Battery Charge Time Calculator
Time to restore a battery from its present state with a given charger current and efficiency. With the formula, a worked example and typical values.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
This page calculates charge time and charge delivered by the charger from battery capacity, fraction of capacity to restore, charger current and charge efficiency. Change any value or its unit and the result updates. The steps below show the working for the default values, so you can check each line against your own method.
Limits of this calculator
Real chargers taper the current near full charge, so the last 10 to 20% takes longer than this estimate. Follow the battery maker's limits on charge current.
Cross-check: 8 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Indicative
Indicative. Estimation only; further engineering checks are required. Basis: Constant-current charging estimate.
What this calculator checks
- The arithmetic of the simplified model for the values you entered.
What remains unverified
- Whether the simplifying assumptions listed on this page hold for your case.
- Any standard, manufacturer or site data. Treat the result as an estimate and confirm it by a further engineering check.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | Not applicable: A simple capacity/current calculation is not a universal charging model. Charge profile, chemistry, temperature, absorption stage and charger control are manufacturer-specific. |
| Standard review state | Not applicable |
| Scope-screening note | not recorded |
| Cross-check test cases | 8 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Indicative |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Battery capacity | 100 Ah |
| Fraction of capacity to restore | 0.5 |
| Charger current | 20 A |
| Charge efficiency | 0.85 |
- Charge needed = 100 Ah × 0.5 = 50 Ah
- t = 50 / (20 × 0.85) = 2.941 h
- Charge delivered = 20 × 2.941 = 58.8 Ah
Assumptions
- Constant charger current for the whole time
- No allowance for the slower absorption stage near full
Real chargers taper the current near full charge, so the last 10 to 20% takes longer than this estimate. Follow the battery maker's limits on charge current.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Battery capacity | Ah | greater than 0 Ah | |
| Fraction of capacity to restore | (number) | greater than 0, up to 1 | 0.5 for a battery discharged by half |
| Charger current | A | greater than 0 A | |
| Charge efficiency | (number) | greater than 0, up to 1 | Typically 0.8 to 0.9 for lead-acid; higher for lithium |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Charge time | h | Shown to 2 decimal places | |
| Charge delivered by the charger | Ah | Shown to 1 decimal place |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
How the result changes with battery capacity
| Battery capacity (Ah) | Charge time (h) | Charge delivered by the charger (Ah) |
|---|---|---|
| 10 | 0.29 | 5.9 |
| 25 | 0.74 | 14.7 |
| 50 | 1.47 | 29.4 |
| 100 | 2.94 | 58.8 |
| 200 | 5.88 | 117.6 |
| 400 | 11.76 | 235.3 |
| 1,000 | 29.41 | 588.2 |
Other inputs are the same as in the worked example above.
Standards and further reading
- IEC 60909-0 (short-circuit currents)
- IEEE Std 519 (harmonic limits)
Check results against your utility's connection requirements and the equipment data sheets. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter battery capacity, fraction of capacity to restore, charger current and charge efficiency. Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Constant charger current for the whole time. No allowance for the slower absorption stage near full.
Which calculators should I use next?
Common next steps are Battery Bank Sizing and UPS Battery Runtime. The related links at the bottom of this page open them.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 8 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Battery Charge Time Formulas: Battery charge time formulas: ampere-hours to replace, charge efficiency, charge time from the charger current
- Battery Formulas: Capacity and Runtime: Battery formulas for energy in Wh, bank capacity from load and depth of discharge, runtime
- Peukert Law and Battery Runtime Formulas: Peukert law formulas: the runtime of a battery at a given current
- Battery C-Rate and Cycle Life Formulas: Battery C-rate formulas: the current for a C-rate, the ideal run time
- Battery Storage Round-Trip Efficiency: Battery storage formulas: the usable energy, the round-trip efficiency
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