PART 2 - CYCLE RATING VS. GRADE A : FINDING ACCURATE CYCLE RATING --

How Does Cycle Life Relate to Grade A?

Grade A simply means the cell meets the manufacturer’s quality and safety specifications of the cell.  One can find Grade A cells at all of the cycle rating levels noted above.  Grade A is not a uniform level of quality.  One really needs to look for Grade A cells that are at the cycle rating level or expected calendar life appropriate for their application.  Higher quality cells have a higher cycle rating.  Often the selection is a value decision, discussed further, below.

Where are the Requirements for Cycle Rating Stated?

There is no consensus standard in the US, but there is in China.  And as that Chinese standard seems to be objectively developed, it has served the industry well.  But, it is not widely known outside of Engineering circles, who have used the criteria established in that standard for many years.  That Chinese Standard is GB/T 31484-2015.  In this standard it specifies:

(1)   For “traction batteries” (i.e., electric vehicles, or EV, applications) cells shall be tested at 1C charge and discharge levels, to 100% DoD, with 80% residual capacity remaining to establish cycle rating, rounded down.  (Note that EV’s are rarely charged or discharged at 1C, often experiencing charge and discharge rates of 2C to 3C, and at higher temperatures.  Active cooling is required to support charge and discharge at 2C or 3C.  In practice, because of these high charge and discharge rates, these cells often see only 3 to 5 years of life, even with liquid cooling and rated at 8000 cycles. More on expected cycle and calendar life is discussed below.)

(2)   For “other applications” (frequently referred to as ESS or energy storage systems).  Lithium applications on boats and RV’s often fall into this category, storing energy for moderate to low discharge rates by electrical systems.  The standard states cells for these other applications shall be tested at 0.5C charge and discharge levels, to 80% DoD, with 80% residual capacity remaining, to establish cycle rating.

That standard also defines other details of the standardized testing, such as number of samples per production run, standard testing temperatures, rest periods between charge and discharge periods, and other test parameters like temperature and humidity.

Where is Cell Cycle Rating Reliably Stated?

Cell cycle rating is required to be done by the cell manufacturer.  These are large companies that are listed companies (on one of the Chinese stock exchanges), and are not producers of the volume of batteries one sees on Temu or Amazon.  The results of cell manufacturer cycle testing are documented in their Cell Specification.  This is a separate document of between 10 to 20 pages, typically.  And can usually only be obtained from the cell manufacturer or the cell distributor one buys the new cells from.  The myriad of traders that sell cells on Alibaba and AliExpress do not have and usually don’t distribute these specifications.  I have heard one YouTube evaluator refer to cell manufacturer cell specification documents as being held as if they were state secrets, and he is correct.

The companies selling the cheap batteries on Temu and Amazon do not want you to see these cell manufacturer specifications, to be able to check on their inflated cycle rating claims.   Or to have the data to check if the cells they built their batteries from might be recycled or reject (Grade B) cells.  The places you cannot trust cycle rating are in the sales documents, webpages and battery stickers of the cheap battery manufacturers.  We have found this in every instance we check.

So, for the general public getting access to those cell specification documents is very difficult.  As a battery manufacturer, we demand copies of those cell manufacturer specification documents so we have confidence in the quality and expected life of the lithium cells we use.  Before buying a battery, ask the selling company for a copy of the cell manufacturer’s specification document.  YouTube evaluators should obtain the cell manufacturers specification, so they understand the true cycle rating of the cells in the battery they are evaluating.  Compare the cell manufacturer’s cycle rating based on testing to the claimed rating by the battery manufacturer.  Don’t accept a short, one-page so-called specification table from the battery manufacturer; that can’t be trusted.  The battery manufacturer should be able to get that detailed cell manufacturer’s specification for you.

PART 3 - BATTERY CYCLE RATING

How Should Cell Cycle Rating Translate to Battery Cycle Rating?

