What this calculator does
This calculator works out how long a battery, such as a car, deep-cycle or lead-acid battery rated in amp-hours (Ah), takes to charge from its current state to a target level, given the current the charger delivers. It is the reverse of a runtime calculation: instead of asking how long a battery can supply power, it asks how long it takes to refill.
Real charging is never perfectly efficient: some of the energy going in is lost as heat rather than stored. That loss varies by battery chemistry and charger, commonly landing somewhere around 80 to 90 percent, so the efficiency figure here is left as an adjustable input rather than a fixed assumption.
The formula
Work out the amp-hours still needed by multiplying the battery capacity by the percentage gap between the current and target charge. Divide that by the charging current multiplied by the efficiency factor, since only part of the current delivered actually ends up stored in the battery.
| Term | Meaning |
|---|---|
| Capacity (Ah) | The battery's rated amp-hour capacity, the total charge it can hold. |
| Charging current (A) | The current the charger delivers to the battery, in amps. |
| Charging efficiency | The fraction of delivered current that actually charges the battery, typically 0.8 to 0.9 rather than a full 1.0. |
The inputs explained
| Field | What to enter |
|---|---|
| Battery capacity (Ah) | The battery's rated capacity in amp-hours, printed on the battery or its datasheet. |
| Current charge (%) | The battery's current charge level as a percentage, estimate if not measured directly. |
| Target charge (%) | The charge level you are aiming for, usually 100 percent for a full charge. |
| Charging current (A) | The charging current supplied by the charger, in amps. |
| Charging efficiency | The fraction of charging current that is actually stored, commonly 0.8 to 0.9 for lead-acid batteries. |
When to use it
Charging a car battery overnight
Working out whether a given trickle charger current will bring a flat car battery back to full before morning.
Sizing a solar charge controller
Estimating how long a deep-cycle battery bank takes to recharge from a given solar charging current helps judge whether the system keeps up with daily use.
Comparing chargers
A higher-current charger cuts charging time roughly in proportion, so this shows the trade-off between charger cost and charging speed.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How long to charge a 60 Ah battery at different currents?
The same battery and charge gap, charged at a range of charging currents.
| Charging current | Charge time | Amp-hours needed |
|---|---|---|
| 2 A | 28 h 14 min | 48.00 Ah |
| 5 A | 11 h 17 min | 48.00 Ah |
| 10 A | 5 h 38 min | 48.00 Ah |
| 15 A | 3 h 45 min | 48.00 Ah |
| 20 A | 2 h 49 min | 48.00 Ah |
| 30 A | 1 h 52 min | 48.00 Ah |
How much does charging efficiency change the time?
The same battery and charger current across a range of efficiency assumptions.
| Charging efficiency | Charge time |
|---|---|
| 70% | 6 h 51 min |
| 75% | 6 h 24 min |
| 80% | 6 h 0 min |
| 85% | 5 h 38 min |
| 90% | 5 h 20 min |
| 95% | 5 h 3 min |
Questions
Why is charging efficiency not 100%?
Some energy delivered during charging is lost as heat and through internal chemical resistance rather than being stored. The fraction that is genuinely lost varies by battery chemistry, age and charger, which is why it is left as an adjustable input rather than a single fixed number.
How is this different from a battery runtime calculator?
A runtime calculation works out how long a battery can power a load before it is empty. This works out the opposite direction: how long it takes to refill the battery from a given charging current. See the runtime calculator at battery-runtime.html for the discharge side.
What charging current should I use for my battery?
That depends on the battery's specifications and the charger's rated output, both of which are printed on the equipment or its manual. Charging too fast can shorten a battery's life or, in the worst case, damage it, so follow the manufacturer's recommended current rather than just the fastest available.
Does this work for phone or laptop batteries too?
The same maths applies, but those batteries are usually rated in watt-hours and charged by wattage rather than amp-hours and amps. For that style of charging, see the device charge time calculator instead.
For the opposite calculation, how long a battery lasts under load, see Battery Runtime. For phone and laptop style charging in watt-hours, see Battery Charge Time.