Battery equalization system and control method thereof

With the development of new energy vehicles, power batteries have entered the public's perspective. In fact, the public has some understanding of this type of battery. Similar lithium batteries have long been widely used in mobile phones. Therefore, everyone will naturally try to use the knowledge of mobile phone batteries to understand power batteries. However, there is a world of difference between power batteries and mobile phone batteries. In the power battery system, there will be new challenges that mobile phone batteries have not encountered-the problem of balancing multiple battery cells in series! This problem is a common problem that plagues all kinds of multi-cell series application scenarios, including electric vehicles. Many conditions, such as inaccurate SOC, sudden stalling, etc., and even sudden damage to the battery pack, are often caused by this balancing problem. Before introducing this invention, let me first analyze and introduce:

1. What is the equilibrium problem? (This is written in a more popular way, basically for popular science, professionals can skip this and go directly to 2)

• Why connect in series?

We know that batteries have specific operating voltages. Dry batteries are 1.5V, lead-acid batteries are 2V, and lithium batteries are 3.7V... Our mobile phones only need a single 3.7V battery cell, but electric cars often require voltages of up to several hundred volts, which requires dozens or even hundreds of batteries to be connected in series to work together. So, everyone should understand that power batteries are different from mobile phone batteries!

• Causes of imbalance: a. Efficiency difference; b. Capacity difference; c. Self-discharge difference.

With so many cells connected in series, are each cell in a consistent state? This is the problem of battery balancing. Battery balancing means that the cells that make up the battery pack maintain a consistent charge in any working state of the battery pack's charge and discharge. However, the actual situation is that the battery pack is unbalanced due to differences in production and aging damage as well as differences in actual temperature and stress. In detail, the following reasons cause the battery pack to be unbalanced:

a. Efficiency difference means that the conversion efficiency of electrical energy and chemical energy is different when the battery cells are charged and discharged. For example, a battery pack consisting of 100 battery cells is balanced at the beginning, and each battery cell has 20% power, so the total SOC is also 20%. Then it was charged, and the battery pack was charged with energy equivalent to 50% of the battery pack. Due to the different charging efficiency of each battery cell, the power of most battery cells is 68%, some battery cells are 68.5%, and some are 67.5%. Some people may ask, where does the remaining electrical energy go? It is mainly lost as heat energy. This is why charging releases heat. It is easy to understand that when discharging, there will also be efficiency differences that lead to inconsistent remaining power.

b. Capacitive difference means that the actual capacity of the battery cell is different. For example, a battery cell with a nominal capacity of 100AH ​​may have an actual capacity of 103AH or 97AH. After a charging process similar to step a, although the power appears to be the same, for a battery cell with an actual capacity of 103AH, the power only reaches 68%*100/103, or 66% of its actual total capacity, while for a battery cell with an actual capacity of 97AH, the power actually reaches 70% of the total capacity of the battery cell.

c Self-discharge difference means that when the battery is not working, it will lose some power over time. And this process will also vary. For example, the battery cell with 68% power mentioned above may drop to 66% after being placed for one month. However, some batteries still have 66.5% power, and some batteries only have 65.5% power.

• The “short board” effect

So, what are the hazards of unbalanced cells? The most significant hazard is the short board effect. We know that cells cannot be over-discharged or over-charged. That is to say, taking the above case a as an example, due to the voltage difference of 68.5% and 67.5%, when discharging, the battery pack can only discharge 67.5% of the power at most, and then it will be forced to cut off the power for protection because the power of individual cells is too low. Then, most normal batteries will have 0.5% of power remaining and cannot be used.

It is easy to understand that during the repeated charge and discharge process, the power gap between the cells will become larger and larger. After 10 repeated charge and discharge cycles, the gap widens to 10% (if the difference is 0.5 each time). For example, in addition to normal charging and discharging, urban roads, repeated braking and accelerator, and frequent uphill and downhill driving will also lead to the expansion of this gap. To be precise, the discharge margin of the battery pack is determined by the cell with the smallest power, and the charge margin is determined by the cell with the largest power, and the discharge margin + charge margin constitutes the capacity of the battery. For example, in extreme cases, if a group of batteries has a short board cell with a power of 10%, and another short board cell with a power of 90%, then the actual capacity of the battery pack is reduced to 20%.

