Year: 2017

For lithium ion cells, there are two facts to consider when determining battery valuation by lifetime capacity:

(1) An energy cell gives you one-stage of life but a power cell gived you two-stages of lives, including the life of a power cell followed by another life as an energy cell.

(2) An 18650 energy cell should not drive a heavy electrical load above 7.0 A while an 18650 power cell can drive high current loads ranging from 10 A to 30 A, depending on its nominal C-rate.

Why can a power cell perform 2-stage lives?

The answer lies in a cell’s DCIR. DCIR or DC internal resistance is an important measurement for differentiating a power cell from an energy cell. The picture below shows two DCIR curves that are measured after 3 months (blue line) and 9 months of storage (red line).

DCIR curves are usually shaped like a valley after discharging the battery. Let’s pay close attention to the trough segment of the curve. The most meaningful DCIR values occur at the trough, which is the minimum DCIR. From now on, we will refer to minimum DCIR as mDCIR.

 

The following two points explain the difference of lifetime capacity between an energy cell and a power cell:

1. A well manufactured 18650 energy cell starts its service with mDCIR around 60 mOhm, and after 500 deep cycles consumption, it retires with mDCIR around 120 ~ 150 mOhm.
2. A well engineered 18650 NMC 5C power cell starts its service with mDCIR around 40 ~ 50 mOhm. After 1000 ~ 1200 deep cycles at 1C discharge rate, its mDCIR may increase up to around 60 mOhm, where it ends its power cell performance. Afterwards, it can cycle continuously as an energy cell. At a discharge rate of 1C, the energy cell stage can last another 700 ~ 800 deep cycles before retiring with mDCIR around 120 ~ 150 mOhm.

Panasonic NCR18650B versus Ameribatt NMC18650-22PL5

The popular Panasonic NCR18650B is a famous premium 2C Li-ion energy cell. Panasonic publishes its cycle life chart as shown below. At 1C rate of discharge, a new NCR18650B delivers 3.35 Ah with mDCIR around 62 mOhm. After about 500 cycles of 1C discharges, a retired NCR18650B delivers about 2.27 Ah (i.e. 68% of its nominal capacity) with mDCIR rising up to 114 mOhm. If you integrate (sum up) the area below the blue curve, its lifetime capacity delivers about 1,400 Ah. Panasonic rates this cell as a 2C energy cell, which means that you may use it to drive an electrical load up to 6.7 A (2C rate) without abusing it.

 

The AmeriBatt NMC18650-22PL5 is a US-engineered 5C Li-ion power cell. At 1C rate of discharge, a new AmeriBatt NMC18650-22PL5 delivers 2.2 Ah/discharge with mDCIR around 45 mOhm. After 1,260 deep cycles at 1C discharges, its capacity drops to 1.76 Ah (78% of its nominal capacity) with mDCIR around 63 mOhm. During this power cell stage, it can drive an electrical load up to 11 A (i.e. 5C rate) without any abuse. Afterwards, you can continue to exploit the cell as a 1.76 Ah energy cell for another 740 deep cycles at the same 1C discharge rate. A retired NMC18650-22PL5 can still deliver about 1.4 Ah/discharge (i.e. 64% of its nominal capacity) with mDCIR around 116 mOhm.

If you integrate (sum up) the area below the blue curve, its lifetime capacity delivers about 2,470 Ah as a power cell (with high current deliverable) and another 1,480 Ah as an energy cell. The total lifetime capacity of 3,950 Ah (about 2,000 deep cycles) is available.

Cycle LIfe Comparison of Panasonic NCR18650B versus Ameribatt NMC18650-22PL5

When you superimpose the cycle-life curve of Panasonic NCR18650B cell over that of AmeriBatt NMC18650-22PL5 cell by aligning their DCIR at 62 mOhm, you begin to notice that the Panasonic cell initiates on cycle 1256 and retires after cycle 1755. Please see vertical line separating the energy-cell phase and the power-cell phase in the next chart below.

To visualize the difference between the two LIFETIME capacities, we just integrate each cycle-life curve and colorize the area under each curve. You may intuitively visualize the battery’s valuation by their LIFETIME capacities. The blue area represents the lifetime capacity that a Panasonic NCR18650B cell can deliver while the green area (including the green area under the blue) represents the LIFETIME capacity that an AmeriBatt NMC18650-22PL5 cell can deliver.

 

Comparison of NOMINAL versus LIFETIME capacity between the Panasonic NCR18650B and AmeriBatt NMC18650-22PL5 is summarized in table below for reference. Note that these figures are based on the 1C discharge rate.

