Battery Characteristics Analysis and Design Scheme in Smart Home

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Battery Characteristics Analysis and Design Scheme in Smart Home [Copy link]

Author: Betty Guo

Nowadays, more and more smart sensors are added to smart building automation, such as smart cat eyes, doorbells, portable cameras, and smoke sensors. These products are usually powered by batteries, such as the common 18650 lithium batteries and AA dry batteries. So in battery-powered products, how to design a reasonable power supply solution is the key issue to extend the battery life.

The article will first analyze the characteristics of commonly used batteries, and then provide a technical design solution using a visual doorbell as an example.

1. Analysis of different battery characteristics

In smart home products, the commonly used battery types are mainly dry batteries and lithium batteries. Lithium batteries are divided into 18650 cylindrical batteries, polymer lithium batteries, nickel-metal hydride batteries, etc. Since the battery voltage will decrease as the battery is used, requirements are put forward for the subsequent circuit design. The following are the characteristics of these three types of lithium batteries:

Battery Type	18650 cylindrical battery	Polymer lithium battery	NiMH battery
characteristic	-High energy density. -High operating voltage platform. -Long cycle life. - Low self-discharge	- Lightweight, thin, high capacity density - Good charge and discharge characteristics - No memory effect	- Low cost - Good fast charging characteristics - Long life, no memory effect - Wide operating temperature range

Regardless of the type of lithium battery, its discharge characteristics and power curve are basically the same. Figure 1 below is the relationship curve between voltage and power at different discharge rates of lithium batteries.

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Figure 1 Relationship between voltage and capacity of lithium battery at different discharge rates

As can be seen from Figure 1, the relationship between battery voltage and power is different for different discharge rates, and is also related to the operating temperature. Therefore, it is not possible to determine whether the battery has power based on the battery voltage. Normally, when the lithium battery voltage drops to 3V, the battery has almost no remaining power. At this time, even through the boost chip, it cannot provide enough energy for the load.

At the same time, Figure 2 below shows the relationship between battery voltage and power under different load conditions. Under different load conditions, the same battery voltage will also have a large difference in the remaining battery power. SOC (State of Charge) is the state of available electrical energy in a lithium battery, usually expressed as a percentage. The relative state of charge ranges from 0% to 100%, with 100% when the battery is fully charged and 0% when fully discharged. The load current is divided into pulse current and continuous current. In the case of pulse current and continuous current discharge, the battery discharge curve will also be different. For example, MCU and Sub-1G loads will generate pulse current when starting, and continuous current for LEDs.

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Figure 2 Relationship between battery voltage and power under different loads

Another widely used type of battery is button cells and dry cells. The common alkaline button cell nominal voltage is 1.5V, and the lithium button cell is 3V. The working voltage of the dry cell is 0.7V~1.5V. When the battery is discharged to a voltage lower than 0.7V, it will not work properly. At the same time, its constant current discharge curve is also related to external environmental factors. The following Figures 3 and 4 are the constant current discharge curves of lithium button cells and dry cells respectively:

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Figure 3 Relationship between voltage and power of lithium button battery

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Figure 4 Relationship between battery voltage and charge

From the above two figures, we can see that, taking the 3V system as an example, for a button battery, when the battery voltage drops to 2.5V, the battery still has some power left for the system to use. However, since the battery voltage is too low to provide enough voltage, in order to further extend the battery life, the 2.5V voltage needs to be boosted to meet the system working level.

2. Power supply design application examples

Considering that the current battery-powered smart home products usually use wireless methods to transmit data, such as WIFI Bluetooth. As mentioned above, when the communication device starts instantly, there will be a large startup current (usually 1A~2A) that instantly pulls down the battery voltage. When the battery voltage is too low, the power chip behind it enters UVLO and the system stops working. Combined with the above analysis of battery voltage and power applications, when designing a power supply, you can use a buck-boost chip to meet common application scenarios.

Combined with the demand for low battery power consumption, an IC with low static current is required, usually at the nA/uA level. Combined with the use of WIFI/BLE including speakers and LEDs in smart homes, a larger operating current is required, generally around 1A~2A. The low Iq 2A output current buck-boost chip TPS63802 is very suitable for this type of application.

TPS63802 is a buck-boost chip with a quiescent current of 11uA and an output of 2A. The input voltage range is from 1.3V to 5.5V, which can cover the application of most smart home products and help users further reduce the design volume. According to TI's reference design, the overall design size can be reduced to ~28mm. For more information, please refer to TI's official website and TPS63802 technical data.

Figure 5 below takes the visual doorbell as an example. TI provides a complete design solution, and all technical information can be downloaded and queried on the TI official website.

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Figure 5 Visual doorbell reference design

MartinFowler

The post by the OP is always well illustrated! Thanks for sharing

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MartinFowler posted on 2020-5-13 15:33 The post by the OP is always well illustrated! Thanks for sharing

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