The formula
Efficiency is about 95% for lithium and 85% for lead-acid, because some energy is lost as heat and gassing.
Why the last 20% takes longer on lead-acid
Lead-acid chargers run at full current (bulk) until the battery reaches about 80%, then hold a fixed voltage while the current tapers off (absorption). That last 20% typically takes 2–4 hours whatever the charger size. LiFePO4 batteries take nearly full current almost to 100%, so they charge much faster.
Worked example
A 100 Ah LiFePO4 at 30% with a 20 A charger: 100 × 0.7 ÷ (20 × 0.95) = 3.7 hours. The same charge on an AGM battery takes about 3 hours for the bulk stage to 80%, plus roughly 3 hours of absorption.
Frequently asked questions
How long does it take to charge a 100Ah battery with a 10A charger?
From 50%, about 5.3 hours for LiFePO4. For lead-acid, roughly 3.5 hours of bulk to 80% plus 3 hours of absorption.
Can I charge faster with a bigger charger?
Up to the battery's maximum charge current. LiFePO4 often allows 0.5C (50 A on 100 Ah); lead-acid prefers 0.1–0.3C.