There is a lot that can determine the longevity of rechargeable batteries. It can be affected by the way the battery is charged and discharged, the operating temperatures, and more.

That’s particularly true of industrial grade lithium-ion batteries that you may use to power remote or portable devices. These cells possess high energy density and can operate under extreme temperatures for a long life – but there are key things to consider if you want to optimize their lifespan.

Here we’ll share our five top tips and delve a little deeper into ‘why’ and ‘how’ each is important

1. Understand the battery terminology
2. Respect a CCCV charging process (especially when on floating mode)
3. Carefully design your Battery Management System
4. Lower your charging C rate
5. Control the charging temperature

Understand the battery terminology

Knowing how a battery works will help you optimize the way you charge and discharge.

Lithium-ion batteries are made of two electrodes: a positive one, and a negative one. When you charge or discharge your battery, electrons leave the battery through the electrical current and ions flow from one electrode to the other.

When the battery provides current, electrons move from the anode to the cathode outside the battery. Applying reverse current allows the battery to recharge itself: the electrons are sent back to the anode and the lithium ions re-intercalate themselves in the cathode. This restores the battery’s capacity.

The whole charging and discharging process is defined as a cycle. The number of cycles that your battery can perform varies depending on the manufacturing process, the chemical components, and the actual usage.

The capacity of a rechargeable battery is measured in Ah – and is directly influenced by:

• The charging and discharging rate of the battery – known as the ‘C rate’. Charge and discharge currents are typically expressed in fractions or multiples of the C rate.

For example, a C charge/discharge means you will charge or discharge the battery in an hour. A C/2 charge/discharge takes two hours, a 2C charge/discharge takes 30 minutes.

• The voltage level - this reflects the charge level.
• The charging, discharging and operating temperature.
• The number of cycles. With time, a battery loses capacity due to the physical and chemical degradation of the electrodes, and the electrolyte

Strong management of the depth of discharge (DoD) - the percentage of the capacity which has been removed from the fully charged battery - and of the maximum charging voltage can enhance the number of cycles that the battery will be able to perform and therefore, its operating life.

Respect a CCCV charging process (especially when on floating mode)

Rechargeable batteries need to follow a specific charging process, usually handled by a carefully selected charger.

Charging a lithium-ion battery is not that simple.

The charger you select has a key role, as the way you set up parameters impacts your battery lifetime. Don’t just plug it into any power supply or use a charger designed for another technology (e.g., Nickel-Cadmium or Lead). Doing so could lead to a number of safety issues.

Charging properly a lithium-ion battery requires two steps: Constant Current (CC) followed by Constant Voltage (CV) charging. This is a CCCV charging process.

A CC charge is first applied to bring the voltage up to the end-of-charge voltage level. You might even decide to reduce the target voltage to preserve the electrode. Once the desired voltage is reached, CV charging begins and the current decreases. When the current is too low, the charge is finished, and the current must be removed.

The capacity of the battery depends on the end-of-charge voltage, so lowering the voltage will lower the battery capacity. You’ll have to find the right trade-off between the autonomy needed, the minimum voltage at which your device can operate, and the longevity of the battery.

Leaving a battery on a permanent charge under a floating current after the CV mode of the charging process is called the ‘floating mode’. Most manufacturers don’t recommend the floating mode as it damages the battery over time. Moreover, maintaining a charge rate into a poorly-designed battery without the right safeguards could overcharge it and lead to an explosion.

Saft’s MP xtd range is specifically designed to operate in floating mode in safe conditions, with a limited cell aging on a wide temperature range.

Carefully design your Battery Management System

Whatever the application, Li-ion cells must be associated with the electronics.

This key electronic component is called a Battery Management System (BMS). The mandatory safety features interrupt the discharge/charge to protect the battery against overvoltage or undervoltage. The BMS monitors the temperature and disconnects the battery when it is at risk of overheating.

The BMS can also incorporate electronics to achieve a consistent charge across each cell in a battery pack (known as balancing). Without a BMS balancing feature, a battery pack with several cells connected in series may find each cell ages differently. As the life duration of the pack is directly related to that most ‘aged’ cell, a good balancing system will improve the battery’s lifespan.

The BMS can be further tailored to your application - some can display the State of Charge and the State of Health to help you understand the state of your battery.

Lower your charging C rate

At low charging speed, ions smoothly intercalate themselves in the electrode, extending the battery’s lifetime.

At a low charging speed (C/2, C/5, or even less), lithium ions smoothly intercalate (insert) themselves in the graphite sheets without damaging the electrodes. When the charge rate increases, this intercalation gets harder and harder.

If the rate is too strong, lithium ions have no time to penetrate the electrode properly and simply deposit on its surface. This causes the battery to age prematurely.

Fast charging rates like 4C or 10C are possible (for example in electric vehicle batteries) but the electrode constructions are different in these cells and, crucially, the expected lifespan is shorter.

Developers need to find the right trade-off between the necessary charging time, and the speed of aging of the battery. A C/50 charging rate is better for the electrodes but not every application can afford more than 50 hours charging time! At Saft, we recommend limiting the charge rate of our MP range to C or less.

Control the charging temperature

Batteries work best when charged at ambient temperature. High or low temperatures lead to premature ageing of the battery.

Most Li-ion batteries use graphite-type material in one electrode. An elevated charging temperature provokes the exfoliation of the graphite sheets which hastens permanent capacity loss in the battery. This phenomenon can be worsened by a high charging rate - the charging current further increases the temperature and accelerates of the exfoliation phenomenon.

A high voltage level coupled to a high temperature causes the electrochemistry to generate gases inside the cell which accelerates chemistry ageing.

Depending on the cell construction, high temperatures can also generate cell swelling. Such a deformation can cause safety hazards if the battery casing or device location have not been designed to support it. Be sure not to exceed the limits set by the battery manufacturer.

If the battery design does not include the mandatory safeguards to avoid overcharge, over-discharge and overheating, a cell internal temperature higher than 130°C could lead to a thermal runaway.

Most Li-ion batteries can only withstand a maximum temperature of 60°C and the recommendation is to charge them at a maximum of 45°C under a C/2 charge rate.

At the other end of the scale, few batteries can be charged below 0°C. The electrode sheets contract and the electrolyte electronic conductivity gets lower, complicating the intercalation of the ions in the graphite. Again, lithium deposit can be generated which causes permanent capacity loss. To compensate, and to allow for the ion to intercalate properly, some manufacturers recommend charging the battery at very slow rate (C/20) when operating below 0°C.

However, Saft’s MP range can handle a broader range of temperatures and charging rates. These batteries can sustain a C charge rate up to 60°C (C/5 up to +85°C for the xtd range) and at very cold temperatures (as low as -30°C) they can still be recharged when applying C/8 and even C/5 rates.

*This is an updated version of an article first published in January 2022.