Coulombic Efficiency

Coulombic efficiency is the ratio of the charge you can take out of a battery to the charge you put in. The rest is lost during charging.

Battery University puts lithium-ion above 99%, lead acid at about 90%, and NiCd and NiMH at up to 90% with fast charge and about 70% on slow charge.

Efficiency adds time to every charge. A 100 Ah lead-acid battery at 90% needs 111.1 Ah to fill from empty, about 11 hours 7 minutes on a 10 amp charger. The battery charging time calculator sets efficiency by battery type, and the guide to lithium vs lead-acid charging compares them.