A tool for estimating how long it takes to charge an electric vehicle. Adjust the battery capacity, start and target state of charge, charger power and charging efficiency to see the energy needed, the energy drawn from the grid, the charge time, the added driving range and the charging cost update in real time. The model includes the tapering of the charge rate above 80%.
Parameters
Total battery capacity
kWh
Rated capacity of the battery pack in the EV
Start SOC
%
State of charge when charging begins
Target SOC
%
State of charge you want to reach. 80% is typical for long trips
Charger power
kW
A few kW for AC charging, 50-350 kW for DC fast charging
Charging efficiency
Fraction of grid energy that ends up in the battery
Results
—
Energy needed (kWh)
—
Energy drawn from grid (kWh)
—
Charging time
—
Average charging power (kW)
—
Range added (km)
—
Charging cost (yen)
—
EV charging — battery and charge-power profile
The battery gauge fills from the start SOC to the target SOC. The curve below is the charge-power profile — flat at full power up to 80%, then tapering down.
Charging time t. P: charger power [kW], C_batt: total battery capacity [kWh], ΔSOC: SOC range charged [%]. The charge rate slows sharply above 80% SOC, so that region is treated as charging at half the power (0.5P) to protect the battery.
Energy drawn from the grid E_grid (η: charging efficiency) and average charging power P_avg. Electricity is billed against E_grid.
What is the EV Charging Time Simulator?
🙋
I thought charging an EV was like filling a fuel tank — the time is just capacity divided by power. Is it not that simple?
🎓
As a starting point that is right. Take the energy you want in the battery and divide by the charger's power for a rough time. For example, to put in 36 kWh with a 50 kW charger, 36 / 50 is about 0.72 hours, roughly 43 minutes. But two real-world effects sit on top of that: charging losses, and the way the charging rate changes.
🙋
By losses, you mean the electricity from the outlet does not all reach the battery?
🎓
Exactly. About 10% of the energy you draw is lost as heat in the charger, the cabling and the battery's internal resistance. That is the charging efficiency, usually 85-95%. So if you want 36 kWh in the battery, you draw 36 / 0.90 = 40 kWh from the grid, and your electricity bill is based on that 40 kWh. Drop the efficiency slider on the left and you will see the grid energy and the cost both rise.
🙋
I see. So what about the second effect, the changing charge rate?
🎓
This is the important one. An EV does not charge at a constant rate. Up to a state of charge of about 80%, the battery accepts the charger's full power. But above 80% the charging current is deliberately reduced to protect the battery — this is called tapering. Look at the curve on the canvas below: it is flat up to 80%, then drops away sharply.
🙋
Why deliberately slow it down above 80%? I want it full as fast as possible.
🎓
As the cell approaches full charge its voltage rises, and forcing a large current at that point makes lithium plate out as metal on the negative electrode. That causes degradation, heating and, in the worst case, fire. So the battery management system reduces the current from around 80%. As a result the last 20% can take almost as long as the first 80%. That is why at a fast charger on a road trip the rule is to charge to 80% and move on. Try setting the target SOC to 100% and see how much longer it takes.
🙋
So if I want to charge faster, I just pick a higher-power charger?
🎓
Basically, yes. Look at the "Charge time vs charger power" chart: at low power the time falls steeply. But raising the power further has diminishing returns, because of the vehicle's own acceptance limit and the tapering above 80%. The realistic approach is AC charging at home (a few kW) overnight day to day, and DC fast charging (50-350 kW) only for road trips to top up quickly to 80%.
Frequently Asked Questions
At its simplest it is the energy you want in the battery divided by the charger's power. The energy needed is battery capacity multiplied by (target SOC - start SOC)/100. But a real EV does not charge at a constant rate: up to about 80% state of charge it accepts the charger's full power (the constant-current phase), and above 80% the current is reduced to protect the battery (tapering). This tool splits the charge at 80% and treats the region above 80% as charging at an average of half the charger power.
