The "battery-acid" flavor is due to cold brewing's imprecise process that can take anywhere from overnight to an entire day, and the ratio of coffee to water can fluctuate without consequence. "You’re like, 'Five pounds-ish [of coffee grounds], to, whatever, five gallons of water, for I don't know, 12 hours,'" said Tooker. Forgot to strain the grounds from the water? Oops, but steeping for 24 hours is also fine. "All the rules go out the window. It’s obviously a compromise."
The origins of the process are also telling: Cold brew started as a graveyard for beans that were about to expire instead of letting them go to waste, according to Tooker. "The freshness at that point doesn't matter," he said. "[The cold brew] would taste the same [regardless of whether] you put in four-week old or five month old coffee in the system." The primary culprit here is the lengthy exposure to oxygen, which puts the steeping coffee at risk of ruin through a reaction called oxidation. (A sliced apple turns brown for the same reason.)