Understanding your second life value estimate
Why three factors, not just remaining capacity
Remaining capacity alone doesn't set resale value, since buyers also weigh chemistry, safety reputation, and how much useful life is realistically left. A battery at 85% capacity but 15 years old carries more uncertainty than one at the same capacity but only 3 years old, even though the capacity number looks identical, an uncertainty gap NREL's energy storage research factors into its own second-life battery valuation studies. The capacity guarantee vs actual tracking tool is a useful reference for documenting that capacity figure before a sale.
Why LFP dominates the second-life market
LFP's long remaining cycle life and strong safety track record make it the most attractive chemistry for repurposing into backup or off-grid applications, such as the kind of light, non-critical load covered in the system size for workshop addon tool. Lead acid, by contrast, often has so little remaining cycle life by the time it's considered for resale that recycling for lead content is usually worth more than reselling the unit intact.
How this fits into your original ownership math
A meaningful resale value at the end of a battery's primary service life effectively lowers the true cost of ownership, since some of the original purchase price comes back. The total cost of ownership comparison can factor in an estimated resale value alongside purchase price and expected replacements, and the financing for battery addition calculator covers the other side of that math if a full replacement rather than resale is the direction you're leaning.
What to check next
Once you have an estimate, the replacement timing tool helps plan when you'll actually need to make that resale decision. Second-life battery markets are also an active area of federal research, tracked by the DOE's Office of Electricity as more batteries reach the end of their primary service life.