In this section, we discuss chip NTC thermistors for temperature parameter control, which are embedded in high-performance rechargeable battery packs for different types of mobile devices.
Current applications of NTC thermistors in battery packs
Among rechargeable secondary batteries, such as nickel cadmium batteries, nickel-metal hydride batteries, lithium-ion batteries and polymer batteries, the market for lithium-ion batteries has experienced massive growth, led by the expansion of the market for portable devices such as mobile phones and digital cameras.
Battery pack configurations, such as those used in ever-increasingly miniaturized cell phones, most commonly consist of a single rectangular cell combined with a control circuit for temperature control. These battery packs typically use chip-type NTC thermistors. On the other hand, battery packs for notebook PCs come in different shapes and sizes, since the number of cells used varies from four to twelve. Some models, such as the so-called “intelligent battery packs”, have a built-in communication control circuit. Typically, round-shaped cells are widely used, and reed-type or chip-type NTC thermistors are mounted in the space between the cells.
Essential recharge voltage control function
It is said that at temperatures ranging from around 0 to around 60°C, batteries can be recharged without risk and without notable effects on battery life. If the temperature is outside this range at the beginning of or during charging, some measures, such as an alarm signal or forced termination of charging, are necessary.
Accuracy in temperature detection and reliability in operation are vital, especially at low temperatures, since it is very dangerous to charge batteries with frozen electrolyte in the cells.In both types, the recharge voltage is thus controlled by making use of the property in which a battery has a higher temperature when it is being charged. In addition, temperature information from NTC thermistors is used to correct calculations for battery life indication.
Requirements of NTC thermistors for battery packs
NTC thermistors for battery packs need to have a temperature detection accuracy greater than is required for general purposes. This is because they play an important role in preventing dangerous circumstances, and in preventing battery life degradation and incorrect operation of battery life indicators due to overcharging.
As you probably know, there are two values that express the basic properties of NTC thermistors. The first is resistance at a standard temperature, and the second is B constant, which is the temperature coefficient between two different specified temperatures. Naturally, both values have a certain degree of tolerance. So, as shown in the next chart, the corresponding accuracy between the resistance of NTC thermistors and the ambient temperature (in other words, the detection accuracy across the entire temperature range in which actual detection is made) very much depends on the tolerances of the two basic values. This means that lower tolerances of resistance and B constant at standard temperatures represent sharply higher detection accuracy in practical use. The specific target is to narrow the tolerances to ±1%.
Also, it is necessary that NTC thermistors for battery packs have immunity to irregular voltages caused by electrostatic discharge, because the electrostatic discharge from a human body may destroy some of the electronic parts in the battery pack if it is attached to a device or connected to a battery charger. NTC thermistors must therefore have reliable resistance characteristics.
In response, we have developed 1608 and 1005 types of multilayer chip NTC thermistor models that have excellent anti-static properties, as well as narrow tolerances of ±1% both for resistance and for B constant.
Striving to reduce resistance deviation
Since they are with microcomputers, NTC thermistors for battery packs have to have matched resistance vs. temperature characteristics (If there is any difference in resistance value, there will be a gap between the temperature detected and the actual temperature).
As explained in an earlier section, NTC thermistors are semiconductors made of oxides of transition metals such as manganese (Mn), cobalt (Co), nickel (Ni) and copper (Cu). With further research into the optimum composition of these materials and microstructure control using additives, we are striving to provide target properties in resistivity, B constant, and other aspects. Needless to say, fine-tuning the sintering conditions is an important factor in controlling the generation of crystal structures. As an example of this, the next chart shows experiments in which resistance vs. temperature characteristics are controlled using additives.
First, we added three different amounts of additive (21wt%, 26wt%, and 35wt%) to the main ingredient to form three materials with different mixing ratios. Then, we compared the resistance deviations of the three in a temperature range of -40 to +110°C (the three materials have the same resistance value at a standard temperature of 25°C). So, even though the materials have the same main ingredient and additive, changes to the mixing ratio lead to substantial changes in temperature characteristics.
These studies were repeated, and based on the result of the final definition of characteristics, we have developed and released the 1608 type and the 1005 type, both of which have the same level of resistance vs. temperature characteristics as high accuracy thermistors for battery packs that are considered industry standards.