Battery Capacity Tester Li-Ion-Calculation and Testing

How do you calculate li-ion capacity?

Lithium-ion cells have a capacity that is measured in ampere-hours (Ah). An ampere-hour generally means that you are capable of extracting one ampere from the battery for an hour. Practically, if you have a 1 Ah cell, it means that you can draw either one ampere for an hour or 2 A for 0.5 hours and even 0.25 A for 4 hours.

Ah, capacity represents a measure of stored coulombs within the cells. If you were to multiply ampere × time, you would get coulombs since one ampere is 1 coulomb per second. For example, because one hour = 3600 seconds, then 1 Ah would be 3600 ampere-seconds. To get coulombs, you would have to multiply this by the total time used or the number of seconds: 3600 coulombs/ second × seconds = 3600 coulombs. However, for smaller cells, one may find that their capacity is measured in milliamp-hours (mAh).?

Cell capacity can also be measured in watt-hours (Wh), which is a measure of stored energy. One watt is equal to one joule per second, and if you were to multiply watts × time, you would get joules. For 1 hour = 3600 seconds, then 1 Wh = 3600 watt-seconds as you would’ve increased one × 3600. This resulting figure is also equivalent to 3600 joules of stored energy within the cells.

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What is a battery capacity tester?

Most conventional battery packs do not come with built-in meters that show the capacity of the battery or tell you when they are out of power and thus, need to be replaced without having to test them first. Without the presence of a battery capacity tester or multimeter, which can also be used as a suitable alternative, the only way to tell that a battery still has charge is to pop it back into its intended electronic device and try turning it on. Such a process is time-consuming and may not yield accurate results since some batteries may only have a low charge but the electronic device may not function thus, indicating that the cells are dead.

A battery capacity tester is, therefore, an electronic device designed for testing the capacity of any battery pack and determining the amount of charge left in the cells. These devices work by checking the current that comes from the cells of a battery. Conductive wires are connected to both terminals on the battery and this stimulates the release of current. Each battery capacity tester also comes with a gauge that indicates the level of charge when the battery is connected to the tester.

Battery capacity testers only measure the current and use resistors to do so. Depending on the type of tester, the gauge is moved by other components, which is typically a pin. As the resistor receives more amperage, the pin moves further on the gauge, indicating more charge within the cells. However, batteries such as lithium cells, may, on many occasions, give inaccurate readings. These results are because the current in such batteries does not wear down evenly, as is the situation in alkaline cells. Instead, at specific charge levels, the current takes a significant drop and thus, the batteries might read fully charged even when they only have half the charge remaining.

Regular battery capacity testers are incapable of testing rechargeable batteries like lithium-ion cells, and telling whether or not they can be recharged. For such situations, you will need to get a battery tester that is specifically designed to serve rechargeable batteries.

How do you test a li-ion battery?

Before testing any li-ion battery, you should consider the testing frequency. How often should you do a capacity test on a lithium-ion cell? Studies show that the best way to determine when to perform capacity tests is by monitoring the intended batteries with occasional resistance measurements. Doing this reducing the need for unnecessary testing and, thus, only incorporates testing when it is essential. For your testing frequency plan to be effective, you need one that is condition-based and not calendar-based. A condition-based testing frequency plan would accommodate all other variables and factors that may affect battery life, while a calendar-based one tends primarily depends on assumptions that may be inaccurate.

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While an internal resistance test shows you the presence of potential weakness in the cells, a capacity test would tell you whether the defect is severe enough for action to be taken. However, one should note that capacity tests tend to reduce battery life and should, therefore, be done occasionally to create a reasonable loss in exchange for knowing the battery’s condition. Such capacity tests are carried out mainly because the users are more concerned with improving the reliability of the cell rather than its longevity. Once a battery is incapable of providing the specific amount of energy required, then the period to which it can continue to supply any power at all becomes irrelevant because such energy would be insufficient.

Testing the battery

Turn on your battery capacity tester and set it to measure voltage readings. Connect the tester’s sensors to their designated battery terminals of your Li-ion cell accurately. Check the voltage reading and note it down and then disconnect the sensors from the terminals. If the battery charged, then the voltage reading you noted down should resemble that of the label on the battery or slightly higher for a fully charged battery. Li-ion batteries are designed to maintain a constant voltage during most of the discharge period or process. This means that even if the battery if discharged to half of its total capacity, the voltage reading is likely to be slightly lower than the one indicated on the battery label.

However, if the voltage reading indicates a figure that is more than 20% lower than the one on the label, then it means that the battery should be recharged. Change the settings of the battery capacity tester to measure mAh. Repeat the above procedure while noting down the readings again. If your battery is fully charged, the readings from the tester should be similar to the ones you noted down.

Final Thoughts

Battery capacity testing gives you a sneak peek of the current condition of the cell. It however, doesn’t certify the battery will remain the same or perform as expected for the next few weeks or months regardless of appropriate maintenance. For this reason, battery capacity testing should only be done occasionally to avoid reducing the battery’s life and get information on the required replacement times.