Battery Charge Time Calculator
Hours to full battery charge from capacity and charger watts.
What it does
Charge time links three nameplate numbers. Capacity sets the job, present charge sets what remains, and charger watts set the pace.
The tool above nets 90 % of charger watts for loss. A 500 Wh pack at 20 % on 100 W needs 400 Wh at 90 W net, or 4.4 hours at 18 % per hour, which fits overnight.
Efficiency is a bulk phase average. Cords, heat, and conversion take about a tenth, while the taper past 90 % takes more time than watts suggest.
Rate per hour guides brick picks. Eighteen percent per hour means a lunch plug adds real range, while 5 % per hour means only overnight works.
Taper closes the plan. The last tenth fills slowly by design to protect cells, so deadlines should target 90 %, not 100 %.
How to use
- Enter battery capacity in Wh.
- Enter present charge percent.
- Enter charger watts.
- Read hours first.
- Read percent per hour next.
- Read overnight fit and plan the plug.
Formula
- capacity
- Nameplate energy.
- now
- Present charge, in percent.
- charger
- Charger output watts.
Now is present charge in percent. The 0.9 factor covers conversion loss in the bulk phase; taper past 90 % adds more.
Worked example
Example: 500 Wh at 20 %, 100 W
Defaults: 500 Wh at 20 % on a 100 W charger. These are also the default values, so the page opens on this answer.
- Capacity: 500 Wh.
- Now: 20 %, so empty share is 1 - 0.20 = 0.80.
- Energy in: 500 x 0.80 = 400 Wh.
- Charge power: 100 W at 90 % efficiency, so 90 W net.
- Time: 400 / 90 = 4.4 hours.
- Rate: 90 / 500 x 100 = 18 % per hour. Tool shows 4.4 hours, 18 % per hour, Fits overnight.
Reference table
| Pack | Charge | Time |
|---|---|---|
| 250 Wh at 0 %, 25 W | 11.1 hours | Needs daytime |
| 500 Wh at 20 %, 100 W | 4.4 hours | Fits overnight |
| 1000 Wh at 50 %, 200 W | 2.8 hours | Fits overnight |
| 1000 Wh at 20 %, 65 W | 13.7 hours | Needs daytime |
Limits
- Bulk phase math. Taper past 90 % adds time.
- Ignores heat, age, and maker current limits.
- Nameplate Wh may overstate usable energy.
- Fast charge needs maker approved bricks.
Common mistakes
- Dividing by brick watts with no loss factor.
- Planning 100 % by deadline instead of 90 %.
- Charging hot packs in sun.
- Mixing volt mismatched bricks.
- Trusting percent lights over watt math.
Try it yourself
Quick checks. Answers sit tucked under each question.
1000 Wh at 50 %, 200 W?
Empty is 1 - 0.50 = 0.50. Energy is 1000 x 0.50 = 500 Wh. Time is 500 / (200 x 0.9) = 2.8 hours.
250 Wh at 0 %, 25 W?
Empty is 1 - 0 = 1. Energy is 250 Wh. Time is 250 / (25 x 0.9) = 11.1 hours. Reads as Needs daytime.
Questions
How long to full?
Multiply capacity by empty share, then divide by charger watts times 0.9. Here 500 x 0.80 / 90 = 4.4 hours.
How fast per hour?
Divide net watts by capacity. Here 90 / 500 x 100 = 18 % per hour.
Why 90 % efficiency?
Near 90 % for the bulk phase. Heat, taper, and old cells slow the last fifth.
Overnight or daytime?
Ten hours or less reads as overnight here. Larger banks or small bricks need daytime.
Does the last bit slow?
Yes for lithium taper. The last 10 % can take as long as the middle 20 %.
Is this a safety rating?
No. Heat, cell age, and maker limits gate real charging.