# Battery Size Calculator

Omni's **battery size calculator** (or **remaining battery capacity calculator**) explains in detail how to check the battery capacity for both lithium-ion and lead-acid batteries.

Our tool has many uses — whether you want to know how much longer your drone will fly after already using it for a few hours, or if you want to compare lead-acid and lithium-ion batteries in terms of their battery capacity, the **battery size calculator** does it all!

## How do I calculate the discharging time of a battery?

You just need to divide the battery's **ampere-hour or amp-hour ( Ah) rating** with the desired load current to estimate the discharging time or run time:

`discharging time = amp-hour / load current`

The amp-hour rating helps us determine the **maximum steady current supplied in an hour**. Suppose your battery has a `20 Ah`

rating on it. Using the battery runtime formula, we find out that it can supply `20 A`

for an hour, `10 A`

for 2 hours, `5 A`

for 4 hours, or `1 A`

for 20 hours.

*Do you know that you shouldn't discharge batteries completely? Learn how to estimate how long you can safely use a battery with our* *battery life calculator*!

## How do I measure a lead-acid battery capacity?

Now that you know how to calculate the runtime/discharge time of any type of battery, let's learn how to measure the capacity of lead-acid batteries after putting them to use.

We calculate the remaining capacity of a lead-acid battery using the following formula:

where:

- $B_\text{Pb}$ – Remaining capacity of the lead-acid battery ($\text{Pb}$ because it's the chemical symbol for lead);
- $I_\text{L}$ – Load current;
- $t$ – Duration for which the power is supplied to the load; and
- $Q$ – Percentage of charge that should remain after the battery is used.

## How do I check the capacity of a lithium-ion battery after use?

For a lithium-ion battery, the formula for the battery capacity is:

where:

- $B_\text{Li}$ – Remaining capacity of the lithium-ion battery ($\text{Li}$ because it's the chemical symbol for lithium);
- $I_\text{L}$ – Load current;
- $t$ – Duration for which the electrical power is supplied to the load; and
- $Q$ – Percentage of charge that should remain after the battery is used.

*Do you want to estimate the full capacity of your battery? Check our* *battery capacity calculator**!*

🙋 Lithium-ion batteries have a higher energy density than lead-acid batteries. So, we can store more energy in a lithium-ion battery using the same physical space. Therefore, a lithium-ion battery can supply more power than a lead-acid battery of the same size!

## How to use battery size calculator?

You can easily find out the remaining battery capacity using our tool by following these simple steps:

**Enter**the wattage of the appliance in the input box for`Application load`

.**Select**the type of your battery from the drop-down list beside`Battery type`

.**Type**the voltage rating specified on your battery in the input box for`Voltage`

.**Enter**the number of hours (or you can select the duration in other units) you intend to use the battery for.**Input**the percentage of charge you want to remain in the battery after use.

*Voila!* The calculator tells you the `Load current`

and `Remaining capacity`

or the battery size!

⚠️ You shouldn't discharge lead-acid and lithium-ion batteries completely. Discharge **lead-acid batteries up to 50%** and **lithium-ion batteries up to 20%** to avoid any irreversible damage and for improved cell life.

**Example:** To find the remaining charge in your UPS after running a desktop computer of `200 W`

for `10 minutes`

:

**Enter**`200`

for the`Application load`

, making sure`W`

is selected for the unit.- Usually, a UPS uses a lead-acid battery. The
`Battery type`

is`Lead-acid`

by default. So you don't need to choose the type manually in this case. **Enter**`12`

for the`Voltage`

as the lead-acid battery provides a total input voltage of`12 V`

.**Select**`minutes`

as the unit from the drop-down list beside the input box for`Required duration`

.**Input**`10`

in the`Required duration`

input box.**Type**`50`

in the`Remaining charge`

box.

The calculator displays both the load current `16.7 A`

and the remaining capacity or the battery size `9.2 Ah`

!

💡 Learn more about the units of amperes and voltage combined together in our kVA calculator. There, you'll also understand the relationship between the kilovolt-ampere (kVA) and kilowatt (kW) units of power.

*Read on to find answers to some of the questions on a battery's capacity and charging. Also, send us your questions on battery capacity in the comment box displayed when you rate the calculator.*

## FAQ

### What is the difference between ampere and ampere-hour?

Ampere (`A`

) is the **SI base unit of the electrical current** while ampere-hour or amp-hour (`Ah`

) denotes the **electric current produced or consumed in an hour**. We use amp-hour to measure the **capacity of a battery**. We also use watt-hour to measure battery capacity.

### What is battery reserve capacity?

The reserve capacity (RC) of a battery tells us how many minutes it can supply a current of `25 A`

at a potential difference above `10.5 V`

.

### How do I calculate the battery size for inverters?

You can calculate the battery size for inverters using the formula **B = P × t / V _{dc}**, where B is the battery capacity in ampere-hour, P is the inverter's power rating, t is the duration of power supply in hours, and V

_{dc}is the DC voltage of the inverter.

### How do I calculate the lithium-ion battery size for a varying load?

To calculate the battery size for a varying load which requires **I _{1}** in the interval

**t**and

_{1}**I**in the remaining time:

_{2}- Estimate the average load current —
**I**._{av}= (I_{1}× t_{1}/ t) + (I_{2}× [t - t_{1}/ t]) - Substitute
**I = I**in the equation for battery capacity of lithium-ion._{av}**B = 100 × I × t / (100 - q)**where B is the battery capacity, I is the load current, t is the duration of power supply, and q is the percentage of charge which should remain in the battery after the discharge.

### How long will a 100 Ah battery run an appliance that requires 1,000 W?

You can calculate the run-time using the formula, **t = (amp-hour × V)/P**, where amp-hour is the battery's maximum capacity, V is the voltage of the power supply, and P is the appliance's wattage.

In the US, the household power supply's voltage is 120 V. Therefore, a 100 Ah battery can supply power for 12 hours in the US for a 1000W-appliance.

### How do I calculate the charging time of a battery?

If we ignore the losses during charging, divide the battery capacity by the charging current to obtain the battery's charging time in hours.