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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 while in use to extend their lifespan. Our rechargeable batteries will have no more secrets for you!
But before going any further, let’s sum-up the factors influencing the capacity of a Li-ion battery.
At high-rate discharge, eg 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.
Strong 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 it age faster.
On new “fresh” batteries, a 1.5C only impacts the capacity of the battery (ie. its autonomy (see chart below)). For batteries that have endured many cycles though, because of the increase of the internal resistance, not only the 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 (eg: C/8) there’s less dissipation, the internal resistance doesn’t build up as much and the cell’s capacity is extended.
This graph represents the discharge rate capability of a new “fresh” MP 176065 xtd battery.
The voltage level reflects the charge level: 4.2V indicates a full charge, 2.7V indicates that the battery is completely discharged.
As you can see, at a C/8 discharge rate (purple line), the cell offers a 5.8 Ah capacity, at 1.5 C, the cell capacity goes down to 5.5 Ah (green line).
The temperature in which a device operates is the main factor impacting a 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 lithium-ion exchanges.
At low temperatures, the electrode contracts and as a result, ions cannot extract. The electronic conductivity of the electrolytes decreases too. Ions move more slowly between the 2 electrodes.
Have you ever had the feeling that your mobile discharges faster during the winter? That’s because cold temperatures make the battery request more power to maintain the required voltage level to send an SMS or take a picture. Try to keep it warm in your pocket, wait and see!
Under the effect of the heat, the electrode expands, making it easier for the ions to move. The electrolyte is also more conductive. However, extreme warm temperatures damage the 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.
The graphic below represents the discharge capacity (Ah) of an MP176065 xtd at C/5 at various temperatures.
Saft MP176065 xtd proposes an unrivalled operating temperature range from -40°C to +85°C but at -40°C, you have to make concessions on the capacity and adapt the operating voltage.
The full charging/discharging cycle is called Depth of Discharge (DoD) where 100% is a full cycle.
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 battery. The shallower the DoD, the exponentially higher the number of cycles given by a battery. By restricting the possible DoD in your application, you can dramatically improve the cycle life of your product.
As an example, Saft MP 176065 xtd’s full charge DoD is 4.2V, the lower charge is 2.7V. A 100% DoD at 25°C, using a C charge and a C/2 discharge rates will allow 4,000 cycles.
A 30% DoD allows 16,000 + cycles, ie 4 times more! A perfect example that fine-tuning DoD, temperatures and C rates can definitely extend your 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, Li-ion 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 battery will last, the more cycles it will be able to do.
Indeed, a 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 discharged (below 2.7 V), an internal chemical reaction occurs, the electrode oxidizes and retracts, the elasticity changes and the battery ages more quickly.
This graph showcases how lowering the DoD will allow you to increase the numbers of cycles.
A partial charge and discharge will therefore reduce stress and prolong battery life. It is recommended to avoid full cycles and stay between 100% and 50% DoD (0-50% SoC).
When several cells are connected in series in a battery pack, an imbalance might occur. The cells behave unevenly over time: charging/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.
This can be limited by requiring that the producer assembles only homogeneous cells in the battery pack, just like we do at Saft.
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 cell balancing can include by-passing some of the cells during charge/discharge to focus on the weakest cells.
We also recommend, for batteries assembled from our MP li-ion range, to proceed with periodic balancing by forcing a full discharge with a low current. Forcing a full cycling helps the battery to recover the available capacity, especially after a long storage time.
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 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 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 battery reaches approximately 70% of capacity (in Ah). For Saft xlr range, the end-of-life SoH is 70%, and 60% for the xtd range. Then the capacity loss accelerates, and the autonomy goes down. The State of Health can be displayed on the Battery Management System as an option. A key maintenance indicator to follow-up!
Let’s summarize our 10 top tips on how to care for your industrial-grade lithium-ion batteries during charge and while in operation to optimize their lifespan:
How to charge your industrial-grade lithium-ion batteries to optimize their lifespan:
How to discharge your industrial-grade lithium-ion batteries to optimize their lifespan:
Now, you know everything there is to know about how to optimally charge or to discharge your Li-ion battery to extend its lifetime!
For more information on Saft lithium-ion rechargeable range, visit the product page: https://www.saftbatteries.com/products-solutions/products/mp-small-vl
And if you’d like to read more about how our batteries operate, check out our case studies:
Fuji Tecom is preventing water leakage and offering more efficient operation thanks to an innovative water leakage detector.
Kongsberg Seatex AS : An autonomous Saft battery solution to monitor the seas despite extreme cold in the Svalbard archipelago
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