Ah and Wh are confusing, which one represents battery capacity?

How to distinguish between capacity and energy?

In fact, there is a very simple way to distinguish between capacity and energy, which is to look at the unit. The unit of capacity is Ah, you can look at mobile phone websites or rechargeable batteries, they all use capacity to measure the size of the battery. Common rechargeable treasure has 5000-10000mAh, and Huawei’s latest Mate60Pro has a 5000mAh battery.

While in new energy vehicles, we use the indicator of energy to describe the battery, the unit is kWh. For example, the battery energy of the long range version of the Tesla Model 3 is 78.4kWh, which is often referred to as 78 degrees of electricity. But I don’t suppose you’ve ever heard it said how many Ah the Model 3’s battery capacity is? Mobile phones and cars are both batteries, so why is there no standardisation of commonly used units? When do we use capacity Ah and when do we use energy kWh?

The relationship between capacity and energy

In fact, it seems confusing between the two units, energy and capacity, there is a connection – as long as you know two parameters of a battery cell, you can calculate the third. And what do these three values mean?

kWh=v * Ah

Let’s look at the most familiar first – voltage. This is something we come into contact with all the time, and different batteries have different voltages. An alkaline battery is 1.5V, a lead-acid battery 12V, a lithium iron phosphate cell 3.2V, and a ternary cell around 3.7V. But what I want to emphasise to you is that these are rated voltages. And in reality, no matter which type of battery cell, the voltage is different when fully charged and discharged – high voltage in the fully charged state and low voltage in the deficit state.

Let’s talk more about capacity

The process of charging and discharging a battery is actually the process of moving charged ions between the positive and negative electrodes. The more ions that can flow after a full charge, the greater the discharge capacity of that battery.
So, how do you measure the discharge of a battery cell? As we just said, the voltage of the battery is different every moment. Therefore, engineers need to use a constant current to test the capacity of the battery during research and development, so that the results obtained will be more accurate.

Using a certain current and discharging for a certain amount of time spells out Ah, which is the unit of capacity.
When a battery cell is developed, engineers will use the size of the capacity to distinguish between different battery cells, which has formed a convention over time. For example, a piece of Xiaomi’s battery cell 20000mAh; a piece of automotive-grade battery cell 203Ah from Ningde Times.

Series and parallel connection

No matter how many times the difference between the two, the reasoning is the same.

On 3C products like mobile phones that use only one battery cell, habit becomes nature, and people directly use the capacity parameter printed on the battery to call the size of the battery.
However, not all batteries that use only one cell. We can also increase the number of cells by series or parallel connection, basic physics we have learnt, series can increase the voltage, parallel can increase the capacity. By adjusting the number of series and parallel connections, it is possible to obtain the final required battery energy.The working voltage of new energy vehicles is 300 or 400V, or even higher 800V, while the output voltage of a Li-ion battery cell is usually around 4V.

In this way, we need to put about 100 sections of battery cells, to the series connection, this time the capacity remains unchanged, the voltage rises to 400V.

Let’s conclude

When engineers develop a single battery cell, they talk about capacity. Because the 3C products around, such as mobile phones, rechargeable treasure with a single cell in general. we have long been accustomed to using capacity to call the battery.
But the battery pack consists of many cells connected in parallel, the capacity has been unable to distinguish, the external performance indicators, is energy.
The battery of new energy vehicles is a typical multi-cell battery pack, so we use energy to measure.
But in the end, we are most concerned about the battery can release the external energy, which is the real measure of the battery “size” of the index. Don’t get it wrong next time.