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Home> Articles>News>How To Optimize Battery Capacity — Lithium Iron Phosphate Rechargeable Battery

How To Optimize Battery Capacity — Lithium Iron Phosphate Rechargeable Battery

From:CTECHI GROUP Limited     Release time:2018-10-31

Overview:To save energy and protect environment has always been a hot topic, and electric and hybrid vehicles provide an excellent way to save energy and reduce carbon dioxide emissions. However, the major weaknesses of electric and hybrid vehicles lie in their battery capacity resulting in distance constraints. Because the maximum battery size that can be installed into a car is often limited by volume and weight, it is becoming increasingly important to optimize the use of existing battery capacity.

To save energy and protect environment has always been a hot topic, and electric and hybrid vehicles provide an excellent way to save energy and reduce carbon dioxide emissions. However, the major weaknesses of electric and hybrid vehicles lie in their battery capacity resulting in distance constraints. Because the maximum battery size that can be installed into a car is often limited by volume and weight, it is becoming increasingly important to optimize the use of existing battery capacity.

To provide the hundreds of volts needed for modern high-performance and high energy density small lipo battery for electric vehicles which usually need to connect several separate battery cells in series. Each high energy density small lipo battery in the lifepo4 lithium iron phosphate battery packs has its own capacity, self-discharge rate, temperature characteristics and battery impedance, and the differences will increase with the aging of the battery. When a cell is charging, this difference leads to a situation in which some cells are not fully charged, but others are already charged. Unless additional measures are taken, the charging process must be terminated because if a battery cell is overcharged, it can be damaged or even completely destroyed.

Similar cases also occur during discharge. On the contrary, the situation is that one battery cell has already fully discharged, while the others still have enough energy to continue powering the car (theoretically). However, it is impossible for the car to continue running at this time, because this will cause overdischarge of the weaker battery cells, resulting in damage to the lifepo4 lithium iron phosphate battery packs. In order to avoid these two situations, an active balance between individual cells is necessary.

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Passive equilibrium method transforms available energy into heat loss.

The widely used method is passive balancing, which uses resistors to re-discharge cells already charged so that other cells can continue to charge. The disadvantages of this method are obvious:

* For balance purposes, batteries can only be discharged.

* Discharge current caused by bypass resistance causes power loss.

* Precious energy will be converted into heat and can not provide power for cars.

* Reduce vehicle travel distance.

The passive balance method can only convert the energy stored in the cell into heat, while the active balance method can transfer the charge from one cell to another cell. There are several ways to achieve charge transfer, such as switching capacitor or inductor. When using the capacitance method, the capacitor will be connected in parallel with the battery unit with higher voltage. Once the battery cell is charged, it is connected in parallel with a lower voltage battery cell and can continue to charge it. This process will be repeated until all battery units reach the same voltage.

The use of capacitors is cost-effective, but one drawback is that the average balance current is limited to less than 50 mA. This limitation does not exist with inductance method, and in this case, it is easy to achieve a balanced current of 1A or more.

Fast and almost lossless charge transfer using active balancing method

Active balancing is achieved by parallel inductors and batteries that need to acquire charge. This result leads to a continuous increase in the current in the coil.

Once the coil has been decoupled from the battery cell discharged through the transistor, the energy stored in the inductor can be charged to the adjacent battery by a diode. Thus the charge can be moved back and forth between two separate cells, achieving extremely high efficiency and almost no loss. This method has some decisive advantages:

* The balance current may reach 1A or above.

* Balance is essentially lossless.

* Balance is extremely fast.

* Increased efficiency and battery capacity.

* Increases the distance traveled by vehicles.

Active balancing using inductors is not a low-cost method compared with other methods mentioned, because relatively expensive inductors are used. However, this is not entirely a problem. The current cost of modern high-performance batteries is close to $10000. With the inductive balance method, even if only an additional 10% of the capacity is obtained, it represents the value of $1,000, which can be used to purchase large quantities of inductors.

Lithium-ion batteries must be monitored for safety reasons, because overload can cause burning and, in extreme cases, even explosion. As with overvoltage, undervoltage, and temperature monitoring, additional functions such as accurate charging condition measurements are needed. In the semiconductor market, components that can achieve all these functions and different balancing methods have been provided. Using advanced active cell balance solutions (e.g. Atmel ATA 6870 Battery Management Circuit) each cell has a separate electronic monitor to provide functions such as charging status determination, active/passive balancing or overvoltage, undervoltage and temperature monitoring

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