A battery charge time calculator has to deal with something the obvious formula ignores. Divide capacity by charger current and you get a number that is always too small, because charging is not one process at a constant rate. It is two: a fast constant-current phase that does most of the work, followed by a constant-voltage phase in which the current falls away steadily and the last portion of the charge takes disproportionately long. That second phase is why a battery reaches 80% quickly and then seems to stop.
Arb Digital keeps a free tools library for the practical arithmetic behind technical decisions. This page models both phases separately, applies a charging efficiency, and gives you the total time along with the energy the charger will draw from the wall.
What This Battery Charge Time Calculator Does
Enter the battery's capacity in whichever unit the label uses, its nominal voltage, the state of charge you are starting from and the one you are aiming for, and what the charger delivers in amps or watts. Set a charging efficiency and the point at which the taper begins. The headline is the total charge time. The grid separates the constant-current phase from the taper phase, shows the energy the supply will deliver, and reports the charge rate as a C figure.
The two adjacent tools on this site answer questions on either side of this one. The battery capacity calculator converts between mAh, Ah and watt-hours and sizes a series-parallel pack — it tells you what the battery holds, which is the input here. The battery life calculator goes the other way, taking a capacity and a load and returning how long the battery runs before it needs this page again. Filling, holding and emptying are three different calculations.
How to Use It
- Enter capacity and voltage together. Amp-hours cannot be converted into charging energy without a voltage, so both fields matter even if the label only gives one.
- Use the charger's output rating. A charger marked "100 W" on its input may deliver rather less to the battery. If the specification gives an output current and voltage, use those.
- Set a realistic start and target. Charging from 20% to 80% is a very different job from 0% to 100%, and the difference is not proportional.
- Adjust the taper threshold to match your system. Many chargers switch out of constant current somewhere between 70% and 85%.
- Read the phase split, not just the total. If the taper phase is longer than the constant-current phase, stopping at the threshold saves most of the time for a small part of the charge.
The Formula / How It's Calculated
Energy needed is capacity in Wh × (target % − start %) ÷ 100, then divided by the charging efficiency, because some of what the charger delivers becomes heat rather than stored charge. That energy is split at the taper threshold. The constant-current portion is charged at full charger power; the taper portion is charged at the reduced average power you set. Each phase's time is simply energy ÷ power, and the two are added.
Worked example, using the values the page loads with. A 100 Ah battery at 12.8 V holds 1,280 Wh. Charging from 20% to 100% means delivering 80% of that, or 1,024 Wh into the cells. At 90% efficiency the charger must supply 1,138 Wh. A 20 A charger at 12.8 V is 256 W. The constant-current phase covers 20% to 80% — 768 Wh into the cells, 853 Wh supplied — which at 256 W takes 3 h 20 m. The taper covers the last 20%: 256 Wh into the cells, 284 Wh supplied, at 45% of 256 W, or 115 W, which takes 2 h 28 m. Total: 5 h 48 m, drawing 1.14 kWh at a charge rate of 0.20 C.
Look at what that split says. The final 20% of the charge takes 43% of the time. Stopping at 80% would have the battery ready in three and a half hours instead of nearly six.
Why the Last Twenty Per Cent Takes So Long
Most modern charging follows a constant-current, constant-voltage profile. In the first phase the charger pushes a fixed current into the battery and the battery's voltage climbs. Once that voltage reaches the charger's set point, the charger holds the voltage there instead, and the current it can push falls away as the battery's own voltage approaches it. Less current means less power, and less power means more time for the same amount of energy.
The effect is visible in every published charging specification. The US Department of Energy's fueleconomy.gov page on all-electric vehicles notes that fully recharging a battery pack can take from 3 to 12 hours, while even a fast charge to 80% capacity can take about 30 minutes — the gap between those two figures is almost entirely the taper. It is also why EV charging is universally quoted "to 80%": beyond that point the rate has fallen so far that the number stops being useful.
This is why the naive formula fails so badly. Capacity divided by charger current on the example above gives 100 Ah × 0.8 ÷ 20 A = 4 hours. The real answer is nearly six. The missing two hours are the efficiency loss and the taper, in that order of significance.
Charge Rate, Written as C
Charging speed is usually expressed relative to the battery's own capacity rather than in amps, because the same 20 A means something completely different to a 10 Ah battery and a 200 Ah one. The C rate is charge current ÷ capacity in Ah. Charging a 100 Ah battery at 20 A is 0.2 C; at 100 A it would be 1 C, in principle a one-hour charge in the constant-current phase.
