Lithium batteries are gradually becoming more popular. Why is the probability of accidents much lower than that of pure electric vehicles?

Let's talk about a topic. With the penetration of lithium batteries in electrification, 48V, HEV and PHEV all use a lot of lithium battery systems. Why is their accident probability overall much lower than that of pure electric vehicle systems?

The material cited here comes from "Cost-effective 48V lithium-ion battery thermal management technology solution and its application in hybrid vehicles", which contains some things that we can compare with pure electric vehicles.

Margin for 48V Systems

In this report, in order to reduce the cost of the system as much as possible, the thermal management of the Pack and Pack is minimized. Therefore, in the following system block diagram, the author attempts to use a completely passive management strategy, mainly heating through the air conditioning system + heating the battery by self-heating by charging and discharging as much as possible within the allowable charge and discharge power range of the LFP battery (the 48V lithium iron phosphate battery is located under the passenger seat).

System Block Diagram

So although the 48V system does not need to design a complex thermal management system based on the characteristics of the battery, the 48V battery is not the protagonist when considering the strategy. The vehicle can fully meet the original functional design of the vehicle through a 48V generator and 48V DC-DC with a 12V battery. When considering the battery, it is easy to accommodate the needs of the battery.

When the temperature is very low, at -30 degrees, the battery's charging and discharging power is limited.

When the battery temperature rises to 0 degrees, it can provide 7Kw discharge power and 3Kw charging power; when the battery temperature further rises to above 30 degrees, the charging and discharging capacity will reach the maximum value.

48V battery charge and discharge power limit

So the biggest advantage of this design is:

When the vehicle starts, if the 48V battery cannot be charged or discharged, the relay needs to remain disconnected, which can also work; if the battery's charge and discharge capabilities are restored, the relay can be closed to connect the battery. It is also because of this that in low temperature conditions, the actual power of the 48V battery can be controlled to limit the power output. The design goal can tolerate the 48V battery to recover its charge and discharge capabilities through heating for a period of time. In the reference lecture, the data set for the demand is to restore its charge and discharge capabilities within one hour to support hybrid functions (such as brake energy recovery, 48V motor start-stop function, etc.), so there is actually a lot of room for the entire system.

Strategies for heating 48V battery systems
Based on the above premise, the article analyzes several strategies, including:

The relay is in the disconnected state, and the 48V battery is not used, so it can be heated naturally with the battery.

The relay closes, limiting the battery current as much as possible (by controlling the generator)

The relay is closed, and the generator is controlled to charge and discharge the battery in a cycle. The battery temperature is increased by charging and discharging, while the battery SOC is maintained.

The data of the actual vehicle results in this report may not be complete. The results of several selected cases are as follows. In fact, it can be seen that charging and discharging the battery itself within the limited power range can effectively bring about temperature rise and power climb.

The following figure is not complete, it is at 33 minutes, here it is completely following the discharge power.

48V battery charge and discharge power limit

The situation of HEV is similar to 48V. The battery of PHEV is larger, the entire allowable power range is wider, and there is more room. I don't know if

the battery heating strategy of BEV
is correct. At present, the biggest bottleneck of pure electric vehicles is the increasingly large battery and the user's expectation of charging speed under various conditions, especially the DC charging speed under low temperature. In this regard, we have to overcome several problems:

What users need is a charging time comparable to that at normal temperature

Using various preheating methods, it is necessary to balance the contradiction between temperature difference, temperature rise rate, allowable power and overall time.

Since the battery is the only source of power, it cannot be too cowardly. A certain discharge power and energy recovery must be achieved, and the power consumption cannot vary too much.

So the problem has evolved into, in order to constantly balance the specific performance under low temperature (including winter fast charging, winter power and winter range data that all media want to go to), although the battery is very sensitive to temperature, the needs of vehicle users are more direct, and there are many challenges in balancing the two. It is easy for major and minor injuries to accumulate here (the temperature difference here, whether to consider the battery cell with the lowest temperature, this is a big balance point, and it takes time to wait for it to rise)

In this Taycan's soft-pack battery cell temperature and SOC charging power spectrum, the permissible charging power is 50kW at 10 degrees and 60% SOC, which is only about 0.5C. The lower 0 degrees and -10 degrees are not actually given, which is actually very conservative.