How to Choose the Right Discharge Rate for Battery Capacity Testing?

When testing battery capacity, the discharge rate (or discharge current) is a key factor that greatly affects the results. Different brands of battery chargers set various discharge currents for their capacity grading functions, and ordinary users often don’t know which one is better.

So, in order to make sure the measured capacity is precise and reasonable, how do you choose the right discharge rate? In the following, we’ll discuss the recommended discharge rates for different batteries to help you better understand this important parameter.

Why is the discharge rate so important for battery capacity testing?

The discharge rate directly affects the battery capacity testing and overall performance. A well-known battery reviewer, Admiral134, tested the capacity of the new XTAR 18650 3500mAh battery at different discharge rates. The results showed that as the discharge rate increased from 0.2C to 2.85C, the battery capacity gradually decreased.

This happens because at higher discharge currents, the internal temperature of the battery rises, causing the chemical composition to decompose more quickly. Some of the battery capacity may be lost as heat or crystallization, which ultimately reduces the battery capacity and can even shorten its lifespan.

An inappropriate discharge rate can lead to misleading test results, while the right discharge rate not only ensures accurate capacity testing, but also improves the overall performance and reliability of the battery.

What is the discharge rate of a 3.6/3.7V lithium-ion battery?

For 3.6/3.7V lithium-ion batteries, the International Electrotechnical Commission (IEC) specifies a“standard discharge rate”- 0.2C. This means that at 20±5°C and with a cutoff voltage of 2.5V, discharging at 0.2C will measure the actual capacity of the battery.

Taking a 18650 3500mAh battery as an example, the recommended discharge current can be calculated as :

• 0.2 x 3500mAh=700mA

The test results from Admiral134 also confirm this. At a discharge rate of 0.2C, the capacities of two batteries were 3494mAh and 3489mAh, which are very close to their nominal capacity of 3500mAh. However, when the discharge rate was increased to 2C or 2.85C, the measured capacities dropped significantly to less than 3400mAh. Similarly, for 3.2V LiFePO4 batteries, a 0.2C discharge rate is also recommended for capacity testing.

What is the discharge rate of a 1.2V Ni-MH battery?

NiMH batteries have a relatively limited capacity, so high discharge rates are usually not suitable for capacity testing. Generally speaking, it’s recommended to use a discharge rate that fully depletes the battery in five hours, which is 5-hour rate (C/5). For example, for AA NiMH batteries with capacities between 2000-3000mAh, the right discharge current is 0.4-0.6A. For AAA NiMH batteries with a capacity of around 1000mAh, the right discharge current is about 0.2A.

The figure below shows the typical discharge curve for Ni-MH batteries at a 5-hour rate. The initial voltage of the battery quickly drops from about 1.4V to 1.2V and then stabilizes at a relatively stable level. From the flatness of the curve, it can be seen that the discharge capacity at C/5 is very close to the rated capacity. This makes the C/5 a reliable discharge rate for testing Ni-MH battery capacity.

What is the discharge rate of a 1.5V rechargeable lithium-ion battery?

1.5V rechargeable Li-ion battery is an emerging battery, and there is no unified discharge rate. For this kind of battery, the capacity of AA size is generally around 2000mWh-4000mWh, and the capacity of AAA size is generally below 2000mWh. According to the testing experience of XTAR lab engineers, it is recommended to discharge AA batteries at 0.5A and AAA batteries at 0.2-0.3A for capacity testing.

When Admiral134 tested four XTAR AA 4150mWh (2500mAh) Li-ion batteries, he also found that 0.5A discharging provided the most accurate test results, closely matching the rated 2500mAh, with  capacity fluctuation not exceeding 70mAh.

However, for ordinary users who do not have load machines or a capacity grading machines, there is a lack of household devices on the market for testing the capacity of these batteries. To address this issue, XTAR developed the first VX4 charger that can testing the capacity of 1.5V rechargeable lithium-ion battery. It uses a constant discharge current of 0.3A, effectively protecting battery life and providing accurate capacity testing results.

Conclusion

When testing battery capacity:

• 3.6/3.7V Li-ion battery: 0.2C discharge is recommended.

• 1.2V NiMH battery: C/5 discharge is recommended.

• 1.5V rechargeable Li-ion battery: The discharge rate should be selected according to the battery size. It is recommended to discharge AA size at 0.5A and AAA size at 0.2-0.3A.

By the recommended discharge rates above, you will get more reliable capacity results, helping you use the battery better. This will also help you avoid trouble caused by different discharge current settings when choosing a battery charger.