Heating element selection calculation

a) Battery heating elements are primarily available in three forms: silicone heating elements, PTC heating elements, and PI heating films. The heating element size design is finalised after the selection of the appropriate material is made based on the heating power requirements, cost, weight, and space.

1) PTC heating elements contain a ceramic heating element and an aluminium plate that is typically 5-8mm thick and has a load-bearing capacity. The battery module's bottom is typically where they are located, as illustrated in Figure 1(a).

2) Silicone heating elements are composed of a silicone body and a heating core that is approximately 1.2mm in thickness. They are typically positioned on the side of the module and are incapable of supporting weight. Alternatively, a silicone heating element may be accommodated at the module's base, as illustrated in Figure 1(b).

3) PI heating films are composed of a heating core and a PI film, which are typically approximately 0.38mm thick. They are typically positioned on the side of the module and are incapable of supporting weight. Additionally, they may be positioned at the base of the module if there is adequate space.

b) Calculation of Heating Power

The heating power of the heating element is determined by the following formula:

F = 60 × R × W

Where, F denotes the heating power of the heating element, W;

The specific heat capacity of the module is denoted by R/(K ∙ K).

The module's weight, in kilograms, is denoted by W.

60 refers to the customer-required battery heating rate, °C/°C.

In practical applications, considering factors such as heat conduction between the module and other components, heat dissipation on the other side of the heating element, and the reduction in heating element power due to changes in battery system voltage, an additional 20% margin is added to the design selection. Therefore, the actual required heating power is:

F₀ = F₀ × (1 + 20%)

c) Power Density Selection

In practical applications, since the batteries at both ends of the module are in direct contact with the end plate, heat dissipation is relatively fast through the end plate. Additionally, due to the folded edge of the end insulation plate, some areas of the battery close to the end plate lack heating elements. These two factors lead to an increased temperature difference between the batteries at both ends of the module and the battery in the middle of the module during heating. Therefore, in the actual heating element design, the power density of the heating element closer to the module end is appropriately increased.

PTC heating elements possess self-temperature control. As shown in Figure 2, the ceramic PTC element heats up after voltage is applied, and the internal temperature rises rapidly, transitioning from the NTC effect to the PTC effect. Subsequently, as the temperature rises, the resistance increases sharply, eventually reaching a balance between heating and heat dissipation, achieving automatic temperature control. Generally, the self-controlled temperature is designed between 60°C and 80°C. When used with a battery module, the surface thermal equilibrium temperature of the heating element is typically between 20°C and 40°C at room temperature. The heating element surface does not glow red or emit an open flame, ensuring safety and reliability. The commonly used power density for PTC heating elements is 0.5~0.6 K/K².

Silicone heating elements and PI heating films have different heating characteristics than PTC. Higher power density results in higher dry-burning temperatures, as shown in Table 1. While meeting heating power requirements, the power density should be minimized. Commonly used power densities for silicone heating elements and PI heating films are 0.25~0.4 K/K².

After increasing the power density at both ends of the heating element, the total heating power of the heating element needs to be recalculated. The calculation formula is as follows:

𝑃𝐻0′ = 𝑃𝐻0 + 𝑃x

Where 𝑃𝑥 refers to the heating power of the increased power density at both ends of the heating element, in W.

d) Heating Element Voltage

The PTC heating elements are connected in parallel within the battery pack, while the silicone heating elements and PI heating films are connected in series. The heating element voltage has two scenarios: First, the heating voltage is provided by the battery pack, in which case the PTC heating element voltage is consistent with the battery pack platform voltage, and the voltage value of each silicone heating element and PI heating film is determined by the number of elements connected in series. Second, the heating element voltage is provided by an external charger, in which case the PTC voltage is consistent with the charger voltage, and the voltage value of each silicone heating element and PI heating film is determined by the number of elements connected in series.

e) Heating Element Resistance

The heating element resistance is determined by the calculated heating element power and voltage, as shown in the following formula:

F = F^2/F0′

Where F refers to the voltage of a single heating element (V);

F0′ refers to the heating power of a single heating element (W).

f) Heating Element Fixing Design: PTC heating elements are fixed to the bottom of the housing with bolts, achieving tight contact with the module through thermal pads or adhesive; silicone heating elements and PI heating films are usually attached to the side or bottom of the module with single-sided adhesive.

g) Heating Harness Fixing: The wiring harness is generally fixed to the end plate or housing, typically with a spacing of 150mm~200mm.