As a lithium-ion battery approaches full charge the cell voltage rises, and pushing a large current at that point causes lithium to plate out on the negative electrode, leading to degradation and heating. To avoid this, the battery management system deliberately reduces the charging current from about 80% onward. This is the constant-voltage phase, or tapering. As a result the last 20% can take almost as long as the first 80%, which is exactly why drivers on a long trip are advised to charge to 80% and move on.
Charging efficiency is the fraction of the energy drawn from the grid that actually ends up stored in the battery. The rest is lost as heat in the charger, the cabling and the battery's internal resistance. Efficiency is typically 85-95%, and this tool defaults to 90%. So to put 36 kWh into the battery you must draw 36/0.90 = 40 kWh from the grid. Electricity is billed on the energy actually drawn from the grid, so the charging cost is computed from the grid energy.
AC home charging delivers low power (around 3 kW in Japan, 7-11 kW on a 240 V North American circuit) and suits overnight charging to a full battery. A DC fast charger (50-350 kW) delivers high power and is built to top up to 80% quickly during a long journey. Sweep the charger power in this tool from 1.4 kW to 350 kW and you will see how dramatically the charge time changes. The basic strategy is AC charging at home day to day, and DC fast charging only for road trips.
Real-World Applications
Planning home charging: If you use an EV for a daily commute, the key question is whether overnight AC charging alone is enough. If you drive 40 km a day at an efficiency of 6 km/kWh, you need to replace about 6.7 kWh each night, which a 3 kW AC charger handles in two to three hours. Enter the battery capacity, start and target SOC and charger power in this tool to estimate the overnight charge time and electricity cost. Combined with an off-peak tariff, this can cut fuel cost well below that of a petrol car.
Charging plans for long drives: On a highway trip, how far you charge at each fast charger shapes the whole journey. Setting the target SOC to 80% keeps you out of the slow tapering region and keeps stops short. By varying the start SOC, target SOC and charger power in this tool, you can estimate the charge time at each service area and build a realistic plan around your rest breaks. Set the target to 100% and you immediately see how much time the last 20% costs you.
Charging infrastructure and facility design: An operator installing chargers at a commercial site or apartment block must estimate the occupancy time per vehicle and the required electrical contract from the expected battery capacities and SOC ranges. High-power fast chargers turn vehicles over quickly but carry a high peak-demand contract cost. Charge-time estimates like this tool's support the early decisions on the number of chargers, their power and the pricing.
Fleet operations (company cars and logistics): For fleets where uptime drives revenue — delivery trucks, taxis — charging must fit into the gaps in the working day. From the battery capacity, the daily SOC range and the available charger power, you can check whether charging completes during overnight or loading downtime. Accounting for tapering may even lead to a decision to operate at 80-90% rather than 100%, increasing total available uptime.
Common Misconceptions and Pitfalls
The most common misconception is treating the charger's rated power as the charging speed. Plugging into a 50 kW fast charger does not mean the vehicle is always accepting 50 kW. When the battery is cold, when the SOC is high, or when the temperature has risen too far, the vehicle reduces the power it accepts. The tapering above 80% is especially pronounced, so the charger's advertised maximum power and the actual average charging speed diverge widely. Read this tool's "average charging power" as an effective figure that already includes tapering.
Next, the assumption that you should always charge to 100%. Charging to 100% every day means more time spent stored at full charge, which accelerates lithium-ion degradation. Many manufacturers recommend 80-90% as a day-to-day ceiling. Set the target SOC to 100% in this tool and you can feel how long the last 20% takes. From the standpoint of both battery life and charging time, 80% is enough except before a long trip. Running the SOC too low (down toward 0%) also stresses the battery, so keeping it roughly within 20-80% is the safe practice.
Finally, the misconception that efficiency (km/kWh) is constant. This tool computes added range using a guide figure of 6 km/kWh, but real efficiency varies strongly with the model, speed, ambient temperature, air-conditioning use, road surface and gradient. In winter especially, cabin heating and the cold-temperature behaviour of the battery commonly worsen efficiency by 30-40%. The added range shown is only a guide for standard conditions; in cold regions or with a lot of high-speed driving you should estimate conservatively. Build charging plans with margin, so that even at worst-case efficiency you reach your destination.