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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.

When to use

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).

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.

Inputs

Parameter	Type	Unit	Required	Description
`capacity_ah`	number	Ah	yes	Rated capacity in ampere-hours (mAh ÷ 1000, e.g. 2500 mAh = 2.5 Ah). Range: > 0
`charge_current_a`	number	A	yes	Charging current delivered to the battery in amperes (for a USB charger, its rated output current). Range: > 0
`charge_efficiency_percent`	number	%	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). Range: ≥ 30, ≤ 100
`start_charge_percent`	number	%	default 0	State of charge at the start; 0 for a fully discharged battery. Range: ≥ 0

Outputs

Output	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.

Example

2,500 mAh phone battery on a 1 A charger: {"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: {"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

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/battery-charge-time (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/battery-charge-time · Markdown: https://tttkmbb.com/engineering/battery-charge-time.md · JSON definition: https://tttkmbb.com/engineering/battery-charge-time.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="battery-charge-time"
OpenAPI operationId: estimate_battery_charge_time
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Battery University – BU-409: Charging Lithium-ion (reference)
Battery University – BU-402: What is C-rate? (reference)
Wikipedia – Battery charger (reference)

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 calculators

Battery Runtime Calculator — How long the charged battery will run a load.
Solar Panel Output Calculator — Daily solar energy available for charging.
Electric Power Calculator — Charger power and energy cost from voltage and current.