The low price of lead-acid, the most popular battery, is often used in setting cost targets for emerging
energy storage technologies. Future cost reductions in lead acid batteries could increase investment and
time scales needed for emerging storage technologies to reach cost-parity. In this paper the first documented
model of cost reductions for lead-acid batteries is developed. Regression to a standard experience
curve using 1989–2012 data yield a poor fit, with R2 values of 0.17 for small batteries and 0.05 for
larger systems. To address this problem, battery costs are separated into material and residual costs, and
experience curves developed for residual costs. Depending on the year, residual costs account for 41–86%
of total battery cost. Using running-time averages to address volatility in material costs, a 4-year time
average experience curve for residual costs yield much higher R2
, 0.78 for small and 0.74 for large leadacid
batteries. The learning rate for residual costs in lead-acid batteries is 20%, a discovery with policy
implications. Neglecting to consider cost reductions in lead-acid batteries could result in failure of energy
storage start-ups and public policy programs. Generalizing this result, learning in incumbent technologies
must be understood to assess the potential of emerging ones