I recently converted my private car into an electric vehicle (EV). As an engineer, I have to handle it anyway.
Figure 1 shows the batteries I received; I only know that the batteries were removed from a modified Porsche 944 EV and charged/discharged about 200 times. Battery sellers are not satisfied with the modified performance. He attributed the unsuccessful reform mainly to the shape of the unselected vehicle. Because there is not enough budget to carry out a complete transformation, the vehicle's balance and handling are poor and there are security risks. Therefore, he intends to start from scratch and design a new vehicle. His original battery is not suitable for his new project, but it is enough for me, and the price is only one-third of the new battery. As a low-volume user, I didn't have access to the best lithium-ion battery technologies. If I chose used batteries, I don't know if they were used in the same battery pack or environment. I can only do as much as I can. I don't know if several serially connected batteries that make up a battery pack are produced at the same time! But I can find second-hand batteries that can save a lot of money and they will not be lost.
Figure 1 - Used 160Ah LiFePO4 Battery
As you can see, these batteries belonged to different groups when they arrived. The reason was that they were all over the body of the vehicle, as long as there was a place, both under the lid and at the back door. I don’t know when these batteries were purchased (bought or purchased separately), at what temperature they were used, monitored and/or managed by the battery management system (BMS), and their charging. the way.
Under normal circumstances, due to the battery production line, the new battery packs produced for use by Original Equipment Manufacturers (OEM) EVs consist of well-matched single cells and are assembled directly. These single cells will always be charged at the same time, used together, and managed with the same algorithm. Most OEM applications also have some type of thermal management functionality to ensure that all single cells in the battery pack remain in the same operating state in the operating environment.
Due to the change in cell impedance, the temperature has the greatest effect on the performance of the cell. If equal loads are applied to all cells, but some cells are at different temperatures, they will be affected by the additional expansion and contraction of the cell material. This will lead to very different aging characteristics. The result is a mismatch in charging, which is due to the different aging rates of the cells; soon capacity mismatch occurs. If left unchecked, the total capacity of the battery pack will be limited by the minimum battery voltage, which will lead to a decrease in vehicle mileage.
Basic battery design principles:
When the first cell is fully charged, charging must stop.
When the first cell has no electricity, the discharge must be terminated.
A weak battery section ages faster than a strong battery.
The battery section with the highest degree of weak storage will eventually limit the available battery capacity (the weakest link).
The system temperature gradient in the battery pack weakens cell sections that operate at higher average temperatures.
Without equalization, the voltage difference between the weakest and strongest cells will increase during each charge and discharge cycle. In the end, one of the single cells will always be close to the maximum voltage, while the other cell will be close to the lowest voltage, thus impeding the charge and discharge capacity of the battery.
Since these batteries will no longer match each other as they were originally used, and because my installation will place them in different temperature environments, I must balance the cells.
There are two major mismatches in lithium-ion batteries; mismatched charging and mismatched capacity (see Figure 2). Charge mismatch does not occur when the charge capacity of the single cells of the same capacity gradually differs. The mismatch occurs when using battery sections with different initial capacities. Since battery packs are usually assembled from single cells that are produced almost at the same time, the manufacturing processes of these single cells are almost the same, so single cells are usually well-matched, and only charging is more common. However, if the battery pack is assembled from single cells of unknown origin, or if the manufacturing process is very different, there may be a mismatch in capacity.
There are two main types of battery equalization: passive equalization and active equalization. Here are the basic features and their respective advantages and disadvantages:
Passive equilibrium:
Simple to implement (hardware and software)
cheap
Reduced charging mismatch
Small equalization current (less than 1A)
Heat - wasting energy!
Active equilibrium:
higher efficiency
Increase available capacity
Reduce charge and capacity mismatch effects
Faster battery pack charging time
Can work during charging and discharging
Larger equalization current (greater than 1A) to quickly balance large batteries
Longer battery life
Mix/Match Brand New/Old Modules
Can use mismatched cells in module (increased yield)
It seems that active balance is the right way!
I decided to use the most proactive TI BMS at hand. In order to ensure that I always get the maximum charge from the battery pack, the voltage difference between all single cells is kept within millivolts. The battery managed by the TI EM1401EVM circuit board uses all TI components to provide 5A active battery equalization (the way I work).
Figure 3 shows the basic architecture. One of the BMS boards is installed next to the battery section and manages each module or battery pack.
Here are the main technical specifications of this vehicle:
The 51x160Ah Thundersky LiFePO4 battery is arranged in the following five modules in the vehicle:
Under the hood: a 12-section module and a 6-section module (see Figure 4)
Under the back of the car (instead of the mailbox and spare tire): 3 modules, 11 battery sections per module (see Figure 5)
About 170V full charge voltage: 27kWh
1000A Water-cooled DC Motor Controller
About 150kW full power: about 200hp, 250ftbs
Car net weight 2900 lbs
The cruising range is about 80 miles
Dashboard changed to Android ODROID board and 7-inch touch screen displaying real-time power, voltage, current, and power per mile
The amount of electricity consumed is approximately between 250Wh per mile and 325Wh per mile
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