In the world’s intensive efforts to ensure a greener future, going from fossil fuel power to renewable energy sources, batteries are becoming a vital tool for that transition. Especially rechargeable lithium batteries, which are aimed to prevail in Electric Storage Systems (ESS), are part of everyday electronics, and play a significant role in the transport sector with the increasing number of Electric Vehicles (EV). The term lithium battery (LIB) refers to an entire family of battery types, with varying chemical compositions, where cathode and anode materials serve as hosts for lithium ions, and the battery contains an organic
conductive electrolyte.

In addition to discharge in regular use, occasionally it is necessary to perform controlled battery discharging for various purposes:

Capacity testing

An essential part of battery maintenance and the most reliable indicator of a battery’s State of Health (SoH) is the battery capacity test, where controlled battery discharging is performed to determine the amount of available battery energy. In accordance with the standards that define the conditions for discharging batteries, it is preferable to carry out ten-hour discharges with currents of 0.1C.

Transport regulations

Due to the fact that most lithium batteries contain a flammable electrolyte and have a very high energy density, and that they can overheat and ignite under certain conditions, in transport, they are considered dangerous goods and special regulatory requirements must be complied with. Among other things the restriction of the amount of energy contained in the batteries that are being shipped.

The ICAO regulations require a controlled State of Charge not exceeding 30% for the shipment of LIB as cargo on passenger flights [3]. It implies the necessity for quick and easy discharge of batteries whose capacity exceeds the permitted level prior to any shipment.

End-of-life management

Significant price increase in the last decade of, primarily, lithium and cobalt as a result of bigger battery consumption in the automotive industry, yields the fast development of battery refurbishing and recycling plants.

Taking into consideration that batteries in EV become unfit for further usage when the pack loses 15% – 20% of its initial capacity, they are often repurposed for a “second life” as stationary energy storage, where the lower current density is required from the battery.

Nevertheless, in this end-of-life management phase, controlled battery discharge is necessary to check its SoH, to determine the remaining capacity and make the decision whether the battery can be refurbished and reused or is ready for recycling. If the battery is declared as non-refurbished, valuable lithium, cobalt, manganese, nickel, copper, aluminum, and graphite, recovered in the recycling process are reused for battery manufacturing again.

Recycling

Unlike the process for previously described applications where discharge is stopped, at the latest, when the battery drops to the cut-off voltage, battery discharge for recycling purposes is done more thoroughly. To avoid possible short-circuiting of the cathode and anode during the crushing phase of recycling and potential self-ignition of lithium cells the deep discharge of the battery is crucial. A deep discharge implies discharging the battery below its cut-off voltage, i.e. below 2.5 V per cell.

Depending on the application, batteries received for recycling can be in a different form. From individual cells (cylindrical, prismatic or pouch), modules – larger groups of cells connected in series and parallel, all the way to battery packs – multiple modules connected in series. Discharging can be done prior to or post disassembly.

Deep discharge of battery modules and packs

Deep discharging of packs and modules, with nominal voltages of 50–800 V, is most efficiently done with electronic loads, a combination of power electronics converters and a group of powerful resistors. For the discharge process to be performed in safe conditions, besides gathering information about the battery’s capacity, SoC and SoH at the beginning of the process it is necessary to monitor the temperature and voltage of individual modules, preferably even groups of cells, as well as to control the discharge current.

Deep discharge of battery cells

Deep discharging individual cells is a somewhat simpler and faster process. The aim is to discharge the cell to or below 0.5 V, deemed safe for further cutting and crushing phases in the recycling process. If done with a low enough C-rate to avoid heating the cell, the discharge process poses no safety risk.

Discharge current and temperature monitoring

Whether the discharge is performed on the pack or cell level, monitoring of discharge current and temperature of the cells is crucial. A higher discharge current shortens the discharge process, but it must be maintained low enough to prevent batteries from overheating. Research has shown that the solid electrolyte interphase (SEI) starts to decompose if the battery temperature exceeds 60 ˚C, exposing anode to the direct contact with the electrolyte, causing an exothermic reaction and thermal runaway. Besides
safety risks, even if the likely battery explosion and fire are prevented, such high temperatures also damage the material and structures in the battery.

Voltage recovery challenge

Deep discharging of cells to the range between 1 V and 0 V irreversibly destroys the cathode active material but it is necessary to overcome the challenge of the voltage recovery effect. When being discharged rapidly it can appear that the entire battery energy was consumed when the voltage drops to 0 V, but due to slow chemical reactions within the battery after the load has been disconnected from the battery, minutes to hours later, the voltage on the battery can recover, even over the critical 2.5 V.

In order to fight such recovery different methods can be used:

• Discharging the battery to 0V and short-circuiting it for a certain amount of time to safely discharge all the residual energy
• Discharging the battery to the chosen negative voltage, causing pole reversal and avoiding voltage recovery

As mentioned earlier, depending on the specific type and model of the battery/cell, as well as it’s SoC, SoH and nominal capacity, the recycling discharge conditions need to be determined to obtain fast but safe deep discharge. This includes choosing the optimal discharge current(s), minimum discharge voltage level, as well as time frames for potential short-circuiting of the battery.

Battery discharge load units

Previously described electronic loads can control the discharge current during the entire discharge process and offer common different discharge modes – constant current, constant power, constant resistance, as well as preset load profiles, depending on needs. They can be stationary or portable, both offering different advantages. Portable ones are lightweight solutions with a design where discharged energy is dissipated as heat over the resistive load, while stationary ones can be designed with energy recovery.

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February 22, 2023