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Electric charge (Q) is the amount of electricity that flows, measured in coulombs (C). Current is the rate of charge flow, so charge is simply current multiplied by time: one amp flowing for one second moves one coulomb. Batteries are rated in amp-hours (Ah), which is just charge expressed over hours.
| Quantity | Formula / Value |
|---|---|
| Charge | Q = I × t |
| Amp-hour | 1 Ah = 3600 C (1 mAh = 3.6 C) |
| Electrons in a charge | n = Q / e, e = 1.602×10−19 C |
| Runtime from capacity | t = capacity(Ah) / current(A) |
Because one electron carries a tiny 1.602×10−19 C, even a single coulomb is about 6.24×1018 electrons.
Charge in coulombs can come from three different starting points, and picking the wrong one is the usual source of confusion.
| Known quantities | Formula | Typical use |
|---|---|---|
| Current and time | Q = I × t | Battery discharge, plating, coulomb counting |
| Capacitance and voltage | Q = C × V | Capacitor charge, sample-and-hold, charge pumps |
| Number of electrons | Q = n × e | Physics, semiconductor carrier counts |
The elementary charge e is 1.602176634×10−19 C exactly — a defined constant in the SI system since 2019, not a measured value.
For a varying current the correct statement is Q = ∫i dt, the area under the current-time curve. Using I×t with an average current is fine where the current is genuinely constant, but it quietly misleads on pulsed or switched waveforms. This is exactly what a coulomb-counting fuel gauge does continuously in hardware, and why it tracks charge correctly through load transients that a simple average would mishandle.
A 1,000 µF capacitor charged to 12 V holds Q = 0.001 × 12 = 0.012 C. A modest 2,000 mAh battery cell holds 7,200 C — six hundred thousand times more. That gulf is why capacitors buffer and filter rather than store: they supply large currents for very short periods, which is exactly what a decoupling capacitor or a DC-link capacitor is for, while sustained energy delivery remains the battery’s job.
The complementary strength is speed. A capacitor can deliver its charge in microseconds, where a battery’s internal resistance and chemistry limit how quickly it can respond — which is why the two are often used together, with capacitors handling transients and the battery handling the average.
Electric charge is the quantity of electricity, measured in coulombs (C). It is the total amount of electrons that have flowed, equal to current multiplied by time.
Q = I × t, where Q is charge in coulombs, I is current in amps, and t is time in seconds. Rearranged, I = Q/t and t = Q/I.
The SI unit of charge. One coulomb is the charge moved by one amp of current flowing for one second, and equals about 6.24×10¹⁸ electrons.
Multiply by 3600, since one hour is 3600 seconds. So 1 Ah = 3600 C and 1 mAh = 3.6 C.
It is the charge the battery can deliver: a 2000 mAh cell can supply 2000 mA for one hour, or 1000 mA for two hours, and so on (ideally).
Divide the capacity by the current: runtime(h) = capacity(Ah) / current(A). A 2 Ah battery at 0.5 A lasts about 4 hours.
About 6.24×10¹⁸, because each electron carries 1.602×10⁻¹⁹ C of charge.
The elementary charge, e = 1.602×10⁻¹⁹ coulombs (negative for an electron, positive for a proton).
No. Current is the rate of charge flow (coulombs per second = amps). Charge is the total amount that has flowed over a time.
Real batteries deliver less at high discharge rates and low temperatures (the Peukert effect), so actual runtime is often shorter than capacity divided by current.
The mass of metal deposited is proportional to the charge passed (Faraday's law), so controlling current and time controls the coating.
Yes — choose the time unit next to the time field and the calculator converts to seconds internally.
Q = I × t for a steady current, Q = C × V for a capacitor, and Q = n × e when counting elementary charges.
Exactly 1.602176634×10−19 coulombs. It has been a defined SI constant rather than a measured quantity since the 2019 redefinition.
No. For a changing current, charge is the integral of current over time, which is the area under the current-time curve. Using an average current with I×t is misleading on pulsed or switched waveforms.
Far less. A 1,000 µF capacitor at 12 V holds 0.012 C, while a 2,000 mAh cell holds 7,200 C — about six hundred thousand times more.
Because it delivers its charge in microseconds, where a battery is limited by internal resistance and chemistry. Capacitors handle transients and filtering; batteries handle sustained energy delivery.
Continuously integrating the current flowing in and out of a battery to track the charge remaining. It handles varying load correctly, unlike a simple average, but accumulates drift and needs periodic recalibration.
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