Smart materials and sensors for estimating a battery state of charge STUDENT NAME: MOK MING HEI STUDENT ID: D SUPERVISOR: DR. S.BOLES PROJECT ID: FYP_4006A_39

Methodology This project investigates in two main problems: 1. Try to find out the correlation between the state of charge and the degree of swelling of a lithium-ion battery with the use of accelerometer. 2. See whether accelerometer is viable to use as the safety device of lithium-ion battery. Therefore, 2 different tests were carried out.

1. Using Accelerometer to estimate the state of charge

WAYS to place the accelerometer 1. over the centre of the package

WAYS to place the accelerometer 2. use an extension arm to exaggerate any swelling in the package and place the accelerometer over the extension arm

Programming for Arduino Program work for the ADXL345 accelerometer could show the change in m/s^2 of both x and y axis. the delay time for each output signal generation = 10 s +/-2g is selected to use in this project

Setting for the VMP-300 mode GCPL (Galvanostatic cycling with potential limitation)

Experimental Results no significant swelling even the battery goes over-voltage to 5.1V. Most of the data is floated between +/-0.04m/s^2, these floats are believed that mainly come from the disturbance of external environment and the normal error of the accelerometer.

Take test 22 as an example Accelerometer ADXL345 Delay: 10s Gravity: +/-2g VMP-300: Charging current: 500mA Discharging current: -500mA Max voltage reached: 5.0V (overvoltage; max v of battery: 4.2V) Min voltage reached: 3.6V It took 6 hours and 7 minutes for the whole charging and discharging process in test 22.

Possible reasons that the tests carried out could not assist to estimate the state of charge of a lithium-ion battery 1. Low C-Rate 2. Accelerometer used not sensitive enough

Low C-Rate The lithium-ion battery used in this project is a 2500mAh battery While the VMP-300 could only provide a max charging and discharging current of +/-500mA means that the max C-rate can use is only 0.2C

Low C-Rate As found by the researchers of University of Michigan, the higher C-rate operation in a lithium-ion battery would lead to a larger temperature increase, hence result in a greater thermal swelling of the battery Moreover, even using a household battery charger, the charging current could be up to 1A or more. Therefore, it is believed that low C-rate would be one of the reasons why the swelling is not observed in this test.

Accelerometer used not sensitive enough ADXL345, its sensitivity of x, y, z axis is 256LSB/g under +/-2g, and its resolution is 0.04m/s^2 high-precision contact-type displacement sensors which are for displacement measurements are used in most study of swelling of lithium-ion battery, they are with 1 μm accuracy and 0.1 μm resolution

2. Accelerometer as a safety device to lithium-ion battery To simulate the serious swelling of a lithium-ion battery, which may lead to malfunction of the battery and risk of fire. By using the simulation, to see whether accelerometer could be possible to use as a safety device of lithium-ion battery.

Setup

Procedure The inflatable bag is empty at first and air would be pumped inside, in order to simulate the situation which the gas is warmed and expanded inside the lithium-ion battery due to overcharging or malfunction of the battery. delay time of 100ms is used

Experimental Results 6 simulations were conducted under the expansion time of 0.5s, 1s, 1.5s, 2.5s, 3s and 4s it is found that the accelerometer would be effective in measuring the rapid expansion

SimulationExpansion time Slope ( △ y /no. of dots) 10.5s s s s s s0.0725

Degree of tilting angle of tilting could be calculated through the following formula: angle = sin^-1(h/21) An increase of 1m/s^2 in y direction is equal to an increase of 1.7cm in actual height

Take simulation 2 as an example: PointHeight(cm)Angle(degree)

Take simulation 2 as an example: The change in volume could be one of the index to the safety condition of battery as well. To calculate it, method in the below figure could be used:

Take simulation 2 as an example: To approximate the volumetric change of the battery, assume highest point the accelerometer reached is both the centre point of the battery and expansion, and consider the swelling as two identical right-angled triangle as showing below.

Take simulation 2 as an example: To find the angle A, using the equation: angle A = 180 – 90 – tilting angle By letting W be the width of the battery, then the approximate change in volume could be calculated by (2 x 1/2 x 21 x h x sinA x W). PointVolume change(cm^3)(W=width) W W W W W W W

Experimental Result With the use of accelerometer, we could collect and calculate the information of tilting angle, height of swelling, slope of magnitude of y direction over time, approximate change in volume during swelling. These data can all be the index to monitor the physical change and the safety condition of lithium-ion battery.

Experimental Result Therefore, accelerometer would be viable to use as a safety device for lithium-ion battery since it is effective in measuring rapid swelling rate, once a significant changes in motion recorded, it could alarm the user to stop using it anymore and lower the risk of fire and explosion.

Accelerometer as a safety device to lithium- ion battery -Economic Consideration Cost of the equipment needed for detection: Arduino Board:HK$250 Accelecrometer:HK$70 TotalHK$320 ApplicationCostCost Increase SmartphoneHK$150~47% (adding accelerometer only) Hybrid electric carUS$8,000-18,000~0.5%

Economic Consideration Smartphone battery a large scale of increase in the price of the battery, customer may not be willing to pay such more to purchase it. Therefore, it may not be viable to equip inside a cell-phone used battery due to this economic consideration.

Economic Consideration Hybrid electric vehicles application The scale of cost increasing is certainly small users are having relatively high spending capability the life time of a HEV battery is much longer than smartphone one (10-15 years) Therefore, it would be worth to install an accelerometer on the HEV in a long term basis, which could alarm the users when the battery comes malfunction.

Conclusion The sensitivity of small accelerometer may not be high enough to measure the tiny expansion, therefore it is not a suitable sensor to estimate the state of charge On the other hand, accelerometer could measure the rapid expansion effectively and it is therefore viable to use as the safety device of lithium-ion battery.