• SOC Error

After understanding the inconsistency of the actual power of the battery cells, it is easy to understand the error of the SOC of the power battery pack. The short board effect tells us that the actual power of the battery pack is determined by the worst short board battery. However, SOC is counting the total power and total voltage of the entire battery pack, so when the inconsistency of the battery pack becomes larger, the error of SOC will also become larger. Therefore, there will be a stall: the SOC shows 25% power in the previous second, but this second it suddenly warns that the battery is low on power, the power is cut off for protection, and the car breaks down directly. This is a common stall problem in electric vehicles. However, manufacturers always blame this situation on the owner's failure to pay close attention to the remaining power. In fact, there is no evidence. After each power protection, as long as the SOC is automatically reset to zero through the software, no evidence will be left. As long as it is charged, everything will be fine again. The owner can only complain that his eyes are broken and he has seen a ghost...

• The “short board becomes shorter” effect

The harm caused by inconsistent cells is not only the aforementioned short board effect, but also directly causes the cells to age faster. We know that cells have a lifespan, and each charge and discharge will actually cause a certain attenuation of the actual capacity of the cells. After thousands of charge and discharge, the capacity of the cells will drop to 90% or even 80%. Furthermore, the aging damage of charging and discharging to the cells is different under different power levels. Charging a cell that is close to full power will cause greater damage, and vice versa, discharging a cell with very low power will also cause greater damage. This is why mobile phone manufacturers often recommend shallow charging and shallow discharge to extend the life of the battery. But electric cars are not like this. Mobile phones are single cells, while power batteries are multiple cells in series. I have seen some suggestions online that electric cars should not be fully charged, and only charge to 80% each time. In fact, this will seriously damage the cells (which will be analyzed later). Since the power level of the short board cell is different from that of other batteries, it always enters the full or low power state earlier than other batteries. Therefore, in the process of repeated charging and discharging, the aging degree of the short board cell is more serious. This is the "shorter short board" effect!

At this point, we have seen a double "vicious cycle": in the repeated charging and discharging process, not only will the difference in power between the cells be introduced to cause the short board, but the more cycles there are, the greater the power difference will be; further, it will also cause the actual aging and attenuation of the short board cell to become greater and greater ("the short board becomes shorter")

• The current equilibrium

In order to overcome the hazards such as the "short board effect" caused by the imbalance of long battery packs, power battery packs are designed with special balancing circuits. At present, these balancing circuits generally adopt the so-called "passive balancing" circuit, that is, during the charging process, when it is almost full, the SOC calibration program is started. Please note that this SOC calibration is completely different from the voltage calibration of a single cell in a mobile phone! The SOC calibration process of the power battery is to first enter the trickle charge, that is, to charge slowly with a very small current. Why is this so? In order to prevent damage to the battery cell due to overcharging, it is necessary to monitor and measure the voltage of each battery cell with high precision until one battery cell is fully charged first. At this time, the SOC calibration program will enter a process called "passive balancing", which is to use a bypass circuit to skip the fully charged battery cell and continue to charge other batteries. Repeat the above process until all batteries are fully charged.

At this point, you may understand why car manufacturers require users to fully charge at least once a month! Therefore, be careful with electric vehicles. "Shallow discharge" is still advisable, but long-term "shallow charging" is not acceptable. Long-term shallow charging will not only increase the aforementioned short board effect, causing the actual available capacity of the battery pack to continue to decline, leaving a hidden danger of stalling, but more importantly, it will also trigger the "shorter short board" effect, directly damaging the battery pack! ! !

