Experience sharing of smart meter design based on nb-iot

When you are designing a NB-IOT-based smart meter, water meter, or parking detector, one of the first problems you need to solve is the battery life. You may wish to replace the battery for several years.

So, you rack your brains to reduce the power consumption of the product and choose a high-capacity, high-quality battery for the product. When choosing a battery, you will definitely pay attention to the battery capacity. But another important invisible indicator is not exactly what every battery manufacturer can give you, that is, the self-discharge of the battery. A battery with a high self-discharge rate may reduce your product life by 50%!

This is a typical value for different battery self-discharge. However, it should be noted that the self-discharge size will be significantly different under different temperature conditions:

1. As shown in the figure below, the battery cell can be equivalent to the energy storage unit Ceff, the series resistance Rs, and the parallel resistance Rp.

2, series resistance Rs: usually we call the internal resistance of the battery (often said battery AC internal resistance or DC internal resistance), the equivalent resistance in the battery charge / discharge when the terminal voltage has a greater impact: Vcell = Vocv - ( I * Rs )

3. Parallel resistance Rp: When the battery is open, the battery's power will consume the charge of the battery through Rp. Under normal circumstances, the Rp resistance is large, and the self-discharge current is generally several to several tens of uA.

Test about self-discharge

This is really not a very difficult test - by measuring the open circuit voltage Voc of the cell over time

Fast and accurate measurement of the self-discharge characteristics of a cell is indeed a huge challenge for us because of the open circuit voltage of the cell.

However, significant changes in OCV can take weeks or even months. No one can stand the months or years to understand the self-discharge characteristics of the battery.

Moreover, the measured value of K does not directly reflect the self-discharge size of the battery (especially for electronic product development engineers - what is the value of K, how much is uA?).

BT2191A battery core self-discharge system

Use N6705B power analyzer and DMM meter

Self-discharge test completed in a few hours

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A battery test real-time data

Test time 1.5 hours, 2 hours about 159uA

No need to wait

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