Battery Life Calculator

Updated Aug 18, 2026 By Jehan Wadia

Introduction

This Battery Life Calculator tells you how long a battery will run your device. Just enter the battery capacity (in mAh or Ah) and how much current your device uses. The tool does the math and shows the answer in hours or minutes.

You can also flip it around. Pick "Battery Capacity" to find out what size battery you need for a job. Pick "Load Current" to see how much current your device can use and still last as long as you want.

Real batteries do not give you every drop of their power. So this calculator lets you choose your battery type, like lithium, AGM, gel, flooded lead acid, or NiMH. Each type has a different safe discharge limit built in. You can also add buffers for cold or hot weather, an older battery, and losses from an inverter or DC-DC converter. That gives you a runtime number closer to real life.

If your device draws different amounts of power at different times, turn on Multi-Mode Load. Add each mode, like active and sleep, with its current draw and how much of the time it runs. The tool finds the average current for you.

Every answer comes with a full step-by-step solution, a chart of where your capacity goes, and a list of the changes applied. It works for phone power banks, solar panel setups, RVs, drones, IoT sensors, and any other DC project.

How to use our Battery Life Calculator

Enter your battery size, how much current your device draws, and your battery type. The calculator shows how long your battery will last, how big a battery you need, or how much current you can draw, plus a full step-by-step solution.

Solve For: Pick what you want to find: Battery Life, Battery Capacity, or Load Current. That box turns off and becomes your answer.

Battery Capacity: Type the size of your battery from its label. Choose mAh for small devices or Ah for big batteries.

Load Current: Type the average current your device pulls. Choose mA or A. If your device is rated in watts instead, run the number through the Watts to Amps Calculator first.

Battery Life: Type how many hours or minutes you need the battery to run. If you are solving for battery life, this menu just picks the unit for your answer.

Battery Type: Pick your battery chemistry, like Lithium, AGM, Gel, Flooded, or NiMH. Each type has a different usable capacity, so this makes your result more real-world.

Load Profile (Multi-Mode Load): Click this if your device uses different power at different times. Add each mode, its current, and the percent of time it runs. All percents must add up to 100%. The tool uses a weighted average to blend the modes together.

Temperature Adjustment: Check this box if the battery works in very cold or very hot places. It adds a 5% safety buffer.

Battery Age Adjustment: Check this box if your battery is more than 6 months old. It adds a 10% safety buffer.

System Efficiency: Check this box if you use an inverter or DC-DC converter, then type the efficiency percent. Most systems fall between 85% and 95%.

Calculate and Reset: Click Calculate or press Enter to see your answer, a capacity breakdown chart, and the math steps. Click Reset to start over.

What Is Battery Life?

Battery life is how long a battery can power a device before it runs out. It depends on two main things: how much energy the battery holds (its capacity) and how fast the device uses that energy (its load current).

Capacity and Load Current

Battery capacity is measured in milliamp-hours (mAh) or amp-hours (Ah). A 2,000 mAh battery can give 2,000 mA for about one hour, or 200 mA for about ten hours. Load current is how many milliamps or amps your device pulls while it runs. Divide capacity by load current and you get run time in hours. If you only know voltage and resistance, the Ohm's Law Calculator will get you the current, and the Amp Calculator works from a wattage rating.

Why Real Life Is Shorter

You almost never get the full number printed on the battery. A few things cut it down:

Depth of discharge (DoD): Most batteries should not be drained all the way. Draining them fully can damage them or shorten their lifespan.
Temperature: Very cold or very hot air makes a battery weaker. Cold is the bigger problem.
Age: Every charge cycle wears a battery down. An older battery holds less than a new one.
System losses: Inverters, DC-DC converters, and even wires waste some power as heat. A 90% efficient inverter means 10% of the energy never reaches your device. Long or thin wiring adds its own loss, which you can check with the Voltage Drop Calculator and the Wire Size Calculator.

Usable Capacity by Battery Type

Battery Type	Safe Usable Capacity
Lithium (LiFePO4 / Li-Ion)	About 95%
AGM (Sealed Lead Acid)	About 80%
Gel	About 80%
NiMH / NiCd	About 70%
Flooded Lead Acid	About 50%

So a 100 Ah flooded lead acid battery really gives you about 50 Ah of safe power. A 100 Ah lithium battery gives about 95 Ah. That is a big reason lithium costs more. To compare that gap in plain percentage terms, 95 Ah is nearly double the usable energy for the same label rating.

Duty Cycle and Mixed Loads

Many devices do not pull the same current all the time. A sensor might use 500 mA while sending data and only 20 mA while sleeping. To find the true average, multiply each mode's current by the share of time it runs, then add the results. If a device is active 10% of the time at 500 mA and asleep 90% of the time at 20 mA, the average is 50 + 18 = 68 mA. This average is what sets real battery life.

Tips for Longer Battery Life

Lower the load current. Turn off screens, radios, and sensors when they are not needed. A Power Consumption Calculator helps you spot the biggest draws.
Keep batteries near room temperature.
Do not drain lead acid batteries below half.
Add a safety buffer of 10–20% when sizing a new battery bank.
Use fewer conversion steps. Every converter loses some power.

Where This Matters

These same rules apply to phones, drones, RC cars, solar power systems, RV and boat house banks, backup power supplies, e-bikes, and battery-powered IoT sensors. The math is the same whether you are sizing a coin cell or a full off-grid battery bank. For related sizing jobs, try the Generator Sizing Calculator for backup power, the PSU Calculator for computer builds, the LED Resistor Calculator for small circuits, and the kWh Calculator or Electricity Cost Calculator when you want to know what the recharging actually costs.

