Junction box for battery modules

The junction box is the part that connects the internal output circuit of the battery module with the external circuit. The appearance of the commonly used junction box is shown in Figure 1. The positive and negative bus bars (wider interconnection bars) drawn from the battery board enter the junction box, plug in or solder them to the corresponding positions in the junction box, and the outer leads are also connected to the junction box by methods such as plugging, welding and screw crimping. The installation position of the bypass diode is left in the junction box or the bypass diode is directly installed to bypass the battery module. In addition to the above-mentioned functions, the junction box must minimize its own consumption of the output power of the battery module, minimize the impact of its own heat on the conversion efficiency of the battery module, and maximize the safety and reliability of the battery module.

Junction box for battery modules
Figure 1 – Appearance of common junction box

Some junction boxes are also directly equipped with output cable leads and cable connector plugs to facilitate quick connection of battery modules or square arrays. When the lead length is not enough, you can also use an extension cable with a connector plug for connection.

In addition to the specifications and dimensions, the specifications and models of the junction box also include the applicable power range, which should be matched with the power of the module when selecting. In addition, it is necessary to determine the size and internal structure of the junction box used in conjunction with the number of module leads (two, three or four) and whether to connect a bypass diode.

In the production of battery modules, in order to ensure the performance and efficiency of battery modules, when selecting the junction box, we should not only select the specification and size, bearing power, working current and working voltage of the junction box, but also consider the contact resistance and heat dissipation performance of the junction box itself, as well as the conduction voltage drop, node temperature and thermal resistance coefficient of the bypass diode in the junction box. The following analyzes the influence of various performance parameters of the junction box on the performance of the battery module for reference when selecting applications.

①The influence of the connection contact resistance of the junction box on the output power of the battery module. At present, most of the connections between the lead wires of the battery module and the junction box and the connection between the bypass diode and the junction box adopt crimping methods. This method will produce a larger contact resistance, which consumes a small part of the output power of the module, and at the same time generates a certain amount of heat.

Since the junction box is just installed on the back of one of the two cells of the battery module, the heat generated by the contact resistance and the heat generated by the bypass diode during operation, part of it is radiated into the air through the junction box, and the other part of the heat is transferred to the battery sheet on the back. The conversion efficiency of the cell decreases as the temperature increases. When the temperature rises by 10°C, the conversion efficiency decreases by 5%. Since the cells of the battery module are generally connected in series, the reduction of the conversion efficiency of one or two cells will inevitably result in a decrease in the output power of the entire module.

In addition, as the service life of battery modules increases, the plating layer at the crimping part may appear rusty, peeling, etc. (the guarantee period of nickel plating on the surface is generally 15-20 years), which will lead to a continuous increase in contact resistance, an increase in power consumption, a decrease in battery conversion efficiency, and aggravate the decrease in output power. Therefore, it is best to choose products with welding connection (also called ohmic connection) when choosing a junction box.

②The influence of the conduction voltage drop of the bypass diode of the junction box on the output power of the battery module. There are not only PN junction resistance in the diode, but also junction capacitance and so on, so the diode will produce a certain forward voltage drop when it is turned on. The forward voltage drop of different types of diodes is different. Generally, the forward voltage drop of low-power Schottky diodes is around 0.3V, the forward voltage drop of high-power Schottky diodes is around 0.55V, and the forward voltage drop of ordinary rectifier diodes are all around 0.7~0.9V. Since the forward voltage drop of different diodes is quite different, when the bypass diode is working, the power consumption generated by the battery module is also quite different. For example, if a battery module uses 156×156 cells and the current is 8A, if the above three diodes are bypassed, the power consumption will be:

Low power Schottky diode P=IU=8×0.3=2.4W

High-power Schottky diode P=IU=8×0.55=4.4W

Ordinary rectifier diode P=IU=8×(0.7~0.9)=5.6~7.2W

It can be seen that although the maximum forward voltage drop of the diode is only 0.6V, the power consumption can be worse by 4-5W.

③The influence of the junction temperature of the bypass diode of the junction box on the reliability of the battery module. The higher the junction temperature of the bypass diode, the higher the operating temperature of the diode, and the better its safety and reliability. Therefore, when selecting the junction box, choose diode products with high junction temperature.

④The influence of the heat dissipation of the junction box on the battery module. I have already mentioned the influence of temperature on the conversion efficiency of the cell. Therefore, when choosing the junction box, not only the thermal resistance coefficient of the diode in the junction box is required to be small, but also the junction box with good heat dissipation must be selected. At present, some manufacturers have developed aluminum alloy junction boxes, integrated bus ribbon junction boxes, etc., which have better heat dissipation. In particular, the integrated bus strip junction box adopts the printed circuit board structure, the circuit copper foil of the circuit board has very good thermal conductivity and is easy to dissipate heat, and does not overlap the battery during installation, so the heat is not easy to transfer to the battery, so as to minimize the impact of heat on the battery module.