PACK includes battery pack, copper bar, nickel sheet, protection board, outer packaging, output (including connectors), highland barley paper, plastic bracket and other auxiliary materials. These items are combined to form a PACK lithium battery pack through series-parallel design. When designing a lithium battery pack, it has always been the focus of debate among engineers whether to use series-then-parallel or parallel-then-series design, so let's take a look at the advantages and disadvantages of series-then-parallel and parallel-then-series design of battery.
Ⅰ. Analysis of characteristics of the series and parallel design of lithium battery PACKs
1. The PACK battery pack requires the battery to have a high degree of consistency (capacity, internal resistance, voltage, discharge curve, service life).
2. The cycle life of the PACK battery pack is lower than that of a single battery.
3. Use under limited conditions. (including charging, discharging current, charging method, temperature, etc.)
4. After the lithium battery packs are formed, the battery voltage and capacity have been greatly improved, and they must be protected. The monitoring of charge balance, temperature, voltage and overcurrent should be carried out.
5. The PACK battery pack must meet the requirements on voltage and capacity required by the design.
Ⅱ. Analysis of the advantages and disadvantages of series-then-parallel and parallel-then-series designs of lithium battery PACKs
1. Disadvantages of first parallel and then series connection
Parallel connection will affect the cycle life of battery due to differences in internal resistance and uneven heat dissipation.
2. The advantages of first parallel and then series connection
(1) When a single battery fails, it automatically exits. Except for the capacity reduction, it does not affect the use after parallel connection. The parallel connection technology is stricter.
(2) When a unit battery in parallel is short-circuited, the parallel circuit current will be very large, which is usually avoided by adding fuse protection technology.
3. Analysis of the advantages of first series and then parallel connection
(1) Connect in series first according to the capacity of the whole pack of batteries, such as 1/3 of the capacity of the whole pack, and finally connect in parallel, which reduces the failure probability of large-capacity battery packs.
(2) The design of first series and then parallel connection is very helpful for the consistency of the entire lithium battery pack.
Ⅲ. The common characteristics of series-then-parallel and parallel-then-series designs of lithium battery PACKs
1. Good battery consistency is required. Whether it is a soft-package battery or a cylindrical battery, multiple series connections are required. If the consistency is poor, the battery capacity will be affected. The battery with the lowest capacity in a pack determines the capacity of the entire pack of batteries.
2. High current discharge performance is required. Should lithium battery pack be connected in series first or in parallel first? The starting current of the motor is three times that of the normal working current, and the power performance of the motor can be improved only by high current discharge.
3. The battery is required to have good heat dissipation. With a large number of batteries, the heat caused by the temperature rise of the batteries inside the battery box is not easy to dissipate, resulting in uneven temperature among the batteries, different discharge characteristics, and long-term degradation of battery performance.
4. The production technology level is high. The battery must be able to withstand the vibration shock of bumpy road surfaces. The production technology, especially the spot welding technology, has high requirements. After welding, tests should be carried out to prevent virtual welding and de-soldering.
Ⅳ. The wiring harness requirements of series-then-parallel and parallel-then-series designs of lithium battery PACKs
Usually, we also have certain requirements for the wiring harness when we make lithium battery packs in series or in parallel, mainly for output current or connection to customer product ports. The normal series and parallel wiring harnesses can be classified into power lines, grounding lines, network lines, signal lines, etc., and lithium battery packs like 6.4V emergency lights are used in series and parallel at the same time.
Power lines connected in parallel first and then in series: power lines refer to the positive and negative lines of power supply below 36V for each electrical load. The wire diameter of the power cable is based on the matching relationship between the cross-sectional area of the wire and the current carrying capacity to determine the cross-sectional area of the wire. In actual design, the cross-sectional area should have a margin. For example, when the load current is less than 5A, choose 1 mm², and when the actual load is 5 to 9A, choose 1.5 mm², and so on, so as to ensure the current carrying capacity due to aging of the wiring harness or higher temperature. When we are determining the cross-sectional area of the wire, the voltage drop and the heating of the wire should be considered, and the wire diameter should be increased in the following cases:
(1) If the wire is too long, the wire diameter can be appropriately enlarged.
(2) When the wire is connected to the electrical appliance after being transferred by multiple connectors, considering the large voltage drop of the terminal, the wire diameter can be appropriately enlarged.
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