How to tell a shoddy power bank

High light: You may notice that market prices of power bank products vary widely. It might be hard for a consumer to tell how good or bad a power bank is by just looking at product appearance. This article shows a way of estimation on what a normal power bank should be able to perform which you can simulate to identify those shoddy power bank products.

Power bank is mainly consisted of Lithium battery cells and boost up circuits, Let’s look at them one by one.

First--Battery cells. Most ultra low price power bank might use three types of battery cells:

(1) B, C grade cells or rejected cells from production line which are usually much cheaper than A grade cells (quality product),

(2) used and refurbished cells which are at end of its life span.

(3) under specification cells (another word: power bamk is over marked) in capacity.

Good news is all three types of cells can not provide as much energy as its nominal capacity claimed in packing or by seller. So a simple charging test can easily disclose true capacity of a power bank.

Here is an example test using iPhone 4S and a power bank product with nominal capacity of 2000mAh.

Step 1: Use your iPhone 4S until about 5% battery level left, meanwhile make sure the power bank under test is fully charged.

Step 2: Let the power bank to charge your iPhone 4S and see if iPhone 4S can be charged to full.

Step 3: Refer to below diagram, assuming 5% energy loss on internal cell and USB connection, we may consider an overall energy transferring effectiveness factor F as 0.86*0.9*0.95 = 0.73. This factor means your mobile device being charged can receive 73% of energy stored in power bank cells that is 2000*0.73=1460mAh. This should be good enough to fully charge an exhausted iPhone 4S with built in battery capacity of 1430mAh (to know other Apple product’s built in battery capacity details, look at “Apple product’s battery capacities” post.)



Considering you have about 5% energy left over to support iPhone’s power consumption during charging hours, there should be no reason for a true 2000mAh power bank to fail such test.

In the real world, overall power transferring factor may vary in range of 0.70--0.76 with products due to different circuit design and different current output capability, the higher power bank capacity with higher current output, the lower effectiveness factor usually has. Another thing need to take note: most of power bank design do not allow its battery cells discharging to zero level to protect cells’ life span (look at "FAQ on power bank” post to know more), discharging stop threshold is usually set at 2%-4% of cell nominal capacity depending on design, therefore, we suggest a 0.68—0.74 transferring factor range for simulating above test.

Nevertheless, such test with a proper transferring factor selected can fairly gauge a true capacity of a power bank product. For example, a 7800mAh power bank should be able to charge your Galaxy SIII phone at least 2.5 folds. (7800*0.7/2100=2.6).

Second—Boost up circuit. Some cheaper power bank product might use cheaper and low quality passive components to built boost up circuit, as a result, such circuit will behave lower current output capability then what it claimed. For example, a marked 1A USB port can not supply 1A current output at rated voltage 5V. To verify this, you may need to connect a 8W, 5 ohm resistor (or two such resistors in parallel for a marked 2A port) to that USB port +5V output and then measure a voltage crossing over the resistor, the lower current output than marked current, the lower voltage over the resistor than 5V.  From formula V/R=I, you can derive its real current output capability.

But, it might be difficult for a consumer to perform such test without proper tools. Most of smart phones are requiring minimum 0.5A charging current only, an ideal device to test a marked 1A or 1.2A output port is iPad3 which require 1.2A minimum charging current when its battery level fall below than 5%. If a power bank under test is not able to supply such current at 5V, its output voltage will drop to below 4.9V where iPad3 will stop charging process.


Wrapup: Power bank is not like clothes or shoes which you can roughly know their quality by touching and feeling its surface material, plus, many consumes have totally no idea about how a certain power capacity should be performing, so there are many dishonest power bank products being over marked on capacity and selling at lower price than those honest products. We believe that consumer has right to know the true capacity of power bank before he/she comparing product price. In fact, by letting power bank to charge your mobile device with proper procedure and a factor selected (0.68—0.74), a consumer can precisely quantify a power bank true capacity.


Author: Product Manager from

31th May 2013