The process of assembling the lithium battery cells into groups is called PACK, which can be a single battery or a battery module in series and parallel connection. Under the background of the new national standard, the demand for lithium batteries is increasing, and many lead-acid battery companies have also launched lithium battery products.
PACK composition
PACK includes battery packs, bus bars, flexible connections, protective plates, outer packaging, output (including connectors), barley paper, plastic brackets and other auxiliary materials to form PACK.
Features of PACK
① The battery pack PACK requires that the battery has a high degree of consistency (capacity, internal resistance, voltage, discharge curve, life).
② The 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 After the lithium battery pack PACK is formed, the battery voltage and capacity have been greatly improved, and it must be protected to perform charge balancing, temperature, voltage and overcurrent monitoring.
⑤ The battery pack must meet the voltage and capacity requirements of the design.
PACK method
①Serial and parallel composition: The battery is formed by single batteries passing through and connected in series. The capacity is increased in parallel, the voltage is unchanged, and the voltage is doubled in series, and the capacity is unchanged. For example, a 3.6V / 10Ah battery is composed of a single N18650 / 2Ah through 5. Parallel to serial: Parallel connection due to the difference in internal resistance and uneven heat dissipation will affect the battery cycle life after parallel connection. However, when a single battery fails, it will automatically exit. Except that the capacity is reduced, it will not affect the use after parallel connection, and the parallel process is strict. When a unit battery is short-circuited in parallel, the current of the parallel circuit is very large, which is usually avoided by adding a fuse protection technology. Series first and then parallel: According to the capacity of the entire group of batteries, the series is first connected in series, such as 1/3 of the entire group, and finally connected in parallel, which reduces the probability of failure of large-capacity battery packs.
② Battery requirements: Select the corresponding batteries according to your own design requirements. The parallel and series batteries require the same type and the same model, and the difference in capacity, internal resistance and voltage value is not more than 2%. Under normal circumstances, after the batteries are connected in parallel and in series, the capacity loss is 2% to 5%. The more the number of batteries, the greater the capacity loss. Regardless of whether it is a flexible battery or a cylindrical battery, multiple string combinations are required. If the consistency is poor, affecting the battery capacity, the battery with the lowest capacity in a group determines the capacity of the entire group of batteries. High current discharge performance is required. The starting current of the motor is 3 times the normal working current, and the high current discharge can improve the power performance of the motor. Requires good battery cooling. The number of batteries is large, and the temperature rise of the battery inside the battery box is not easy to dissipate, resulting in uneven temperature among the batteries, different discharge characteristics, and a long-term decline in battery performance. High level of production technology. The battery must be able to withstand the vibration and shock of bumpy roads. High requirements on the production process, especially the spot welding process. After the welding is completed, the test is carried out to prevent the virtual welding and de-soldering.
③PACK process: The PACK of the battery is realized by two ways, one is by laser welding or ultrasonic welding or pulse welding, which is a commonly used welding method, the advantage is better reliability, but it is not easy to replace. The second is the contact through the elastic metal sheet. The advantage is that no welding is needed, and the battery is easy to replace. The disadvantage is that it may cause 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 by the discharge time or the hour coefficient required to discharge the rated capacity with a certain discharge current. Among them, discharge rate = rated capacity / discharge current
Assembly of busbar soft connection
In the PACK process, such as nickel sheets, copper-aluminum composite bus bars, copper bus bars, total positive and negative bus bars, aluminum bus bars, copper soft connections, aluminum soft connections, copper foil soft connections, etc. are also used. The processing quality of busbars and soft connections needs to be evaluated from these aspects.
①Whether the material material is compounded or not, if the busbar material is not up to the standard, the resistivity will be increased, and it is especially necessary to confirm whether it is compounded with ROHS related requirements.
② Whether the key dimension processing is in place. The over-tolerance of key dimensions may lead to insufficient safety distance between high-voltage devices during the assembly process and cause serious safety hazards.
③ The bonding force of the hard zone of the soft connection and the stress absorption status of the soft zone.
④Whether the actual processed soft connection and the overcurrent capacity of the busbar meet the design standards, and whether there is any damage to the insulated thermoplastic sleeve.