FAQ about Power bank product

 

Question 1: How to understand battery cell life span by such specification as “500 cycles on 100% DOD”?

Answer: First, let us understand some terms, Term DOD stands for “Depth of Discharging”. Term C is nominated discharging current. For example, 0.8C discharging current of 1000mAh (C=1000mA) battery is 800mA. Now 100% DOD means discharge a battery to zero energy level at discharge current of 1C. A Lithium ion or Lithium polymer battery can be discharged to certain depth level and recharged to its full capacity, such discharge-recharge procedure is called one cycle.

A battery full capacity will drop down (or fade) along with many factors. The deeper level of discharge; more cycle times; higher discharging current; higher temperature; more overcharged; longer storage time, the faster fading speed of battery full capacity. Among these factors, DOD of each cycle and cycle times are major factors in application.

The spec of “500 cycles on 100% DOD” is saying “after 500 cycles of 100% depth discharging at 1C current and recharging to full capacity, battery full capacity is at least 60% or above of its original capacity at first cycle”, which is industry standard destroying test and quality benchmark for A grade battery. It provides an effective quality measurement for battery cell in short time.

Since battery full capacity will fade much faster after falling below 60% of original capacity, so a battery when its full capacity dropping to 60% of original capacity will be consider at end of its life span.

At last, let us look at how to define a battery full capacity, in above destroying test, charge will be stopped when charging current drop to C/20. That stage represents 98% of nominated capacity. Any further charge will be greatly reducing battery cell life span.

 

 

 

Question 2: How to estimate my power bank life span by reading its battery cell life span specification? Does it mean my product can only be used to charge device 500 times?

Answer: Of course not! 500 times on 100% DOD is an extreme destroying test and you never have chance to use battery cell in that way. Power bank product built in circuit will manage level of DOD and make sure user get much more cycle times. Look at below table illustrating three group of battery cell testing  with different level of DOD.

 

100% DOD

80% DOD

50% DOD

Percentage of DOD on each cycle

80%

300

500

800

 

60%

500

800

1400

 

40%

600

1000

1700

 

Remaining Capacity after above cycles

 

 

 

Cycle times

Table: At a constant temperature of 25 degree C and 1C discharging rate

In real application, you even do not have chance to discharge power bank at constant 1C current. Most of power bank product design set DOD level at 90%-95%, some design may set charging threshold at 4.1V instead of 4.2V which representing 92% of nominate capacity. Thus, for a well designed power bank, its application cycle times can easily reach 1000 times even you discharge it to auto cutoff in each cycle. It will be much more than 1000 times if you do not discharge it to the auto cutoff each cycle and recharge it more often. What product design compromised is only a balance point on either more cycle times or more real usable capacity, but not both.

Therefore, power bank life span is partially depending on design and mainly depending on how you use it, provided the product you bought is using A grade battery cells.