How to Calculate Battery Amp-Hours for Your Devices – A Guide by Viking Batteries
We’re often asked how to determine how many amp-hours are needed to power a device – or several devices. So here’s a clear, step-by-step guide to help you calculate your battery needs accurately.
🔧 Step 1: Basic Calculation – Amp-Hours (Ah)
First, determine how much current (in amps) your device uses and how long you want it to run.
Formula:
Capacity (Ah) = Current (Amps) × Time (Hours)
Example:
If your pump uses 120mA (0.12A) and needs to run for 24 hours:
0.12A × 24h = 2.88Ah
But you’re not done yet!
🔋 Step 2: Depth of Discharge (DOD)
Batteries last longer if you don’t fully drain them. For example, lead-acid batteries are usually rated for 80% discharge. To compensate:
Adjusted Capacity = Ah / 0.8
Example:
2.88Ah / 0.8 = 3.6Ah
You’ll need a minimum 3.6Ah battery to maintain good cycle life.
⚡ Step 3: Discharge Rate Effects
Not all batteries deliver full capacity when discharged quickly. This is especially true for lead-acid, alkaline, and zinc-based chemistries. At high current (e.g., 1C rate), usable capacity can drop by 50%.
Rule of thumb (for lead-acid):
At a 1C discharge rate → Usable capacity ≈ 50% of rated capacity
Example:
A device drawing 20A for 1 hour:
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Step 1: 20A × 1h = 20Ah
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Step 2: 20Ah / 0.8 = 25Ah
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Step 3: 25Ah / 0.5 = 50Ah
You’d need a 50Ah lead-acid battery to run the device at that draw for 1 hour.
🔁 Step 4: What If the Load Isn’t Constant?
If your device has variable power needs, calculate the average current over time.
Example:
Device draws 20A for 1 second, then 0.1A for the rest of the hour.
Average Current = (20 × 1/3600) + (0.1 × 3599/3600) = 0.1044A
Use this average in Step 1, then apply Steps 2 and 3 as needed. Short bursts of high power are less harmful than continuous high discharge.
⚙️ Bonus: Converting from Watts to Amp-Hours
If your device power is listed in watts, convert it like this:
Watt-hours = Watts × Time (Hours)
Adjust for inverter efficiency (e.g., 85%):
Watt-hours / 0.85
Then convert to amp-hours:
Amp-hours = Watt-hours / Battery Voltage
Example:
Running a 250W device on a 12V battery for 5 hours:
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250W × 5h = 1250Wh
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1250Wh / 0.85 = 1470Wh
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1470Wh / 12V = 122.5Ah
Then apply Steps 2 and 3 to get your final battery size.
If you need help selecting the right Viking Battery, our experts are here to assist! Just reach out with your device specs and we’ll guide you to the best option.
