# Battery Charge Time Calculator

> Estimates the time to charge a battery from its capacity in Ah, the charger current, the charge efficiency (default 85 %) and the starting state of charge, and reports the charging C-rate.

- Calculator id: `battery-charge-time` · Category: Engineering & Automotive (`engineering`) · Tool name: `estimate_battery_charge_time`
- Canonical page: https://tttkmbb.com/engineering/battery-charge-time · This document: https://tttkmbb.com/engineering/battery-charge-time.md · JSON definition: https://tttkmbb.com/engineering/battery-charge-time.json

## Purpose

Estimates the time to charge a battery from its capacity in Ah, the charger current, the charge efficiency (default 85 %) and the starting state of charge, and reports the charging C-rate.

**Use when:** You need to know how long a phone, e-bike, car or solar battery takes to charge with a given charger current, or the C-rate that a charger imposes.

**Do not use when:** You need the runtime of a battery under load (use battery-runtime) or an exact lithium-ion CC/CV profile (the constant-voltage tail makes the last 20 % slower than this linear estimate).

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `capacity_ah` | number | Ah | required | Rated capacity in ampere-hours (mAh ÷ 1000, e.g. 2500 mAh = 2.5 Ah). (> 0) |
| `charge_current_a` | number | A | required | Charging current delivered to the battery in amperes (for a USB charger, its rated output current). (> 0) |
| `charge_efficiency_percent` | number | % | optional, default 85 | Fraction of the charger's output stored as charge: ≈ 85 % for lead-acid and simple chargers, 95–99 % coulombic efficiency for lithium-ion (losses then come from the CV tail). (min 30, max 100) |
| `start_charge_percent` | number | % | optional, default 0 | State of charge at the start; 0 for a fully discharged battery. (min 0) |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `charge_needed_ah` | number | Ah | capacity_ah × (100 − start_charge_percent) / 100. |
| `charge_time_hours` | number | h | charge_needed_ah / (charge_current_a × efficiency). |
| `charge_time_formatted` | string |  | Charge time as hours:minutes:seconds. |
| `ideal_charge_time_hours` | number | h | charge_needed_ah / charge_current_a. |
| `c_rate` | number | C | charge_current_a / capacity_ah; 1C charges the nominal capacity in one hour. |
| `c_rate_description` | string |  | Typical interpretation of the C-rate for consumer cells. |

## Formula

`charge_time_hours = capacity_ah × (100 − start_charge_percent) / 100 / (charge_current_a × charge_efficiency_percent / 100); c_rate = charge_current_a / capacity_ah`

Linear approximation. Lithium-ion chargers switch from constant current to constant voltage at about 70–80 % charge, so the final portion takes longer than the current alone suggests; lead-acid absorption charging behaves similarly.

## Data Sources

- Battery University – BU-409: Charging Lithium-ion — https://batteryuniversity.com/article/bu-409-charging-lithium-ion (reference, retrieved 2026-09-24)
- Battery University – BU-402: What is C-rate? — https://batteryuniversity.com/article/bu-402-what-is-c-rate (reference, retrieved 2026-09-24)
- Wikipedia – Battery charger — https://en.wikipedia.org/wiki/Battery_charger (reference, retrieved 2026-09-24)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/battery-charge-time?capacity_ah=…&charge_current_a=…`
- `POST https://tttkmbb.com/api/v1/calculate/battery-charge-time` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/battery-charge-time · OpenAPI operationId `estimate_battery_charge_time` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "battery-charge-time", "inputs": {…}}`

## Example

- 2,500 mAh phone battery on a 1 A charger: inputs `{"capacity_ah":2.5,"charge_current_a":1}` → `{"charge_needed_ah":2.5,"charge_time_hours":2.941,"charge_time_formatted":"2:56:28","ideal_charge_time_hours":2.5,"c_rate":0.4}`
- 100 Ah battery from 50 % at 10 A: inputs `{"capacity_ah":100,"charge_current_a":10,"start_charge_percent":50}` → `{"charge_needed_ah":50,"charge_time_hours":5.882,"c_rate":0.1}`

```
GET https://tttkmbb.com/api/v1/calculate/battery-charge-time?capacity_ah=2.5&charge_current_a=1
```

## Limitations

You need the runtime of a battery under load (use battery-runtime) or an exact lithium-ion CC/CV profile (the constant-voltage tail makes the last 20 % slower than this linear estimate). Linear approximation. Lithium-ion chargers switch from constant current to constant voltage at about 70–80 % charge, so the final portion takes longer than the current alone suggests; lead-acid absorption charging behaves similarly. All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**Which current do I enter for a USB charger?**

The charger's rated output current (e.g. 2 A) only if the device draws it; most phones limit charging to their own maximum and the charge current falls in the CV phase.

**What is a safe C-rate?**

Standard lithium-ion cells are rated for 0.5–1C charging; lead-acid is usually charged at 0.1–0.3C. Higher rates need cells and chargers designed for fast charging.

## Related

- [Battery Runtime Calculator](https://tttkmbb.com/engineering/battery-runtime.md) — How long the charged battery will run a load.
- [Solar Panel Output Calculator](https://tttkmbb.com/engineering/solar-panel-output.md) — Daily solar energy available for charging.
- [Electric Power Calculator](https://tttkmbb.com/physics/electric-power.md) — Charger power and energy cost from voltage and current.