Your Lithium-Ion Batteries' Final Days

We can hear your horrified screams as you imagine losing access to your priceless LFP battery bank. Premature death is what we want to avoid, and in order to do so, we need to comprehend how lithium-ion batteries degrade.

Cycling and Age

When a battery’s capacity drops to 80% of what it should be, battery makers label the battery as “dead.” As a result, a 100Ah battery reaches its conclusion when its capacity drops to 80Ah. Your battery will eventually die due to two mechanisms: cycling and age. You lose a small amount of capacity and cause a little bit of damage to the battery every time you discharge and recharge it. But even if you store your priceless battery in a lovely glass-enclosed shrine and never cycle it, it will eventually run out of power. Calendar life is the name of the last one.

There is a dearth of exact information regarding the calendar life of LiFePO4 batteries. The results of certain scientific studies on the impact of temperature and SOC extremes on calendar life serve as guidelines. Our research suggests that the maximum calendar life of batteries is roughly 20 years assuming you do not misuse your battery bank, stay away from extremes, and generally just utilize your batteries within reasonable parameters.

BMS is needed

Along with the battery’s cells, the BMS, which is constructed of electronic components, is also inside the battery. Your battery will also fail when the BMS does. BMS ultimately needs to last as long as lithium-ion cells do. Lithium-ion batteries with an integrated BMS are still too young for us to know for sure, so we will have to wait and watch.

Temperature’s impact

Over time, internal battery processes work together to coat the electrolyte-electrode interface with chemical substances that block the passage of lithium ions into and out of the electrodes. Additionally, as a result of the processes, lithium ions are no longer free to flow between electrodes and are instead bound into new chemical compounds. Those processes will occur regardless of our actions, but the temperature has a huge impact on them. Keep your batteries below 30 °C because they warm up slowly. The rate of change increases significantly at 45 degrees Celsius! Heat is by far lithium-ion batteries’ worst enemy!

There are more factors that affect a LiFePO4 battery’s aging rate and calendar life: It also has anything to do with State-Of-Charge. Despite the fact that high temperatures are dangerous, these batteries really, truly do not want to sit at 0% SOC at such high temperatures. Having them at 100% SOC at high temps is also terrible, though not quite as severe as having them at 0% SOC. Low temperatures have a smaller impact. LFP batteries cannot be charged below freezing, as we previously discussed. It turns out that discharging them below freezing has an increased effect on aging as well, though it is technically doable. Although it’s not nearly as terrible as leaving your battery sitting at a high temperature, it’s best to expose your battery to freezing temperatures when it’s neither charging nor discharging and when you still have some gas in the tank but not a full tank. In general, if these batteries need to be stored for a longer period of time, it is preferable to do so when they are between 50% and 60% SOC.

Cycle of life

The next thing need to think about is the cycle of life. Lithium-ion batteries can now frequently last for thousands of cycles, even at a complete 100% charge-discharge cycle.

The cycle life is also influenced by the speed at which those lithium ions are tugged here and there. LFP batteries will typically charge and discharge at 1C, but if you restrict this to more realistic numbers, your battery will last longer. Maintaining a lithium-ion battery’s Ah rating within the range that lead-acid batteries are limited to will help it last longer.

Voltage

Voltage is the final element worth addressing, even though this is primarily what the BMS is intended to control. A small voltage window exists for both charging and discharging lithium-ion batteries. The range for LiFePO4 is about 8.0V (2.0V per cell) to 16.8 Volts (4.2V per cell). The battery should be kept firmly within those bounds by the integrated BMS.