EV battery health: the three numbers that actually matter

Every used-EV listing quotes state of health. It is a single percentage produced by the battery management system, and it is genuinely useful — but on its own it is close to a marketing number. Two cars can both report 92% and be in completely different condition.

The first number worth pulling is cell voltage delta. A healthy pack under light load keeps its individual cell groups within a few millivolts of each other. A pack with a 40–60 mV spread has a weak group, and the weak group defines how the whole pack behaves: it hits the lower cutoff first, so usable range collapses faster than the headline percentage suggests.

The second is thermal history. Packs age on a curve driven by time at high temperature and high state of charge. A car that spent three Los Angeles summers parked outdoors at 100% charge has lived a harder life than one with double the mileage that was garaged and charged to 80%. Some platforms log this; on others you infer it from the ownership pattern.

The third is DC fast-charge count relative to total energy throughput. Fast charging is not poison, but a pack that got most of its energy from 150 kW sessions has taken more calendar-adjusted stress than one charged overnight at 7 kW. Ratios matter more than raw counts.

None of this requires exotic tools — it requires a scan session with the pack awake and a technician who knows what the parameters mean. Before buying any used EV, budget an hour of diagnostic time. It is the cheapest hour in the entire transaction.