The number matters because every battery has a maximum charge rate its manufacturer specifies, and exceeding it is the fastest way to destroy a pack. Lead-acid batteries typically accept much lower C rates than lithium chemistries, and cold cells accept far less than warm ones — many lithium battery management systems refuse to charge at all below freezing. Whatever this calculator says, the manufacturer's rated maximum charge current governs, and it is on the datasheet rather than in any calculator.
The C rate reported here uses the charger's constant-current output, which is the phase where the rate is highest. During the taper the effective rate falls continuously toward zero. The Ohm's law calculator handles the volts-amps-watts conversions if your charger's specification is given in mixed units.
Where the Missing Energy Goes
Charging is not lossless. Some of the energy the charger draws becomes heat in the charger's own electronics, some becomes heat in the battery's internal resistance, and in lead-acid chemistries some is consumed by gassing near the top of the charge. Ninety per cent is a reasonable working figure for a good modern lithium system; lead-acid is meaningfully worse, often in the seventies or low eighties overall.
The practical consequences are two. First, the wall meter always shows more energy than the battery gained, and the gap is the efficiency figure — 1.14 kWh drawn to store 1.02 kWh in the example. Over hundreds of cycles that difference is a real cost, which the electricity bill calculator can price. Second, that lost energy is heat, and heat has to go somewhere. A charger that is warm is normal; a battery that is hot is not, and it is the reason fast charging is limited by thermal management as much as by electrochemistry, as the Department of Energy's vehicle batteries research page describes in setting fast-charge targets alongside cost and range.
Solar and Intermittent Charging
Everything above assumes a charger delivering steady power. Solar charging does not: output varies with the sun through the day and collapses under cloud. For a solar system, the useful approach is to work in energy rather than time. Take the daily energy the array produces, divide it by the energy the battery needs, and you get days rather than hours.
Enter the array's average delivered power rather than its peak rating if you want a rough time from this page — an array rated 400 W might average 150 W across a usable day. The solar panel calculator handles the peak-sun-hours arithmetic that produces that average properly, and the appliance wattage calculator gives the load side, so you can check whether daily generation actually exceeds daily consumption. If it does not, no charge time is long enough.
Arb Digital's free tools library covers the energy, power and cost maths behind hardware and household projects, and our team is happy to talk through anything the tools cannot answer.
Browse Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Dividing capacity by charger current — that answer ignores both efficiency and the taper, and understates a full charge by hours.
- Using the charger's input rating — what matters is what it delivers to the battery, which is lower and usually stated separately.
- Assuming 100% is only slightly slower than 80% — the final fifth of a charge routinely takes as long as the middle three fifths.
- Ignoring the manufacturer's maximum charge rate — a calculator will happily compute a C rate that would damage the pack. The datasheet governs.
- Applying this to solar without adjusting — solar output varies through the day, so work in daily energy rather than a single charge time.
Related Free Tools From Arb Digital
Use the battery capacity calculator to establish what the pack holds in watt-hours first, the battery life calculator for runtime under a load, the energy converter for watt-hours against joules and BTU, the solar panel calculator for an off-grid charging source, and the electricity bill calculator to price the energy a charging routine consumes. Everything else is in the free online tools hub.
Frequently Asked Questions
Work out the energy needed in watt-hours, divide by charging efficiency, then divide by the charger's power. Do it in two parts — full power up to the taper threshold and reduced power after it — because the constant-voltage phase is far slower than the first phase.
Because it assumes full current for the whole charge and no losses. In practice around 10% of the energy becomes heat, and current falls away through the final phase. On a typical full charge the simple formula can be two hours short.
Once the battery's voltage reaches the charger's set point, the charger holds voltage constant and the current it can deliver falls as the battery approaches that voltage. Less current means less power, so the same energy takes considerably longer to move.
Charge current divided by capacity in amp-hours. Charging a 100 Ah battery at 20 A is 0.2 C. It expresses speed relative to the battery's own size, which is why manufacturers specify maximum charge rates in C rather than in amps.
Around 90% is reasonable for a modern lithium system. Lead-acid is usually worse, often in the seventies or low eighties, because energy is also consumed by gassing near the top of the charge. The lost energy leaves as heat.
Only roughly. Solar output varies through the day and collapses under cloud, so enter the array's average delivered power rather than its peak rating, or better, work in daily energy and calculate days rather than hours.
That page calculates how long a battery runs while powering a load. This one calculates how long it takes to refill. They use the same capacity figure from opposite directions, and neither substitutes for the other.
This tool estimates charging times from figures you supply. It is not battery, charger or electrical safety guidance, and the manufacturer's own charging specification always governs.
