Science and technology
Basic knowledge of lithium battery PACK
The process of assembling lithium battery cells into groups is called PACK, which can be a single battery or battery modules connected in series and parallel. Under the background of the current new national standard, the demand for lithium batteries is increasing, and many lead-acid battery companies have also launched lithium battery products; in fact, the lithium battery PACK process is not difficult. Mastering this technology can assemble batteries by yourself instead of just acting as a manufacturer. The role of "battery porter", profit and after-sales is no longer restricted by others; mastering a technology, "lithium" travels all over the world.
PACK includes battery pack, bus bar, flexible connection, protection board, outer packaging, output (including connector), barley paper, plastic bracket and other auxiliary materials together to form PACK.
Features of PACK
①Battery pack PACK requires batteries to have a high degree of consistency (capacity, internal resistance, voltage, discharge curve, life).
②The cycle life of the battery pack PACK is lower than that of a single battery.
③Use under limited conditions (including charging, discharging current, charging method, temperature, etc.)
④ The battery voltage and capacity of the lithium battery pack are greatly improved after the PACK is formed, and it must be protected by charging equalization, temperature, voltage and overcurrent monitoring.
⑤The battery pack PACK must meet the voltage and capacity requirements of the design.
①Serial and parallel composition: The battery is formed by the parallel connection of single cells. Parallel connection increases the capacity and the voltage remains the same. After series connection, the voltage doubles and the capacity remains the same. For example, a 3.6V/10Ah battery is composed of a single N18650/2Ah through 5 parallels. Parallel first and series later: Parallel connection will affect the battery cycle life after parallel connection due to the difference in internal resistance and uneven heat dissipation. However, a single battery will automatically exit when it fails. In addition to the reduction in capacity, it will not affect the parallel connection. The parallel process is stricter. When a unit battery is short-circuited in parallel, the current in the parallel circuit will be very large, which is usually avoided by fuse protection technology. First in series and then in parallel: According to the capacity of the whole group of batteries, it is connected in series first, such as the capacity of the whole group is 1/3, and finally connected in parallel, which reduces the probability of failure of the large-capacity battery group.
②Battery core requirements: According to your own design requirements, select the corresponding batteries. The parallel and series batteries require the same type and model, and the difference in capacity, internal resistance, and voltage value is not more than 2%. Under normal circumstances, after the battery is combined in parallel and series, the capacity loss is 2%-5%. The more the number of batteries, the greater the capacity loss. Whether it is a flexible package battery or a cylindrical battery, multiple strings are required. If the consistency is poor, the battery capacity will be affected. The battery with the lowest capacity in a group determines the capacity of the entire battery group. High current discharge performance is required. The starting current of the motor is 3 times of the normal working current, and high current discharge can improve the power performance of the motor. Good heat dissipation of the battery is required. With a large number of batteries, 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. The level of production technology is high. The battery must be able to withstand the vibration and impact of bumpy roads. High requirements for production process, especially spot welding process. Test after welding to prevent false welding and desoldering.
③The process of PACK: The PACK of the battery is realized in two ways. One is laser welding, ultrasonic welding or pulse welding. This is a commonly used welding method. The advantage is that it has good reliability, but it is not easy to replace. The second is to contact through the elastic metal sheet. The advantage is that it does not need to be welded and the battery is easy to replace. The disadvantage is that it may lead to poor contact.
Charge and discharge time
Charging time (hours) = (battery capacity Ahx charging coefficient) / charging current A
Discharge rate: The discharge rate of the battery is expressed in terms of discharge time or the hour factor required for a certain discharge current to discharge the rated capacity. Among them, discharge rate = rated capacity/discharge current
In the PACK process, nickel sheets, copper-aluminum composite busbars, copper busbars, total positive and total negative busbars, aluminum busbars will be used, as well as copper soft connections, aluminum soft connections, and copper foil soft connections. The processing quality of busbars and flexible connections needs to be evaluated from these aspects.
①Whether the material material meets the requirements of composite material, the busbar material will increase the resistivity if the material does not meet the standard, especially it is necessary to confirm whether the relevant requirements of ROHS are composited.
②Whether the processing of key dimensions is in place. The over-tolerance of key dimensions may cause insufficient safety distance between high-voltage devices during the assembly process and cause serious safety hazards.
③The bonding force of the hard zone of the soft connection and the stress absorption condition of the soft zone.
④Whether the actual processing soft connection and the overcurrent capacity of the busbar meet the design standards, and whether the insulated thermoplastic sleeve is damaged.