How to maintenance lithium battery system for long lifespan?
To maintain a lithium battery system for a long lifespan, it is important to keep it cool, avoid overcharging and overdischarging, store it at a partial charge, inspect it regularly, and use a battery management system (BMS). It is also important to keep the battery connections clean and free of corrosion, and to use the battery regularly.Last week we shared with you our top 5 tips on how to charge your lithium-ion batteries to extend their lifespan. In this article, we will focus on how to care you for your Lithium-ion battery to extend their lifespan.
Lower the C rate when discharging to optimize your battery’s capacity and cycle life
At high-rate discharge, for example 1.5 C, the extraction of lithium ions from one electrode and intercalation to the other is too strong to be efficient. This damages the electrodes’ elasticity. Think about breathing hard and fast all the time, you will lose your breath, without benefitting from the air nor gaining energy. We need to lower the C rate when discharging to optimize your battery’s capacity and cycle life.
Strong lifepo4 battery discharge current rates increase the battery’s internal resistance. The battery will have to strive to deliver high current and use more power to keep the same voltage level, which will therefore make lifepo4 battery system age faster.
On new “fresh” batteries, a 1.5C only impacts the lifepo4 capacity of the battery. For lifepo4 batteries that have endured many cycles though, because of the increase of the lithium cells’ internal resistance, not only the usable capacity will be impacted but also the cycling that is involved. Consequently, only a fraction of the capacity will be usable. At a slow pace there’s less dissipation, the internal resistance doesn’t build up as much and the cell’s capacity is extended.
The voltage level reflects the charge level: 3.55V indicates a full charge, 2.8V indicates that the battery is completely discharged.
Be aware the temperatures at which the lifepo4 battery is being discharged
The temperature in which a device operates is the main factor impacting a lifepo4 battery’s power consumption. This is true for primary batteries but also for rechargeable batteries. At extreme temperatures, electrode and electrolyte no longer have the optimal shape to enable efficient lifepo4 exchanges.
At low temperatures, the electrode contracts and as a result, lifepo4 cannot extract. The electronic conductivity of the electrolytes decreases too. Ions move more slowly between the 2 lifepo4 electrodes.
Have you ever had the feeling that your energy storage system discharges faster during the winter? That’s because cold temperatures make the lifepo4 battery cells request more power to maintain the required voltage level to charge or discharge to your home . Try to keep your lifepo4 battery warm in your house, wait and see!
Under the effect of the heat, the lifepo4 battery electrode expands, making lifepo4 battery easier for the ions to move. The electrolyte is also more conductive. However, extreme warm temperatures will damage the lifepo4 battery electrodes. Some batteries will behave better than others at high or low temperature depending on the chemistry used by the manufacturer and the construction of the cell
Optimize a lifepo4 battery depth of discharge (DoD).
The full charging/discharging cycle is called Depth of Discharge (DoD) where 100% is a full cycle.There is we need to do that Optimize a lifepo4 battery depth of discharge (DoD).
A 70% DoD means that 70 percent of the available energy is delivered, and 30 percent remains in reserve. One cycle of 100% DoD is approximatively equivalent to 2 cycles at 50% DoD, 10 cycles at 10% DOD and 100 cycles at 1% DOD. The depth of discharge complements the state of charge (SoC): as the Depth of Discharge increases, the State of Charge decreases.
There is a direct relation between the depth of discharge and the cycle life of the lithium battery. The shallower the DoD, the exponentially higher the number of cycles given by a lifepo4 battery. By restricting the possible DoD in your application, you can dramatically improve the cycle life of your product.
A 30% DoD allows 6,000 cycles! A perfect example that fine-tuning DoD, temperatures and C current rates can definitely extend your lifepo4 battery's lifetime. To illustrate the idea, a 100% DoD could be compared to breathing hard, using your full lung capacity, thus exhausting your entire body. But if you are breathing normally, you can keep on moving longer.
Contrarily to some received ideas, lifepo4 batteries don’t have a memory. They don’t need regular full discharge and charge cycles to prolong life. It’s actually the contrary: the smaller the discharge (low DoD), the longer the lifepo4 battery will last, the more cycles it will be able to do. Indeed, a lifepo4 battery full charging and therefore high currents boost the cell’s capacity but cause stress to the electrode which stretches, thickens, and enlarges itself to allow all the ions to penetrate. Conversely, when completely lifepo4 battery discharged (below 2.7 V), an internal chemical reaction occurs, the electrode oxidizes and retracts, the elasticity changes and the battery ages more quickly.
A lifepo4 battery partial charge and discharge will therefore reduce stress and prolong lifepo4battery life. It is recommended to avoid full cycles and stay between 100% and 50% DoD (0-50% SoC).
Make sure to go periodic balancing cells in your Lifepo4 battery pack
When cells of your lifepo4 battery pack are connected in series in a energy storage battery system, an imbalance might occur. The lifepo4 cells behave unevenly over time: charging or discharging levels, self discharge and impedance (internal resistance) may vary from one cell to another. Cycling the battery when unbalanced worsen the disparities and leads to voltage loss.
You will also need to proceed to balancing from time to time so that one cell does not wear out more than the others. Here again a smart embedded BMS takes good care of your battery. It will find the weakest cell and make sure it is charged/discharges at the same level than the others.
Methods used to perform lifepo4 cells balancing can include by-passing some of the cells during charge/discharge to focus on the weakest cells.
Monitor the lifepo4 battery State of Health (SoH)
The State of Health reflects the general condition of a battery and its ability to deliver energy over time. It gives an indication of how much of the lifepo4 battery’s lifetime available energy has been consumed, and how much is left.
It is a very interesting parameter to monitor since it can indicate when the lifepo4 battery is experiencing problems or needs replacement. Indeed, although it may slightly vary from one cell manufacturer to the other, it is generally considered that the electrochemistry is at the end of its life when the lifepo4 battery reaches approximately 70% of capacity (in Ah). For Batterlution Ground HV range, the end-of-life SoH is 70%. Then the lifepo4 battery capacity loss accelerates, and the autonomy goes down. The State of Health can be displayed on the Battery Management System (BMS) as an option. A key maintenance indicator to follow-up!