The number of cells needed for a 1MW system is very high and can run into the thousands. When new, each of these cells have undergone strict performance testing before being assembled into a battery pack. Once these cells have been used in an EV, temperature influences, charge- and discharge conditions and other factors will have affected the electrochemical performance of the cells. Some will have 80% capacity left while other might have higher or lower capacities.
After decommissioning an EV battery pack, these packs need to be disassembled, each cell tested against the specifications of the new energy storage application, graded and assembled again into packs with equal performance. For a large scale ESS, multiple packs need to be connected together whereby each pack should be within 1% of the specifications.
Matching these cells and packs so they have equal performance requires extensive testing over the full spectrum of use. Variations between packs will bring challenges to the electronics needed to balance the packs and that in turn results in additional losses. A ‘quick and dirty’ connection of such packs will greatly increase the risk of thermal runaways and other technical and safety problems.
Expected (remaining) functional life
In the current market, where cells are replaced at 70 – 80% of remaining capacity, these cells are recycled. Very little study has been done on the behavior of cells after reaching this point. A point selected for sound reasons since the few studies done show that cell performance start deviating greatly after that. Remaining capacity can drop suddenly, internal resistance increases, mechanical failures (anode and cathodes change mechanically too while charging and discharging) occur and seals, still perfectly good for the mundane EV applications, can crack under the higher ESS loads.
For demanding large scale storage applications, the Return on Investment is crucial. If the 2nd hand batteries would give an additional functional life of say 2-3 years, and again, this needs considerable study and experiments to find the boundaries, a replacement of cells/batteries will be need 6 to 4 times during the functional life of the energy storage application.
Being able to do so quickly and safely might require a suitable mechanical form factor that allows for an easy exchange. Cables, connectors and battery shelves or racks must be
standardized otherwise this process takes too much time, time the ESS is supposed to generate money or provide (backup) power for the owner.
How about the economics?
For many electric vehicles, the battery pack still is a major cost factor. Being able to reduce the cost of ownership by offering customers a rebate for their traded-in battery pack after a number of year is good for the vehicle owner. In fact, it might be a little game changer if batteries where getting more expensive over time. However, the economy of scale of battery manufacturing is such that prices are expected to drop significantly in the foreseeable future. Hence 2nd hand packs need to compete against new packs whereby the cost of the described disassembly, testing, re-packaging and commissioning will only increase over time due to a large percentage of manual labor.
Furthermore, the EV builder will be stuck longer to using the same old technology for its vehicles since upgrading to a different and better cell technology will create compatibility problems for his 2nd use applications.
At face value, supply chain management looks great but in reality it might be difficult to ensure a constant supply of equally ‘good’ battery packs for these 2nd use applications. Sourcing from other EV companies will only increase the issues due to different batteries used and the wear and tear profile of these.
Building storage systems whereby the supply of your most critical component, the batteries, is not constant, will be challenging for both the manufacturer and his customer that has to deal with uncertain delivery times.
Apart from the cells, the battery management system and the energy management system, controlling the whole storage application might need constant changes to adapt to the type and quality of the batteries used. Virtually no system will be the same and maintenance will become more difficult over time. The cost for programming these changes and variations, testing before implementing these and the risk of errors should not be under estimated either.
If the functional life of a battery pack in an EV is set at 5 years, a new replacement pack will have dropped in price to approx. 77% of its original price at an aggregated price reduction of 5%/year as is now seen in the industry. The 2nd hand pack has 80% capacity left but also has a much shorter life expectancy. Any trade-in value for the old pack higher than 30% of the original pack would be a challenge for the ESS-builder since he still has considerable cost to make and he has to exchange the packs a number of times over the functional life of the ESS.
Looking at the above, designing, selling and maintaining large scale Energy Storage Applications based on 2nd hand batteries are not only a technical risk, they also form a great economic risk for both the builder and the end-user. The unpredictability, in most cases the absence of any warranty from the cell manufacturer due to totally different use, and possible safety issues are difficult to factor into an equation and will depend on the risks the user is willing to take for a marginal (if any at all) economic benefit.
Will environmental issues be reduced?
To some extend yes, extending the life of a battery pack is always good but recycling of batteries is pretty well organized now and during the sorting and grading of the used cells, a considerable number of cells will be scrapped too. Therefore, the environmental effect might be overstated and used for marketing reasons mainly.
Does all of this means there is no use for 2nd hand batteries? No certainly not but the approach taken by some companies sound like fitting 4 tires from different vehicles found at a scrapyard to your car and driving 240km/hr on the highways! It might work sometimes but could be disastrous at other times!
Use of these batteries in smaller applications such as the electrification of small boats used for recreational purposes or home storage applications will give the new owner many years of good use. The frequency of use is limited and the charge and discharge profiles are much less aggressive, finding matching cells is easier due to the smaller numbers and therefore the feasibility of such application to become successful is much higher.