Battery Charge Time Calculator — Hours to Fill a Battery from Any State of Charge

Background.

Charging a battery is, on paper, a division: work out how much charge is missing, then divide it by how fast the charger can put charge back. The complications are that not all the charge you push in stays in, and that the last part of a charge does not happen at a constant rate at all. This calculator handles the first honestly and warns you about the second beside every result.

Start with the deficit. State of charge is a percentage of the rated capacity, so a 100 ampere-hour battery sitting at 20 percent is missing 80 ampere-hours. At 12 volts nominal that is 960 watt-hours to put back. If your battery is rated in milliampere-hours, watt-hours or kilowatt-hours instead, the nominal voltage converts between them — the relation is written into US regulation, which defines a lithium-ion battery's watt-hour rating as its ampere-hour capacity multiplied by its nominal voltage.

Next the charger. A charger specified in amps is already telling you what you need; one specified in watts becomes a current when divided by the battery's nominal voltage, so a 120 W charger on a 12 V battery delivers about 10 A. The calculator also reports the C-rate, which is the charge current divided by the rated capacity in ampere-hours. That single number tells you how hard the charger is pushing relative to the battery's size: 0.1 C is a gentle overnight charge, while 1 C would refill the entire rating in an hour and is more than many chemistries will accept.

Then the efficiency. Some of the charge that goes in is lost to internal resistance and side reactions rather than stored, so the charger has to run longer than the raw ampere-hour figure suggests. Arizona State University's PVCDROM teaching reference defines the coulombic efficiency as the ratio of the charges entering the battery during charging to those that can be extracted during discharging, and puts it typically in excess of 95 percent. This page defaults to 95 percent — the conservative end of that statement, attributed rather than invented — and leaves the field editable, because your charger and your battery have their own numbers.

Put those together and the default case comes out at 80 ampere-hours divided by 9.5 effective amps, which is 8.42 hours, or 8 hours 25 minutes. At a perfect 100 percent efficiency it would be exactly 8 hours, so the efficiency correction is costing about 25 minutes.

Now the warning that matters most, and it is beside the result rather than buried in an accordion. This is a constant-current estimate, and real chargers do not hold a constant current all the way to full. As the battery fills, the charger switches to holding a constant voltage and the current tapers away, so the last stretch takes disproportionately longer than any linear figure implies. A lead-acid absorption stage can run for hours at a dwindling current. A lithium pack's final few percent are far slower than its first eighty. That is why this calculator says a charge from 60 to 100 percent takes exactly as long as one from 0 to 40 percent, and why in reality it will not.

Two more limits. The current is capped by whichever is smaller, the charger's own output limit or the maximum charge rate the battery is rated to accept — a 100 A charger on a battery specified for 20 A charges at 20 A, and exceeding the manufacturer's maximum is a safety matter rather than an optimisation. And battery-management systems reduce or stop charging when cells are cold or hot, which no arithmetic here predicts. Treat the answer as a planning estimate, not a countdown.

What is battery charge time calculator?

Battery charge time is how long a charger takes to move a battery from one state of charge to a higher one. State of charge is the proportion of the rated capacity currently stored, so the quantity that actually has to be replaced is the rated capacity multiplied by the difference between the two percentages.

The calculation lives in ampere-hours because chargers are specified by current. A charger delivering I amps replaces I ampere-hours every hour, in principle. In practice it replaces slightly less, because the coulombic efficiency of the charge is below one: some of the charge that enters goes into internal resistance and side reactions rather than into stored energy. Dividing the deficit by the effective current rather than the nominal one is the whole of the correction.

The C-rate is the standard way of describing charge or discharge current relative to the battery's own size: current in amps divided by capacity in ampere-hours. It is dimensionless in effect and lets you compare a 2 A charge on a phone against a 50 A charge on a vehicle pack. A charge at 1 C would in principle refill the whole rating in one hour.

What this arithmetic describes is the constant-current or bulk phase of a charge. Nearly all modern chargers run a two-stage profile: constant current until the battery reaches a set voltage, then constant voltage while the current falls away. The first stage is close to linear and is what this page models; the second is asymptotic and its duration depends on the charger's set point, the chemistry, the temperature and the battery's condition.

How to use this calculator.

1. 01Enter the battery's rated capacity and choose whether the figure is in mAh, Ah, Wh or kWh.
2. 02Enter its nominal voltage. It is needed whichever unit you used, and it also converts a charger's wattage into a current.
3. 03Enter the state of charge you are starting from and the one you want to reach. The target must be higher than the start.
4. 04Enter the charger's output during the constant-current stage, in amps, milliamps or watts. If the battery accepts less than the charger can deliver, enter the battery's limit instead.
5. 05Set the charge efficiency. The 95 % default is the conservative end of PVCDROM's 'in excess of 95%'; use your own figures if you have them.
6. 06Read the time, then check the C-rate to see how hard the charger is working the battery relative to its size.
7. 07Add time for the tapered final stage, which this linear estimate does not include, and never exceed the manufacturer's maximum charge current.

The formula.

