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Things You Need To Know About Battery Cycle Life

Sep. 15, 2020

Some consumers may have that the charge and discharge life of lifepo4 batteries is "3000 times." But what is "3000 times?" It refers to the number of charge and discharge cycles of the battery.

Let us look at an example: Let us say there is a lithium battery that uses only half of its charge in one day and is then charged fully. On the next day, it again only uses half of its power. Although the battery has been charged twice, this does not count as one charge cycle but two.

A charging cycle is when a battery goes from being fully charged to empty and then from empty to fully charged; this is not one single charge. Just based on the previous example, it's clear that it can usually take several charges to complete a cycle.

Every time a charging cycle is completed, the battery capacity decreases a bit. However, the reduced capacity is very small. High-quality batteries will still retain 80% of their original capacity after many cycles of charging. Many lithium battery products will still be used after two or three years. Of course, after the end of the lithium battery life, it still needs to be replaced.

Ultimately, a 3000-cycle life means that a manufacturer has achieved about 3750 recharge times at a constant discharge depth (such as 80%) and reached 3000 charging cycles. In other words, if we ignore other factors that could reduce the Lithium ion battery capacity and we take 80% of 3750, we receive 3000.

However, due to various factors in life, especially considering how the depth of discharge (DOD) during charging is not constant, "3000 charging cycles" can only be used as a reference to battery life.

Overall, it is better to think of the life of the lithium battery as related to the number of times the charging cycle is completed and not as directly related to the number of charges.

Deep and shallow charging

Cycle life as a function of depth of discharge.A partial discharge reduces stress and prolongs battery life, so does a partial charge. Elevated temperature and high currents also affect cycle life.

Note: 100% DoD is a full cycle; 10% is very brief. Cycling in mid-state-of-charge would have best longevity.

Things You Need To Know About Cycle Life

Here is another way to think of the cycle lives of lifepo4 batteries: the life of a Lifepo4 battery is generally 2000 to 3000 charging cycles. Assume that the capacity provided by a full discharge is Q. If the capacity reduction after each charging cycle is not considered, lifepo4 batteries can provide or supplement 2000Q-3000Q power in total during its life. From this we know that if you use 1/2 each time, you can charge 4000-6000 times; if you use 1/3 each time, you can charge 6000-9000 times. By analogy, if you charge randomly, the number of times is uncertain. In short, no matter how a Lifepo4 battery is charged, it is constant to add a total of 2000Q to 3000Q of power. Therefore, we can also understand this: the life of a Lifepo4 battery is related to the total charge of the battery and has nothing to do with the number of charges. The effects of deep charging and shallow charging on lithium battery life are similar.

In fact, shallow discharge and shallow charges are more beneficial to lifepo4 batteries. It is only necessary to deep charge when the power module of the product is calibrated for lithium batteries. Therefore, lithium-ion-powered products do not have to be constrained by the process: they can be charged at any time without worrying about affecting the battery life.

Effects of temperature on battery life

Lithium-ion suffers from stress when exposed to heat, so does keeping a cell at a high charge voltage. A battery dwelling above 30°C (86°F) is considered elevated temperature and for most Li-ion a voltage above 4.10V/cell is deemed as high voltage. Exposing the battery to high temperature and dwelling in a full state-of-charge for an extended time can be more stressful than cycling. Table below demonstrates capacity loss as a function of temperature and SoC.

Things You Need To Know About Cycle Life

If a Lithium-ion battery is used in an environment higher than the specified operating temperature (above 35℃), the battery’s power will continue to decrease. In other words, the battery’s power supply time will not be as long as usual. If a device is charged at such temperatures, the damage to the battery will be greater. Even if the battery is stored in a hot temperature environment, it will inevitably cause damage to the battery. Therefore, it is a good idea to extend the life of lithium-ion batteries by using it under normal operating temperatures as often as possible.

If you use Lithium batteries in a low-temperature environment (below 4℃), the battery life will also be reduced. Some older Lithium batteries of mobile phones cannot even be charged under low temperatures. However, unlike in high temperatures, once the temperatures rise, the molecules in a battery will heat up and immediately return to the previous charge.

Having explored battery performance under these extreme temperatures, the question now becomes if there are any batteries that can be used in environments with low or high temperatures.

How To Extend The Cycle Life?

Lithium ion batteries are currently the preferred energy storage technology for PEVs because of their high energy density, good power capability, high cell working voltage, and relatively good cycle life. Current thinking is that reducing the high current pulses experienced by the batteries in both charge and discharge will reduce the stress on the batteries and thus increase cycle life.

At the same time, try to keep the battery away from places where the temperature is too high. The best working temperature of the battery is 25 degrees Celsius, and the reasonable temperature of the battery room is kept within 22 to 25 degrees Celsius.

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