• Difficulty Analysis

At this point, you may have some questions. Isn't power balancing just measuring the voltage of each battery cell and replenishing power from the high-voltage battery cell to the low-voltage battery cell? In fact, car manufacturers are not stupid. They not only have doctors and masters, but also many academicians. The reason why the problem of power battery balancing has become a world problem and has not been completely solved after decades of research is that there are reasons: First of all, "measuring the voltage" is actually not easy. The voltage of each battery cell measured by the external circuit is actually the external voltage of the battery cell. When the battery cell is working at a high current, this voltage, in addition to the influence of internal resistance, also shows inductive and capacitive interference due to the influence of the physical structure and chemical process inside the battery cell, resulting in the inability to accurately measure the actual real voltage inside the battery cell. If the external characteristic voltage of the battery when working is used to determine the difference in the battery cell and determine the balancing direction, it will make a mistake. Passive balancing during trickle charging is to avoid this situation. Some friends may ask, isn't there active balancing? When parking, isn't there plenty of time to slowly balance? Active balancing means taking some power from cells with high power to compensate for cells with low power. To achieve active balancing, you need to have a separate charging and discharging path that can be connected to any cell (you can't know in advance which cell will have a short board), that is, hundreds of channels. What's more, the voltage of the power battery reaches hundreds of volts, and the cost of high-voltage semiconductor circuits is very high.

In short, the difficulty of balancing lies in the measurement problem and the circuit complexity problem! Therefore, if you search for patents related to battery balancing, you will see countless applications. However, the balancing circuits of electric vehicles nowadays are almost all passive balancing at the end of charging, and there has been no new progress for a long time.

2. Innovative Methods

• Breaking through habitual thinking

When it comes to the problem of power battery balancing, people think of a string of hundreds of cells connected in series. Their power is not balanced, so they need to compensate for the power difference to make their power balanced. Based on this idea, precise measurement and a dedicated compensation path for each cell are necessary. Whether it is active balancing or passive balancing, they are all written in textbooks, and it seems that there is no other way. . .

I have also been troubled by this problem for several years. Finally one day, I got rid of the inertia of thinking: a power battery pack can actually have more than one series-connected battery cell string, in fact, it can have two battery cell strings (in fact, this is still easy to break through); then, further, the number of battery cells in the two strings can be different!!!

• Specific principles

At present, when it comes to balancing, the only things that come to mind are active balancing and passive balancing. In essence, both use external measurement circuits to find the "short board battery cell", and then use compensation circuits to compensate for it so that the power is balanced. That is, measurement circuit, compensation circuit and control logic. However, everyone knows that as long as two batteries are connected in parallel, a balance will spontaneously occur between them: the battery cell with high power will spontaneously replenish power to the battery cell with low power. I call this phenomenon "natural balance". The characteristic of natural balance is that there is no measurement circuit, no compensation circuit. In essence, it is to "measure" the battery cell with the battery cell and "compensate" the battery cell with the battery cell. So the question is, two battery cells are easy to solve, but what about hundreds of cells? Yes, just connect two strings of batteries in parallel!

Obviously, disconnecting all KB switches and connecting all KA switches can realize parallel connection of each battery cell of two battery strings. The same is true in reverse. By continuously switching the switch groups KA and KB, the parallel correspondence between the battery cells EA and EB can be changed to realize staggered parallel connection. Then, by utilizing the staggered parallel connection, the balancing effect is transmitted to achieve the balance of all battery cells. Whether in charging, discharging, or standby, the switch switching is maintained uninterruptedly to achieve full-time natural balance.

• Power MOS circuit
• 
• cost

You may be a little disappointed when you see this: how much does it cost to have so many power MOS? !

This is also a misunderstanding. In fact, I thought of the "natural balance" method almost a year ago, but the circuit at that time encountered obstacles in cost and failed. Now if you look closely at this circuit, you will know that the breaking voltage of these switches is very low, just the voltage of a battery cell. And this kind of low-speed power switch semiconductor with an operating voltage below 10V costs only a few cents! Therefore, the cost of the entire circuit is not large, and for electric vehicles, it is expected to be as low as $1/kWh.