Calculate amps from amp-hours and discharge time using A = Ah / h, or solve for amp-hours or run time, with unit conversion for mAh, Ah, mA, A, minutes, and hours.
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Amp Hours to Amps Formula
Amp-hours measure how much charge a battery holds, while amps measure the current being drawn at a moment. The link between them is time. This calculator uses the charge relationship and gives you three modes depending on what you want to solve for:
A = Ah / h
Ah = A * h
h = Ah / A
Where:
- A is the current, in amps
- Ah is the battery capacity (charge), in amp-hours
- h is the discharge time, in hours
The solve-for selector controls which value is returned. When you solve for amps, you enter amp-hours and the time over which the battery discharges, and the charge is divided by the time. When you solve for amp-hours, you enter amps and time, and the two are multiplied. When you solve for time, you enter amp-hours and amps, and the charge is divided by the current. The unit dropdowns convert milliamp-hours, amp-hours, or kiloamp-hours for charge and seconds, minutes, hours, or days for time into the base units before the math runs. The optional usable capacity setting lowers the amp-hours actually available, which matters for chemistries that should not be fully discharged.
Discharge Current for Common Capacities
The table below shows the steady current a battery can supply for a given capacity and discharge time, using A = Ah / h at full rated capacity.
| Capacity (Ah) | Discharge time (h) | Current (A) |
|---|---|---|
| 50 | 10 | 5 |
| 100 | 20 | 5 |
| 100 | 10 | 10 |
| 100 | 5 | 20 |
| 200 | 20 | 10 |
Many batteries should not be drained all the way down. The usable capacity setting in the calculator accounts for this. The table below lists typical recommended depth of discharge by battery type, which sets how much of the rated amp-hours you can plan to use.
| Battery type | Typical usable capacity |
|---|---|
| Flooded lead-acid | About 50% |
| AGM lead-acid | About 50% to 60% |
| Gel lead-acid | About 50% |
| Lithium (LiFePO4) | About 80% to 100% |
Example Problems
Example 1. You have a 100 Ah battery and want to know the steady current it can supply over a 20 hour period. Divide the capacity by the time: 100 Ah / 20 h = 5 A. The battery can deliver 5 amps for 20 hours.
Example 2. A device draws 4 amps and you want it to run for 6 hours. Solve for amp-hours by multiplying the current by the time: 4 A * 6 h = 24 Ah. You need at least 24 amp-hours of usable capacity, so a battery rated higher than that once depth of discharge is included.
Frequently Asked Questions
How do you convert amp hours to amps? Divide the amp-hours by the number of hours over which the battery discharges. For example, a 50 Ah battery discharged over 10 hours supplies 50 / 10 = 5 amps. You cannot convert amp-hours to amps without knowing the time, because amp-hours measure charge and amps measure the rate of flow.
Why do you need the time to make the conversion? Amp-hours and amps describe different things. Amp-hours are charge, which is current multiplied by time. To recover the current you have to divide the charge by the time it flows. Without a time period there is no single answer.
What is usable capacity and why does it matter? Usable capacity is the share of the rated amp-hours you can safely draw before the battery should be recharged. Lead-acid batteries are often limited to about half their rating, while lithium batteries can use most of theirs. Lowering the usable capacity reduces the amp-hours available, which lowers the current or run time the calculator reports.
