They discuss this in the article. Used batteries are not their bottleneck, at least for this company. The issue is incorporating all the different designs/chemistry's/states safely: >The company has more than one GWh of batteries ready to go and expects to add another five GWh this year. Ultimately, the bottleneck to scale isn’t battery supply — Redwood takes in more than 20 GWh of batteries each year. is the safety qualification process required to put batteries out in the field. Sun explained that the core architecture, racking, pack manager, and software can handle multiple chemistries and form factors, but getting them past hurdles like the rigorous UL 9540A fire safety testing Redwood recently passed in-house is another matter.