Second life batteries

Second life batteries

With increased concern for environment and recycling, companies especially EV companies focused on second life for batteries. Retired EV have cells which preserve around 70-80 % of  capacity. Recycling lithium ion batteries is a costliest affair which includes huge machinery and expertised labor. Economically sending these batteries for other applications reduces their expenses and secondly, environmentally its more clean to use the batteries up-to its complete life.

There are two reasons for discarding working batteries :

• Batteries are used in pack, since many electronics are worked at higher voltages and power, therefore they are connected in various series and parallel configurations. When one or more lithium batteries goes fail, it affects the battery pack in terms of decrease in power and voltage. This makes the connected device fail to work or work at lower performance. So, there is a need to replace the battery pack. 
• Electric vehicles and some devices are used around up-to 80% of the battery capacity since after that the mileage of the vehicle decreases. Since battery packs are made very sturdy with cooling components attached and replacing cells in a pack is not easy, whole pack has to be disassembled for replacing a cell and therefore whole battery pack is discarded.

The discarded battery packs are disassembled and each individual cells are tested and reused for applications which doesn't require high power.

Advantages

• Second life batteries are lot cheaper then corresponding new batteries, 
• Environmentally its more clean to use the batteries up-to its complete life.

Disadvantages

• Estimating the life is difficult
• Different chemistry and different capacity batteries makes the supporting electronics complex because each cell may have different charge discharge behavior.

Applications

• Stand alone or stationary batteries at home or for grid
• Some companies use these batteries for cheap toys and electronic components where they needn't guarantee.

Estimated life:

EV are assumed to work for eight years (80% of SOC) then for SOC to be 50 % it could be 12 yrs.

Many electronics for example laptops have a life of about 1 yr to 3 yrs and fail when one single cell fails and the other cell can work for even more than 10 yrs and with capacity above 80%.

Characterization of used batteries

Failed batteries are easier to detect, either they are physically broken, exploded, swelled,etc or OCV is around 0 - 1 V. Difficulty comes when we need to check the SOC and predict the extent of life.

Steps for characterization

•  Formation Cycles or Preconditioning: A passive layer is formed in the initial cycles around the active cathode as well as anode which is called as surface electrolyte interphase or SEI layer. Lot of physical (porosity/shape), chemical (electrochemical state) changes occur after which they are almost stable.
• Relaxation time: Whenever the cell is charged or discharged there are dynamic changes inside it. The measurable quantity at any state will not be exact if the cell itself is dynamic. A relaxation time has to be ensured for the cell to achieve quasi thermodynamic state.
• Capacity test depends on temperature, C-rate and also number of cycles
• OCV vs SOC @ C-rate and Temp constant
• DC Pulse test @ C-rate, SOC and different temp
• EIS test @ SOC and different temp