As energy storage becomes more prevalent among solar installations, consumers must understand how to prolong the battery life of their energy storage for safety and cost savings. Often, energy storage can account for most of the costs associated with purchasing a solar PV system.
Here, we go over what factors have the most significant effects on the battery life of Li-ion batteries as Li-ion chemistries have become the standard of energy storage installations across the solar market.
What you need to know
Theoretically, a newly purchased Li-ion battery should output 100% of its advertised capacity, but this is not always the case. It is not uncommon for batteries to output at lesser specifications than what is specified by the manufacturer. The consumer will rarely spot-test the specifications to confirm its viability and address related issues with the manufacturer.
Also, the amount of time a battery is unused and sitting on the shelf before being sold can affect its longevity. Such factors can create a wide range of performance acceptability by the manufacturer. This information is vital for consumers to know as many factors will affect battery life longevity, the most prominent health indicator of Li-ion battery performance.
However, the aspects related to the maintenance and operation contexts of Li-ion batteries will typically have the largest effect on performance, and the consumer can generally manage these.
What Effects Li-ion Batteries Performance?
For Li-ion batteries, the following aspects have the most significant effect on performance:
Temperature of operation
Over time, the temperature of battery operation will significantly affect the number of Li-ion battery cycles and will have a more substantial effect than cycling or aging. The general rule of thumb for consumers to understand is that the hotter a battery gets, its performance becomes worse. Most importantly, the lesser the lifetime of the battery will last. When exposed to heat, the battery chemistry becomes stressed, and the performance will deteriorate to the amount of stress put onto the battery.
Below is shown a table comparison of how the Li-ion battery operating temperature will affect the recoverable energy capacity. Note that the operating temperature is fixed for a year for this example.
What is the number of cycles for Li-NMC and LiPO4 Batteries per DoD?
Cycling a battery is loosely defined within the energy storage market, but it essentially refers to discharging to a certain extent (% DoD, or depth-of-discharge). For Li-ion batteries, the deeper the battery is cycled on a constant basis, the shorter its lifetime will become.
For example, a battery discharged to 100% DoD (full cycle) will have fewer cycles over its full lifetime than a battery that is discharged to 50% DoD (half discharge).
Below is shown a table that describes the number of cycles for Li-ion on chemistries, Li-NMC, and LiPO4, as a function of the depth of discharge.
Note that the user can significantly increase the number of cycles through a lower % DoD as the relation is not linear and resembles an exponential relation.
What is the ideal SoC for Li-ion Batteries?
The aging of the battery is unavoidable, but the state of charge (SoC) can slow the battery’s aging at a resting state. Li-ion batteries tend to retain longevity and undergo a “low stress” state when at a partial discharge or charge (i.e., keeping the battery lower than 100%).
For Li-ion batteries, the battery lifetime can double per voltage increments below its max SoC.
See an example table below of this behavior.
In the table above, for every 0.1V below the max charge level voltage (4.2V at 100% charge), the number of discharge cycles doubles. In contrast, as the voltage of the battery increases above its max charge (voltage) state, the number of cycles decreases, and thus the longevity of the battery deteriorates.
With this, the battery SoC mustn’t be kept below 25% as this will have a similar effect when the battery is at full charge for an extended period of time – discharge cycles decrease.
For Li-ion batteries, the rule of thumb is to keep the SoC between 25% to 80% to minimize the stress upon the battery’s internal chemistry and materials.
Users can take advantage of these factors to increase the lifetime and maintain the quality of Li-ion batteries. In the solar context, a battery BMS will prevent the batteries from overcharging, which would prevent the battery cells from exceeding the max voltage state at 100% SoC.
However, inverters and charge controllers can be programmed to keep the batteries at a specific SoC, where a partial charge (25%-80%) would be most optimal for prolonging battery life. You should store the batteries in a cool and well-ventilated area to maintain temperature control.