Rated 100Ah Lead-Acid Battery Often Delivers Under 70Ah at High Load

A 100Ah battery is widely assumed to supply 100 amps for one hour. In reality, under Peukert’s Law—first quantified by German scientist Wilhelm Peukert in 1897—the usable capacity of a lead-acid cell falls non-linearly as discharge current rises. A flooded or AGM unit rated 100Ah at the C20 rate (5A for 20h) typically yields only 55–70Ah when drained at 1C (100A), cutting runtime to roughly 30–40 minutes instead of 60.

The physics is unambiguous. At high current, concentration polarization starves the plate surface of reactive ions; I²R internal resistance converts part of the charge into waste heat; and lead-sulfate crystals clog electrode pores, locking deep active material out of the reaction. Lead-acid Peukert exponents sit at k≈1.2–1.55, whereas LiFePO4 stays near k≈1.02–1.05, making lithium capacity almost rate-independent.

Field data show the cost of ignoring this. Electric mopeds hitting 0.9–1.1C on hills age lead-acid packs in about two years; overloaded e-tricycles above 1C sulfation rapidly. In UPS black-start events lasting 5–15 minutes, naive sizing (nameplate Ah ÷ load) overstates backup time by 30%–40%.

Engineers advise reading the manufacturer discharge table, not the headline number: C10 or C20 labels describe slow drains, not 1C surges. For high-rate duties, either oversize lead-acid banks by 1.3–1.5× or switch to lithium. A battery is not a bucket—its amp-hour rating is a low-rate laboratory figure, not a high-power guarantee.