Electric vehicle (EV) batteries are subjected to hundreds of partial charge-discharge cycles per year at changing discharge rates and possibly at extreme temperatures. These EV batteries degrade drastically during the first years of operation in electric vehicles. EV batteries have an average life of 5-7 years as research from China Commercial Industry Research Institute shows. China’s first batch of large-scale mass-produced EV batteries has reached the critical point where these batteries must be replaced, which led to the recycling of these EV batteries and that created the battery recycling market.
The batteries are removed from the EV when the battery reaches 80 percent of its total usable capacity or State of Health (SOH). The State of Health of a battery describes the difference between the battery being studied and a new battery and it considers the aging of the cell. It is defined as the ratio of the maximum capacity and the maximum capacity of a new cell, expressed as a percentage.
For example, a battery with a usable capacity of 10kW now only have a usable capacity of 8kW. Although the original specifications of the battery states that it is a 10kWh battery it is fact now an 8kWh battery.
Manufacturers of the lithium-ion EV batteries have the options to dispose of, recycle it for the valuable metals or to refit the batteries for use in the renewable energy market, known as second life batteries.
When the batteries are re-used in the renewable energy market, it is known as second life batteries. Second life batteries can reduce the amount of waste and prevent the additional depletion of Earth’s minerals such as cobalt, lithium, and nickel. Lithium extraction can also have negative environmental consequences because the process is water intensive.
Figure 1. The figure above shows the recycling process.
Lithium ion (Li-ion) cells age both with time and usage. Aging generally means a decrease in its energy density and power capability, caused by the loss of capacity and the increase of the cell’s impedance. Lithium cells also have a linear and nonlinear aging curve. The former indicates decelerated aging, whereas the latter describes accelerated aging. By the time that the cells are removed from the EV battery and used for second life batteries the cell have moved into the nonlinear accelerated aging stage of its lifes duration. The below graph shows in red the linear aging curve for a first life battery as well as the non-linear aging curve in green for a second life battery:
Figure 2. Aging of a first life battery vs the aging of a second life battery
The charge/discharge rate, temperature, and Depth of Discharge (DOD) all play a role in the state of the cells when it is removed from the EV battery pack. The higher the DOD the lower the cycle life. The internal resistance of the cells from one EV battery pack will not be the same as the cells from another EV battery pack. There is currently no market-wide regulatory or policy standards for manufacturers or end users of EV batteries for testing and confirming the quality of second life cells. No guarantees exist regarding second life-battery quality or performance, no common standards for battery management systems (BMS) or state of health (SOH) disclosures. It can be safe to assume that standards governing the second life battery technology may take quite some time to materialize.
Therefore, if two second life batteries are purchased at the same time, there is no guarantee that the internal resistance of the two packs is equal or very close to one another. In a system where two or more batteries are connected in parallel, the batteries will not store equal amounts of energy and one battery will drain faster than the other.
A C-rate is a measure of the rate at which a battery is discharged relative to its maximum capacity. A 1C rate means that the discharge current will discharge the entire battery in 1 hour. For a battery with a capacity of 100 Amp-hrs, this equates to a discharge current of 100 Amps. A 0.2C rate for this battery would be 20 Amps.
Power2sa distribute batteries that are assembled using only brand-new A grade cells, that is manufactured by a reputable lithium-ion phosphate cell manufacturer, according to stringent international standards. IPD’s VxI5100 will reach 80 percent of its usable capacity (SOH) after the battery have completed 2500 charge/discharge cycles at 1C. This means that the VxI5100 will operate for more than 6 years before it reaches the 80 percent SOH and 14.4 years till the end of its life at 0.2 C.
As cells age the internal resistance of the cell increases. The internal resistance determines the battery’s capability to deliver current to the load. In applications where high continuous current is required, more second life batteries per installation will be required to deliver the same continuous current as one new (first life battery). The continuous current of second life batteries is limited by battery assemblers to 0.2C, which means that a 100Ah battery will only deliver 20A. For the next example, it is assumed that the efficiency of the DC to AC converter is a hundred percent, and that the system is not connected to the grid, or it is during a load shedding period.
If a 2kW load is connected to the system, the load will draw 9 amp from the inverter if the inverter voltage is set to 220V AC, the current drawn from a 48V DC battery to generate 2kW is 41A DC. In the above example the system will require three second life batteries in parallel, which is limited to 20A each, to deliver the current required by the load.
The VxI5100 can deliver 100A continuous and in the above example, only one VxI5100 battery will be required to deliver the load current.
Not all batteries are assembled with new A Grade cells that is manufactured by a reputable cell manufacturer. Power2sa have selected VTC Power as its supplier for batteries because VTC Power has been manufacturing high quality cells for more than 20 years. Without going into the chemical composition of an individual cell, the quantities and quality of the different raw materials used to manufacture the cell, determines the quality of the cell. There is a difference between a cell manufacturer and a battery assembler. VTC Power only use high quality balanced cells that are manufactured by VTC Power to assemble the VxI5100 battery, whereas a battery assembler will source cells from other manufacturers. These cells could be new, or they could be recycled cells from electric vehicles.
One must therefore ask the following questions when deciding between a battery that is manufactured with new (first life) cells and a battery that is assembled using second life cells.
- What is the continuous and peak current that will be required by the load?
- Where do the cells that are used to manufacture the battery come from?
- Is the cost saving between a second life battery and a new battery worth the 2500 cycles (6 years) that is lost?
- Is the capital expenditure to replace the battery pack sooner worth the investment?
Second life batteries do have their place in the market and Power2sa support all efforts for sustainable mining and the recycling of rare metals; however, it is important that you understand all the specifications in the datasheet to determine whether the battery will deliver to the requirements of the overall system and whether the return on investment is worth it.
Not all batteries are equally manufactured.