Analysis of three battery capacity monitoring schemes

IOT Products such as smart bracelets 、 An electric toothbrush 、 Intelligent door lock, etc , These multifunctional devices require that the battery must be able to provide more power supply capacity and longer operation time . Electronic system designers usually focus on improving power conversion efficiency 、 Configure chip sleep mode 、 Improve the battery capacity . However , However, the problem about the accuracy of battery power detection is easy to be ignored .

ask ： Why should we pay attention to the accuracy of battery power detection ？

answer ： We spend a lot of energy on optimizing power consumption , However, the error range of battery power detection is ±10%, It means that when the system low battery alarm , Yes 10% The battery capacity or running time is not at the point where an alarm is required .

There are several common schemes for battery power detection ——

Coulometer measurement

The basic principle of coulometer counting is to count the inflow 、 The current flowing out of the battery is integrated , In practical application, it is usually used for special purpose IC Conduct test statistics . For rechargeable batteries , This method is very effective , But for non rechargeable batteries , Such as button battery in smart door and window sensor , The designer cannot know which brand of battery the user is using , Therefore, there is no accurate initial battery capacity data , As the disposable battery is used up, it will be scrapped , So there is no so-called charging current . The coulometer can update the capacity of the battery only when it is fully discharged immediately after it is fully charged , This drawback lies in the portable IOT Very obvious in the product . in addition , The power consumption of coulometer can not be ignored , With TI Of BQ76920 Coulometer chip as an example , Turn on ADC In acquisition mode , The power consumption can reach 130uA, For portable IOT In terms of products , Such power consumption may be far more than its own power consumption .

Battery voltage detection

Check the battery voltage , The battery voltage data is used to determine the level of electric quantity . This design usually uses a voltage follower for impedance matching , The designer can adjust the detection resistance to M Level to reduce battery power loss . This design is relatively intuitive , However, due to the internal resistance of the battery , Only when the battery is not pumped with a large current , The detected voltage has a strong correlation with the battery power . After battery aging , And when the operating environment is at a low temperature , The detected power error will be greater .

Impedance tracking measurement

The steady-state circuit model of battery can be simplified as voltage source and resistance , The voltage source corresponds to the battery open circuit voltage , The resistance is equivalent to the internal resistance of the battery . In case of large current pumping , Internal resistance will share part of the pressure drop , For example, a section has been used 1 Year dry cell , At low temperature 0℃ Under the circumstances , Internal resistance may reach 0.5Ω, Suppose we use the “ Battery voltage detection ” The voltage value obtained from the scheme is 1.2V, This voltage is sufficient to drive a from the point of view of voltage alone 1V Powered motor , But actually , If the starting current of the motor is 1A, At the moment of starting, if the current flows through the internal resistance, it will cause 0.5V Pressure drop of . Thus, the output voltage of the battery is 1.2-0.5=0.7V,0.7V The motor cannot be driven by the voltage of , namely ： Even if the battery voltage is 1.2V, But the internal resistance is 0.5Ω It is still unable to drive the working voltage under the characteristic of 1V The motor of .

actually , Discharge the battery with a constant current , The discharge curve is approximately as follows ：

We can see The discharge curve of the battery will be flat in the middle process , When the battery is close to full capacity and low capacity, the rate of power decline is significantly accelerated , This is related to the characteristics of electrochemical reaction . Again , The internal resistance of the battery is not a constant , This value can feed back the dynamic discharge performance of the battery .

About the impedance test , Introduce a method ： First, the battery voltage is detected when the battery is at low current output Ua, Then apply a large current to the battery I Pumping ( In practice , In order to avoid unnecessary power waste , It is suggested that the detection should be carried out in combination with the high current action of the product ), Wait for the moment when the high current pumping is over , Check the battery voltage Ub, Then the internal resistance of the battery r=( Ua - Ub )/I.

Measure the battery voltage 、 internal resistance , Consider static and dynamic situations , Add the discharge rate 、 working temperature 、 The degree of aging 、 Self discharge characteristics and other parameters , Then the calculation of battery power and these parameters can be established into a functional model , This is known as ： Impedance tracking measurement .