to improve it in the initial design of an electric car, and hiding batteries is actually the most expensive option.
Therefore, the issue with Xiaomi's SU7 is most likely due to charging losses in the battery. Is it normal to have over 10% loss? Well, not only is a 10% loss common, it can even be higher.
For instance, there are complaints on a certain platform about Tesla, with its charging losses reaching as high as 11%, far exceeding the 4% mentioned by the individual earlier. However, the reasons behind this are quite complex and could involve factors related to the user, the vehicle, and the charging station.
The most important factors are related to two things: charging method and battery type.
Firstly, if it's slow AC charging, a 10% loss is normal, but with fast DC charging, this loss can be reduced to 5%.
Tests have been conducted on 4 car models using DC fast charging, and the charging loss rates were all around 5%, indicating that the claim that DC fast charging does not easily lose battery capacity is valid.
But the key factor affecting efficiency here is not fast charging versus slow charging, but the difference between alternating current (AC) and direct current (DC).
DC charging is like the charging station directly pushing electricity into the battery cells, with fewer steps involved, making it a more end-to-end charging process. On the other hand, with AC charging, the electricity in the vehicle has to pass through the onboard charger (OBC) to convert AC to DC, leading to more energy losses in the process.
The guy at the beginning is testing whether the Xiaomi SU7 is using direct current or alternating current.
Judging by its efficiency in fully charging in 1 hour, it should also be a fast direct current charging, so the reason for the charging loss is likely not alternating current, but probably the second type, the battery type.
The internal resistance of lithium iron phosphate batteries is much higher than that of ternary lithium batteries, especially the issue of insufficient consistency in lithium iron phosphate battery cells, which can lead to some cells having particularly high internal resistance. As a result, charging losses will also be higher, with energy being dissipated as heat within the battery cells.
Typically, the loss rate for ternary lithium batteries is around 5%, while for lithium iron phosphate batteries, it can exceed 10%.
Interestingly, the base model of the Xiaomi SU7 uses lithium iron phosphate batteries.
To make a comparison, let's also take a quick look at the top-of-the-line Xiaomi SU7 that we purchased.
Firstly, it's important to note that this Xiaomi SU7 uses a 101-degree ternary lithium large battery. Due to time constraints, we were unable to fully deplete the battery; instead, we started charging from 36% capacity and stopped at 80%.
If we disregard the issue of power loss during charging, roughly speaking, it's like charging 44.44 kWh of electricity, while the actual electricity consumed by the charging station is 45.91 kWh. Doing the math, the loss rate is only 3.3%, much lower than 12.5%.
Not only us, but also Xiaomi users online have tested charging the top-spec version SU7, feeling sorry that the battery didn't run dry, and the final test results were similar to ours.
It is reasonable to have a 3% loss when charging Xiaomi SU7 using a combination of ternary lithium battery and DC fast charging.
Like the mentioned 4% for the Model 3, I estimate that it may also be testing the high-end version with ternary lithium batteries, and this number seems reasonable.
Although during charging, factors like running the air conditioning or entertainment system, or aging charging piles with high resistance, may lead to inconsistent loss rates, the main cause of this loss should be lithium iron phosphate.
Today, Xiaomi officials also responded to this incident, confirming that it is not a case of "reverse overstatement" of hidden power.
Furthermore, they also mentioned "discharge rate" in the announcement, in simple terms, being gentle with the throttle can allow the battery to discharge more than the nominal capacity.
In short, it's impossible to "reverse overcharge" the electricity. Are you expecting to get free electricity from the car manufacturers?
However, the most frustrating part is having to pay for the extra electricity when overcharging. Xiaomi saying "there won't be any additional cost for more electricity" leaves me puzzled. Isn't electricity cost calculated based on usage per kilowatt-hour?
Charging more will naturally cost more, so this can't be a case of "reverse overcharging," right?