The deficit comes first. State of charge is defined as a proportion of the rated capacity, so moving from a start percentage to a target percentage means replacing the rated ampere-hours multiplied by the difference divided by a hundred. For a 100 Ah battery going from 20 percent to 100 percent that is 100 × 0.8 = 80 Ah. Expressed as energy it is the rated watt-hours times the same fraction, and the link between the two is the nominal voltage: Title 49 of the US Code of Federal Regulations, section 171.8, defines a lithium ion battery's watt-hour rating as its rated ampere-hour capacity multiplied by its nominal voltage. That single relation is what lets this page accept mAh, Ah, Wh or kWh interchangeably.

Notice a consequence of this definition worth being explicit about: only the difference between the two percentages affects the answer. Charging from 0 to 40 percent and from 60 to 100 percent produce identical times in this model. That is exactly correct for the constant-current phase and exactly wrong near the top of a real charge, which is why the taper warning appears beside every result.

The charger is normalised next. A unit rated in watts becomes a current through OpenStax University Physics Volume 2, section 9.5, Equation 9.12, "P = IV", so I = P ÷ V; a 120 W charger on a 12 V battery is a 10 A charger. Milliamps divide by a thousand. The C-rate is then the charge current divided by the rated capacity in ampere-hours, so 10 A into a 100 Ah battery is 0.1 C.

Efficiency is applied to the current rather than to the time, which amounts to the same thing but is easier to reason about. PVCDROM, the photovoltaics teaching reference from Arizona State University, defines coulombic efficiency as "the ratio of the number of charges that enter the battery during charging compared to the number that can be extracted from the battery during discharging" and notes it is typically "in excess of 95%". At 95 percent, a 10 A charger delivers 9.5 A of stored charge, and the time is the deficit divided by that: 80 ÷ 9.5 = 8.4210526316 hours, or 8 hours 25 minutes. At a perfect 100 percent the same charge takes exactly 8 hours, so the time scales as the reciprocal of the efficiency — halving the efficiency doubles the time, which is asserted as a test.

The result was cross-checked against a source not otherwise used on this page. Idaho National Laboratory's Battery Test Manual for Plug-In Hybrid Electric Vehicles, INL/EXT-14-32849 Revision 3, defines the C1/1 rate as the current corresponding to the manufacturer's rated one-hour capacity, giving the instance that a battery whose rated one-hour capacity is 40 Ah has a C1/1 of 40 A. Driving a 40 Ah battery across its full state-of-charge range at 40 A must therefore be a C-rate of exactly 1 and must take exactly one hour at perfect efficiency. This calculator returns precisely that, and halving the current to 20 A gives 0.5 C and two hours.

No chemistry-specific constant is baked into the page. The efficiency default is attributed to its source and is editable; no absorption time, taper profile or maximum charge rate is assumed, because none of those are universal and guessing one would be worse than leaving it to the reader's datasheet.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Christiana Honsberg and Stuart Bowden, PVCDROM / PVEducation (Arizona State University), "Battery Efficiency". Source of the charge-efficiency figure used as this page's default. Retrieved text: the coulombic efficiency is "the ratio of the number of charges that enter the battery during charging compared to the number that can be extracted from the battery during discharging", and is typically "in excess of 95%". Recorded honestly: the same page gives no numeric voltage-efficiency or overall-efficiency figure, and none is attributed to it here. Access: open. Retrieved 2026-07-29.
2. [2]US Government Publishing Office / US Department of Transportation, Code of Federal Regulations, 2024 edition, Title 49 § 171.8 (definitions). Statutory source for the charge-to-energy conversion this page uses to accept mAh, Ah, Wh and kWh interchangeably: "Watt-hour (Wh) means a unit of energy equivalent to one watt (1 W) of work acting for one hour (1 h) of time. The Watt-hour rating of a lithium ion cell or battery is determined by multiplying the rated capacity of a cell or battery in ampere-hours, by its nominal voltage." Access: open. Retrieved 2026-07-29.
3. [3]OpenStax (Rice University), University Physics Volume 2, §9.5 "Electrical Energy and Power". Primary source for turning a charger's wattage into a current and for the energy-time relation. Equation 9.12 gives "P = IV"; the section states "E = Pt" for power delivered at a constant rate and that "The energy unit on electric bills is the kilowatt-hour (kW·h), consistent with the relationship E = Pt", with "1 kW·h = 3.6 × 10⁶ J". Access: open. Retrieved 2026-07-29.
4. [4]Christiana Honsberg and Stuart Bowden, PVCDROM / PVEducation (Arizona State University), "Battery Charging and Discharging Parameters". Secondary check on the state-of-charge and C-rate definitions used here. Retrieved text: "Battery state of charge (BSOC or SOC) gives the ratio of the amount of energy presently stored in the battery to the nominal rated capacity"; "The Depth of Discharge (DOD) of a battery determines the fraction of power that can be withdrawn from the battery"; and the Cx notation where "x is the time in hours that it takes to discharge the battery". Access: open. Retrieved 2026-07-29.