Why do power banks never deliver the advertised capacity?
Power banks are an indispensable accessory for mobile device users. However, the actual capacity of power banks is often lower than the specifications announced by the manufacturer. So what is the cause of this difference?
If you own a power bank, you've probably noticed that the actual capacity it provides is often lower than the number advertised by the manufacturer.
For example, in theory, a 10,000mAh power bank should be enough to fully charge a phone with a 5,000mAh battery twice. However, in reality, this almost never happens. Have you ever wondered why?
Why doesn't your power bank provide the advertised capacity?
In reality, manufacturers aren't lying to you, at least not technically. The power bank you buy actually contains lithium batteries with the capacity stated.
Unfortunately, not all of this capacity can be used to charge your device. The actual amount of power you can extract, also known as the “rated capacity”, is often lower than the nominal capacity.
The reason for this difference comes from many different factors, including energy conversion efficiency, heat loss, power consumption during transmission, as well as different voltage levels between the power bank and the receiving device.
Voltage conversion efficiency
Power banks typically provide a 5V output to suit the charging needs of most mobile devices. However, the lithium batteries inside power banks store energy at a lower voltage, typically around 3.7V.
To be able to provide the correct 5V to the device, the power bank must use a part called a boost converter, which is responsible for raising the voltage from 3.7V to 5V.
However, this conversion process is not perfect. Converters cannot reach 100% efficiency, but usually only reach around 80-90%. This means that some of the energy from the battery will be lost as heat and electrical loss during the conversion process.
This loss is one of the reasons why the actual capacity you can use to charge your device is lower than the nominal capacity listed on the power bank.
Reserved buffers reduce available capacity
Both smartphones and power banks are designed to reserve a portion of their battery capacity as a safety buffer, to prevent overcharging or complete discharge. This mechanism is important because it helps protect the battery from permanent damage and prolongs its lifespan.
However, the amount of buffer required is different between the two types of devices. Smartphone batteries are optimized for everyday use, with limited capacity and efficient power management. Therefore, the buffer they require is relatively small.
Power banks, on the other hand, operate independently, store large amounts of energy, and typically undergo more charge-discharge cycles. Therefore, they require a larger buffer level to ensure safety and long-term durability.
If the power bank is completely discharged, in some cases it may have a serious fault, causing the protection circuit to activate and refuse to accept a charge, resulting in a state where it cannot be used again.
Inefficient cooling system
Heat is the enemy of all batteries, including the lithium batteries found in smartphones and power banks. When temperatures rise too high during charging or discharging, the battery can suffer performance degradation, shorten its lifespan, and even pose a risk of serious damage.
As a result, smartphone manufacturers have incorporated a variety of advanced thermal management mechanisms to control and reduce heat output. This can include smart charging algorithms, temperature sensors, battery management systems (BMS), heat dissipation materials, and even active cooling systems on some high-end devices.
While power banks do have thermal management measures in place, their performance is often not as optimized as that of smartphones. This is especially true of low-cost power bank models, where thermal management hardware may be cut to reduce manufacturing costs.
As the temperature increases too much, the efficiency of energy conversion decreases, resulting in greater power loss. As a result, some of the battery's capacity is wasted as heat instead of being transferred to your device, significantly reducing the actual amount of power you can use.
Battery degradation
Unlike smartphones, which are updated and released by tech companies every year, power banks do not have such a rapid upgrade cycle.
This means that many power bank models may have been manufactured years ago and sat in storage for a long time before reaching consumers.
During this period of time, even though it is not in use, the lithium batteries inside the power bank will still naturally wear out. This happens because the chemical reaction inside the battery continues to take place even when there is no charging or discharging activity, resulting in a gradual decline in storage capacity.
When users buy and use a power bank, they may notice that the actual capacity is lower than the published specifications, even if the device is new.
Of course, manufacturers have taken many measures to prolong battery life during storage, such as keeping it at the optimal charge level (~50%), maintaining it in a suitable temperature environment, and using protection circuits to minimize energy loss.
However, even with proper maintenance, lithium batteries cannot avoid chemical aging over time, causing the available capacity to decrease compared to the original.
How to find a good power bank?
Manufacturers rarely disclose the rated capacity of power banks, mainly due to competitive pressure in the market. Instead, they just emphasize the larger nominal capacity number to attract customers.
However, in reality, the available capacity is often only about 60-70% of the advertised capacity. For example, a 20,000mAh power bank actually only provides about 12,000mAh, enough to charge a 5,000mAh phone nearly two and a half times.
For a more accurate estimate, you can check the product's marketing materials to see if the manufacturer mentions the number of charge cycles for certain phone models. From there, you can calculate the actual capacity of the power bank yourself. As long as the rated capacity is above 60%, you are not being fooled. The higher the number, the better the performance.
For example, the Ugreen Nexode 20,000mAh backup charger, the manufacturer claims can charge the Galaxy S24 Ultra phone (5,000mAh battery) 3 times, meaning its rated capacity is about 75%, a very impressive number compared to the general level.
However, nothing is more reliable than reading real reviews from users or experts to confirm the real performance of the product.
Additionally, you should avoid unknown brands and only buy from reputable companies, as low-quality brands often cut back on important protection mechanisms, use cheap materials, causing the product to deteriorate quickly and pose potential risks when used.