Charging is never 100% efficient: lithium-ion batteries store about 95% of the charge put in, lead-acid about 85% and nickel batteries 70–80%. Real chargers also slow down near full (the constant-voltage phase), so charging the last 10–20% takes longer than the simple estimate.
How to use the Battery Charge Time Calculator
- Battery capacity
- Current charge level
- Target charge level
- Charging current
- Battery type (efficiency)
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the battery capacity, the current and target state of charge, the charging current and the battery chemistry.
Tips
- Use the figures printed on your product or its manual when you have them.
- Round quantities up so you are never short.
- Measure twice before you buy anything cut to size.
- Everything runs in your browser – try different values to compare options.
Common mistakes to avoid
- Mixing units (inches and centimetres, mAh and Ah).
- Ignoring efficiency losses and safety margins.
- Treating rules of thumb as exact.
Inputs explained
| Field | What to enter |
|---|---|
| Battery capacity | Units: Ah, mAh |
| Current charge level | In % |
| Target charge level | In % |
| Charging current | Units: A, mA |
| Battery type (efficiency) | Choose one: Lithium-ion / LiPo (95%), LiFePO4 (96%), Lead-acid (85%), NiMH (80%) … |
What each result means
- Charge needed = [Battery capacity]×max([Target charge level] – [Current charge level], 0)÷100
- Charging time = [Charge needed]÷([Charging current]×[Battery type (efficiency)])
- Charging time (h min) = [Charging time]
- Charge rate (C-rate) = [Charging current]÷[Battery capacity]
Example values
How the main result changes when Battery capacity changes and everything else stays at the example values above:
| Battery capacity (Ah) | Charging time (h) |
|---|---|
| 2.5 | 1.05 |
| 3.75 | 1.58 |
| 5 | 2.11 |
| 6.25 | 2.63 |
| 7.5 | 3.16 |
| 10 | 4.21 |
Formula
Charge needed = capacity × (target − current state of charge)Charge time = charge needed ÷ (charging current × efficiency)Worked example
Inputs — Battery capacity: 5 Ah; Current charge level: 20 %; Target charge level: 100 %; Charging current: 2 A; Battery type (efficiency): Lithium-ion / LiPo (95%).
Results — Charge needed = 4 Ah; Charging time = 2.11 h; Charging time (h min) = 2 h 6 min; Charge rate (C-rate) = 0.4 C.
Frequently asked questions
How long does it take to charge a 5,000 mAh power bank at 2 A?
From 20% to 100%, about 2 h 6 min, plus a little extra at the end.
Why does the last 20% charge slowly?
Lithium batteries switch to constant-voltage charging near full, so current tapers off.
What is a safe charging current?
Often 0.5–1C for lithium-ion; check the battery's datasheet.
What formula does the Battery Charge Time Calculator use?
Charge needed = capacity × (target − current state of charge).