Formulas used

Usable capacity after depth of discharge

C_{usable} = C_{rated} \times DoD

Capacity after real-world derating

C_{derated} = \frac{C_{usable}}{1 + d}, \quad d = 0.05_{temp} + 0.10_{age}

Effective load current including system losses

I_{eff} = \frac{I}{\eta}

Battery life (run time)

t = \frac{C_{derated}}{I_{eff}} = \frac{C_{rated} \times DoD \times \eta}{I \times (1 + d)}

Required battery capacity

C_{rated} = \frac{t \times I_{eff} \times (1 + d)}{DoD} = \frac{t \times I \times (1 + d)}{\eta \times DoD}

Maximum load current

I_{max} = \frac{C_{derated}}{t} \times \eta = \frac{C_{rated} \times DoD \times \eta}{t \times (1 + d)}

Weighted average current for multi-mode duty cycle

I_{avg} = \sum_{i=1}^{n} I_i \times \frac{D_i}{100}, \quad \sum_{i=1}^{n} D_i = 100

C-rate of the load

C\text{-rate} = \frac{I}{C_{rated}}

Frequently asked questions

Why is my answer lower than the simple capacity divided by current?

Because the calculator subtracts real-world losses. It applies your battery type's safe discharge limit first, then any temperature, age, or efficiency buffers you checked. A 100 Ah flooded battery only gives about 50 Ah of safe power, so the runtime is about half of the simple math.

What is the difference between mAh and Ah?

They measure the same thing at different scales. 1 Ah = 1,000 mAh. Small items like phones, power banks, and drones use mAh. Car, RV, solar, and marine batteries use Ah. The calculator switches between them for you when you change the unit menu.

Does changing the unit menu change my number?

Yes, it converts it automatically. If you type 5000 mAh and switch to Ah, it becomes 5 Ah. The value stays the same size, just written a different way. Nothing is lost.

Do I need to enter voltage?

No. This tool works in amp-hours and amps, so voltage cancels out. Just make sure your battery capacity and your load current are for the same voltage system. If your device is rated in watts, convert watts to amps at your battery voltage first.

What efficiency percent should I use?

Use 85% to 95% for most inverters and DC-DC converters. Cheap inverters run near 85%. Good pure sine inverters run 90% to 95%. If you power your device straight from the battery with no converter, leave the box unchecked.

Should I check both the temperature and age boxes?

Check each one that applies to you. Both together add a 15% buffer. Checking them makes the runtime shorter or the needed battery bigger, which keeps you on the safe side.

Why must my duty cycle percentages add up to 100%?

Because they split up all of the device's time. Every second has to belong to one mode. If your percents total 90% or 110%, the average current is wrong, so the calculator asks you to fix it before it solves.

Can I use Multi-Mode Load when solving for Load Current?

No. Load Current is the answer in that mode, so there is nothing to average. The button turns off. Switch to Battery Life or Battery Capacity to use duty cycles.

How many modes can I add?

Up to 8. That is enough for most devices, like active, idle, sleep, and transmit. The Add Mode button turns off once you hit the limit.

What does the Capacity Breakdown chart show?

It shows where your capacity goes in four steps: rated capacity on the label, usable capacity after depth of discharge, capacity after derating buffers, and what actually reaches your device after system losses. The last bar is the number your runtime is really based on.

Which battery type should I pick if I do not know mine?

Check the label. Most home solar and RV batteries say AGM, Gel, or LiFePO4. Rechargeable AA and AAA cells are usually NiMH. Phone and laptop packs are Li-Ion. If you truly cannot tell, pick Generic, but know that result is the best case and not real-world.

Why does the calculator say my battery life is over 10 years?

That warning means your load current is very small next to your battery size. Double-check that you used mA and not A, or Ah and not mAh. Also remember that batteries self-discharge over months, so no battery really lasts a decade on the shelf.

What is a C-rate and why does it show up?

C-rate compares your load to your battery size. A 100 Ah battery pulling 100 A is 1C. Pulling 10 A is 0.1C. High C-rates heat the battery and cut real capacity more than this tool predicts. Keep lead acid under about 0.2C for best results.

Can I use this for a solar or off-grid battery bank?

Yes. Enter your total bank size in Ah, your average load in A, and pick your chemistry. Turn on system efficiency for your inverter. Add the temperature buffer if the bank sits in a cold garage or hot shed.

How accurate is the answer?

It is a solid estimate, not a promise. Real results shift with temperature, battery age, load spikes, and cell quality. Add 10% to 20% extra when you size a new battery so you are not cutting it close.

Why did my typed number change after I clicked Calculate?

The box you are solving for gets filled in with the answer. If you solve for Battery Capacity, the calculator writes the required size into the capacity box. That is your result, not an error.

Can I calculate battery life in days?

The unit menu offers minutes and hours, but the result line also shows a plain-language version. Anything over 24 hours is written in days and hours for you, like 3 days 6 hours.

Does a bigger battery always mean longer life?

Usually yes, if the chemistry stays the same. But a 100 Ah lithium battery beats a 100 Ah flooded battery by a lot, because lithium safely gives about 95 Ah and flooded gives about 50 Ah. Chemistry matters as much as the number on the label.