The above facts may be new to most people. We hope that you get some insights from the recently advanced Li-ion technology and benefit from it. A well-engineered NMC [Li(Ni_1-x-yMn_xCo_y)O₂] power cell made with proprietary electrolyte and additives delivers 3 key goals:

1. Longer lifespans even when deep cycled
2. Higher LIFETIME capacity
3. Greater battery valuation

The above 3 goals are achieved because the aging rate of a cell’s internal ionic resistance is slowed down completely. Based on that fact alone, we should recalculate the battery’s valuation that power cells deserve. This issue can become quite complex and difficult to resolve since it involves the shift of market demands and supplies.

Different perspectives when doing a battery valuation

From an intuitive perspective, battery valuation of a nominal 3.35 Ah cell should cost 152% contrasting to that of a nominal 2.20 Ah cell because the ratio of run-time per discharge is 152% (i.e. =3.35/2.20). Consumers normally do not sense the battery value from a power cell with longer deep cycle life characteristics. They are willing to pay by nominal capacity because they can easily have faith in the sensible short-term user experience.

From an analytic perspective, battery valuation of a cell with LIFETIME capacity of 3,950 Ah (i.e. green area) should cost 282% to that of a cell with lifetime 1,400 Ah (i.e. blue area) because the ratio of the total (lifetime) runtime is 282% (i.e. =3950/1400). Industrial users consisting of analytical engineers should be more likely to believe in the deep cycle life test data that differentiate the value between a power cell and an energy cell.

However, there are great distortions of what constitutes greater battery valuation ever since lithium ion cells were first commercialized in huge demand as energy cells for the laptop PC industry in 1991. Battery valuation may be corrected step by step, based on market demands that shift from the lithium ion energy cells to the expansive demands of lithium ion power cells in near future. We are pioneers when it comes to determining battery valuation via lifetime capacity. Battery valuation by lifetime capacity is the most rational way to conserve Earth’s finite resources so that human descendants can thrive longer on Earth.

What happens when you abuse an 18650 rechargeable LiMn₂O₄ battery (refer to next photo)? We address this question by explaining why the electronic IR (internal resistance) of a lithium battery becomes much higher if abused. Additional topic about the increasing ionic IR in a Li-ion battery is beyond the scope of this blog post. We may bring that back later on. But for now, let us dissect an abused 18650 rechargeable lithium ion battery.

The reality about most high drain cells sold in the market today

The picture above shows a high drain LiMn₂O₄ battery that’s designed to drive a heavy electrical load up to its advertised 35 A. They are popular in the Vaping & MODS industry. However, the cycle count only lasts approximately 150 ~ 200 cycles. Their lifetime capacity may accumulate no more than 300 Ah, which is only about 12% lifetime capacity of a well-designed 18650 such as the AmeriBatt Heavy Duty NMC cell. There are several factors contributing to the short cycle life of your typical high drain cell. Unfortunately, most consumers don’t understand this fact and continue to pay high prices for it.

Let’s look at the following two photos as we dissect an abused 18650. Note that this dissection is from an abused IMR18650 35A high drain LiMn₂O₄ battery.

Structure of negative electrode of an abused 18650

The above photo shows the abused negative electrode. The black layer consists a mixture of conductive powder, additive, binder and anode active (graphite) material, which should adhere to the RA (rolled copper alloy) copper foil seamlessly. Unfortunately, many large areas of the mixed binder material just simply peel off. The effective areas of the electron collector at negative electrode is substantially lost, which results in higher resistance in the electronic path.

Structure of positive electrode of an abused 18650

The above photo shows the abused positive electrode. The black layer consists a mixture of lithium carbonate, additives, binder and cathode active material, which should adhere to the aluminum foil seamlessly. Unfortunately, many large areas of the mixed binder material just simply peel off. The effective areas of electron collector at the positive electrode is substantially lost, which results in higher resistance in the electronic path.

Binder composition used in 18650 lithium ion cells

The binder material is made of PVDF (polyvinylidene difluoride) and SBR (styrene-butadiene rubber). They are highly abrasion resistant and have good aging stability. When mixed into slurry in a vacuum, they exhibit very strong adhesive property and good tensile strength. But what makes their adhesion so flimsy? The answer is in continuous cycling of the battery at very high discharge currents such as 20 ~ 30 A. When discharged continuously at these high currents, the cell gets hot within a few minutes. A hot cell experiences immense strain between binder and metal foil due to their different temperature coefficients. The effect of this strain is more pronounced as the battery body temperature goes above 75 °C.

How to prolong lifespan of your 18650 high drain cell?

To extend the life of your battery, you need to reduce the electronic IR increase by preventing the battery body temperature from going above 60 °C when you discharge it. Do not take the max continuous discharge current of the advertised 35 A at face value! Always measure the battery body temperature to determine the proper discharge current so that it does not exceed 60 °C. Improper discharge practices may instantly increase the electronic IR by 5~10 mOhm, which has a similar effect as a cell that’s undergoing ionic IR aging by 6~12 months! Both the electronic IR aging and the ionic IR aging are irreversible!