Monitoring the cycles of charge and discharge of a Li-Ion battery

17 March 2013

In this new experiment we have used openDAQ to analyze the cycles of charging and discharging of a lithium battery.

Nowadays, many portable electronics work with lithium-ion batteries. The properties of Li-ion batteries, such as light weight, high energy capacity and high resistance to self-discharge, along with the absence of memory effect, or their ability to work with a large number of regeneration cycles, have enabled a wide dissemination of such batteries in applications for the consumer electronics industry. Its expansion to other markets, however, has been limited by their sensitivity to high temperatures, which could even cause the battery to explode if not using the adequate protections.

In this case, we want to check the voltage and current cycles during the charge and discharge of an electronic device designed by us, which is carrying a lithium battery. The goal is to use that information for programming the curves in the firmware of the device and thus being able to give the user a precise indication of current charge percentage.

One must say that battery manufacturers provide good information about charge and discharge curves for their batteries, but these curves are usually measured at constant current, but this does not always happen in the real world, as discharge current always increases as the battery voltage is falling.

With this purpose, we have connected openDAQ between the battery and the device, and used a 0.1Ω shunt resistor to monitor the current that is circulating between them. Following there is a scheme of the experiment:

 

As can be seen, the differential voltage between the inputs A7 and A8 will give us indication about the current flowing from the battery at each time, while we monitor actual battery voltage at input A8.

The current flows during operation for this device are around 200mA, while its internal charging circuit provides a current up to 500mA battery. Charging lithium batteries usually involves several phases, the main ones being at constant current load and constant potential load. The discharge current is obviously not equal depending on what the device is doing at that time but, for simplicity, throughout this experiment the device remained in the same state doing a typical work cycle.

We used the application EasyDAQ activating two graphs simultaneously, one to monitorize current and another for the battery voltage, and collecting the data every 3 seconds. In the next graph you can see the result of the voltage curve during discharge:

 

Of course, these graphs depend greatly on the temperature. In this case, we can very well simplify by assuming that the normal operation of the equipment will be carried out at building temperature (20-25 °C).