Battery manufacturers in the industry should state their battery cycle rating as no more than that of their cells, based on the standards noted above for ESS applications.  Some give ratings based on only partial discharges; those are not relevant.  Also verbal claims aren’t acceptable.  Ask for the cell specification document by the cell manufacturer that is 10-20 pages or longer; that way you know you are getting the right information.  It is improper to claim a cycle rating for the cells used that is higher than that demonstrated in the cell cycle testing.  Any reference to a cycle rating associated with a battery, really means the cycle rating of the cells used in that battery.  US battery manufacturers are generally more truthful in stating their cell cycle rating, as they can face truth-in-advertising lawsuits.  The Chinese battery companies do not face that risk in their home judicial system; factual accuracy in advertising is at the buyer’s risk.  It is considered the buyers’ responsibility to ensure a product will meet their needs.

Does Cycle Rating Predict Battery Longevity?

In short, the answer is no.  One should not try to correlate the cycle rating to something like the number of days a battery will last.  As noted earlier, the battery configuration and the operating environment may present a more challenging operating environment than occurred in the standardized cycle rating testing.  So, batteries usually do not see the number of rated cycles in practice as the cells demonstrated in the cycle rating tests.  (See notes below on approximate calendar life for different cycle ratings.)

Effect of Cell Cycle Rating on Battery Cost?

Cycle rating definitely affects cell pricing.  Often the cost of a battery with Grade A cells rated for 3000 cycles is much lower than that of a battery with 6000 cycle rate cells.  But, that is if you can find the true cell cycle rating.  Too often the battery manufacturer hides the true rating, or blurs it by claiming a lot higher number, with a footnote that says the rating is based on 40% depth of discharge cycles.  That is dishonest as they are not using the cycle rating standard correctly and are counting a partial cycle as a full cycle.

The economics are simple.  For a battery that is at a substantially lower cost than another, if both have Grade A cells, it usually means the cells of the high-cost battery are a higher cycle rating, and the battery itself is thus a higher quality battery, will have greater reliability in operation, and will have a much longer life. [2]

PART 4 - BATTERY AND CELL CALENDAR LIFE

What is Calendar Life, and is that Important?

Calendar Life is the general term that relates to the expected life of the battery.  It reflects how long a cell, and thus the battery it is in, will last.  There are other chemical processes that occur in a lithium cell that are more related to time and contribute to reaching end of life in the cell, no matter the number of cycles. 

One example is the formation of the SEI layer.  That layer is beneficial in that it has the Li-ions lined up and ready to do their electron transfer function, supporting greater rates of charging or discharging, than if the layer didn’t exist and ions were equally distributed through the electrolyte solution.  But that layer can be detrimental in terms of continued increase in density over time, reducing the electron transfer rate and increasing the internal resistance of the cell.  An increase in IR leads to reduction in the electron transfer rate and also adds to rise in temperature of the electrolyte and electrodes.  This process occurs over time, and is not as highly dependent on cycle count.

From research and practical applications, we find that calendar life is shorter for cells with a low cycle rating, and much longer for cells with a high cycle rating.  High cycle rated cells employ advancements in cathode and anode structure and in electrolyte formulation, which lead to a higher cycle rating, and a corresponding longer calendar life.  Research has found that LiFePO4 cells that have a high cycle life rating generally also have a longer calendar life if not abused. [3]

Calendar life is very important as it may be more relevant to the life the batteries achieve, as opposed to an actual cycle count.  And the calendar life of a battery can be reduced or extended depending on how aggressively or lightly the battery is used. [4]

Based on actual personal practice of using cells in marine and RV applications, we provide below an expected calendar life for cells of different cycle rating, under different use conditions.

PART 5 - EXPECTED LIFE : CALENDAR LIFE --

What Sort of Calendar Life Can I Expect for my Battery?

In brief, as discussed earlier, a higher cycle rated cell will have a longer calendar life, calendar life can be affected by high-vs. low charge and discharge rates, and calendar life is not exactly measured, so consensus on a ballpark level is useful.  The table below is based on the experience of an engineer and sailor who cruised for over 25 years, many of those years on lithium cells, and has also assisted or installed lithium cells on 24 other boats used by full time live-aboard cruisers, with cells that have reached their end of life.  These lasted between 6 to 15 years, with the shorter life being cells of a lower cycle rating and the longer range for cells that had a very high cycle rating.  While this is not a highly precise result, and such should not be expected as precise use of batteries will vary from user to user.  But it does give an approximate sense of calendar life in comparison to cycle rating.