How to test lifepo4 battery？

When we buy lithium phosphate batteries, the most important thing we care about is how long they can last, after all, this directly affects the battery life of the device and our wallet. I have been thinking about how to measure the battery life, and I have tried many methods and stepped on pitfalls. Now I have some experience and I would like to share it with you. ​
In fact, the most common standard for judging the life of lithium phosphate batteries is to see how many times they can be charged and discharged. From full power to empty power, and then fully charged, this is considered a cycle. Generally speaking, when the battery capacity drops to about 80% of the original purchase, it is almost time to consider replacing the battery. To know how the battery life is, the key is to focus on the two indicators of capacity and internal resistance. ​
Let’s talk about capacity testing first, which is the first step in judging the battery life. The easiest way is to manually charge and discharge several times. First, fully charge the battery, and write down the power displayed on the mobile phone, electric car dashboard or device, or look at the capacity data in the battery management system. Then use it normally until the device automatically shuts down, and then fully charge it, and write down the power charged this time, which is basically the actual capacity of the battery now. Repeat several times, and you can clearly see that the battery capacity is slowly decreasing. For my own battery, the capacity dropped from 100Ah to 92Ah in the first 100 cycles, which means that the battery is starting to “not work”. ​
However, manual testing is too time-consuming. If you want to test accurately, you still have to rely on a professional battery tester. This instrument can set parameters such as charging and discharging current and cut-off voltage by itself, and can also automatically record data and generate a capacity change curve. I used a portable one before. When the battery capacity starts to decrease and how fast it decreases, I can see it all at a glance. It is much more convenient than measuring it yourself, and the data is also reliable. ​
Let’s talk about the internal resistance test, which is also an important method to determine the battery life. The longer the battery is used, the greater the internal resistance. If the internal resistance is too high, the battery performance will deteriorate, and there may even be safety hazards. I used a multimeter with internal resistance measurement function to measure it. I set the multimeter to the gear for measuring internal resistance, connected the positive and negative poles of the battery, and the value came out. The new battery has a small internal resistance, and the more times it is used, the higher the internal resistance will be. I have a battery with an internal resistance of 30mΩ when I first bought it. After using it 300 times, it increased to 65mΩ. At this time, I could obviously feel that it charged slowly, consumed power quickly, and the battery life was much shorter.

During the test, I also found that temperature has a great impact on the battery. Lithium phosphate batteries are in the best condition at around 25℃. If the temperature is too high or too low, the battery will age very quickly. If conditions permit, try to find a place with a constant temperature when measuring the battery life. If there is really no other way, you have to remember the temperature at the time when measuring, so that the measured data will have reference value. ​
Another particularly important point is that the current size of charging and discharging will also affect the test results. When charging and discharging with a large current, the chemical reaction in the battery is more intense, and the capacity drops faster. So if you are not in a hurry, try to use a small current to measure. I usually use a current of 0.2C, which can be completed quickly and the data is more realistic. ​
After this test, I understand that there are too many benefits to regularly “checking” the battery. Knowing the battery’s condition, you can change some bad habits that hurt the battery, such as not overcharging and discharging, and choosing a suitable charger and charging environment, so that the battery can be used for a longer period of time.​
In the final analysis, measuring the battery life requires a bit of patience, more observation and more records, and taking into account the data of capacity and internal resistance, combined with factors such as temperature and current, in order to understand the true life of the battery and arrange its use and replacement reasonably.