Electric charge shows up in wildly different units depending on the field: physicists default to coulombs, battery datasheets use amp-hours and milliamp-hours, and chemists reach for faradays. All of them measure the same underlying quantity — they just scale it differently for convenience. This article explains how each unit relates to the coulomb, why the scaling factors are what they are, and when you'd reach for each one.
The coulomb is the common thread
Every charge unit in this converter reduces to a multiple of the coulomb, the SI base unit. A coulomb is defined as the charge transported by a steady 1-amp current in 1 second, which means charge and current are directly linked: charge (C) = current (A) × time (s). Amp-hours and milliamp-hours are really just that same relationship measured in hours instead of seconds, which is why 1 Ah works out to exactly 3,600 C (3,600 seconds in an hour) and 1 mAh works out to 3.6 C. Once every unit is expressed in coulombs, converting between any pair — say, mAh directly to faradays — is a single multiply-then-divide step through that common base.
Why battery ratings use Ah and mAh instead of coulombs
A typical phone battery holds on the order of 18,000 coulombs of charge, but manufacturers print '5,000 mAh' on the label instead. That's a usability choice: milliamp-hours describe capacity in terms a user can directly compare to a charging rate (a 2 A charger fills a 5,000 mAh battery in about 2.5 hours) or a device's power draw, without requiring anyone to do arithmetic with a four-or-five-digit coulomb figure. The trade-off is that mAh and Ah only measure charge, not energy — converting to watt-hours (which the Battery Capacity Converter handles) requires knowing the pack voltage as well.
Faradays and the elementary charge: the chemistry and physics ends
Chemists working with electrolysis reach for the faraday because it directly ties charge to moles of electrons transferred, which is the quantity that shows up in Faraday's laws of electrolysis (the amount of substance deposited or liberated at an electrode is proportional to the charge passed, measured in faradays). At the opposite extreme, physicists working at the level of individual particles use the elementary charge — the charge on a single proton or electron — because it's the smallest free-standing unit of charge that exists in nature. Since the 2019 SI redefinition, the elementary charge is an exactly defined constant (1.602176634 × 10⁻¹&sup9; C) rather than an experimentally measured value, which is also what makes the coulomb itself formally definable in terms of elementary charges.