Battery Charge Time Calculator

Use our battery charge time calculator to easily estimate how long it’ll take to fully charge your battery.

Battery Charge Time Calculator

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Optional: How charged is your battery? If left blank, we'll assume it's fully discharged (0% SoC), except for lead acid batteries which we'll assume are discharged halfway (50% SoC).
Estimated charge time:

Tip: If you’re solar charging your battery, you can estimate its charge time much more accurately with our solar battery charge time calculator.

How to Use This Calculator

1. Enter your battery capacity and select its units from the list. The unit options are milliamp hours (mAh), amp hours (Ah), watt hours (Wh), and kilowatt hours (kWh).

2. Enter your battery charger's charge current and select its units from the list. The unit options are milliamps (mA), amps (A), and watts (W).

3. If the calculator asks for it, enter your battery voltage or charge voltage. Depending on the combination of units you selected for your battery capacity and charge current, the calculator may ask you to input a voltage.

4. Select your battery type from the list.

5. Optional: Enter your battery state of charge as a percentage. For instance, if your battery is 20% charged, you'd enter the number 20. If your battery is dead, you'd enter 0.

6. Click "Calculate Charge Time" to get your results.

3 Battery Charging Time Calculation Formulas

For those interested in the underlying math, here are 3 formulas to for calculating battery charging time. I start with the simplest and least accurate formula and end with the most complex but most accurate.

Formula 1

Formula: charge time = battery capacity ÷ charge current

Accuracy: Lowest

Complexity: Lowest

The easiest but least accurate way to estimate charge time is to divide battery capacity by charge current.

Most often, your battery's capacity will be given in amp hours (Ah), and your charger's charge current will be given in amps (A). So you'll often see this formula written with these units:

charge time = battery capacity (Ah) ÷ charge current (A)

However, battery capacity can also be expressed in milliamp hours (mAh), watt hours (Wh) and kilowatt hours (kWh). And your battery charger may tell you its power output in milliamps (mA) or watts (W) rather than amps. So you may also see the formula written with different unit combinations.

charge time = battery capacity (mAh) ÷ charge current (mA)

charge time = battery capacity (Wh) ÷ charge rate (W)

And sometimes, your units are mismatched. Your battery capacity may be given in watt hours and your charge rate in amps. Or they may be given in milliamp hours and watts.

In these cases, you need to convert the units until you have a 'matching' pair -- such as amp hours and amps, watt hours and watts, or milliamp hours and milliamps.

For reference, here are the formulas you need to convert between the most common units for battery capacity and charge rate.

Battery capacity unit conversions:

Charge rate unit conversions:

The formula itself is simple, but taking into account all the possible conversions can get a little overwhelming. So let's run through a few examples.

Example 1: Battery Capacity in Amp Hours, Charging Current in Amps

Let's say you have the following setup:

  • Battery capacity: 100 amp hours
  • Charging current: 10 amps

To calculate charging time using this formula, you simply divide battery capacity by charging current.

100Ah ÷ 10A = 10 hrs

In this scenario, your estimated charge time is 10 hours.

Example 2: Battery Capacity in Watt Hours, Charging Rate in Watts

Let's now consider this scenario:

  • Battery capacity: 1200Wh
  • Charging rate: 150W

Because your units are again 'matching', to calculate charging time you again simply divide battery capacity by charging rate.

1200Wh ÷ 150W = 8 hrs

In this scenario, your estimated charge time is 8 hours.

Example 3: Battery Capacity in Milliamp Hours, Charging Rate in Watts

Let's consider the following scenario where the units are mismatched.

  • Battery capacity: 3000mAh
  • Charging rate: 10W
  • Charging voltage: 5V

First, you need to decide which set of matching units you want to convert to. You consider watt hours for battery capacity and watts for charge rate. But you're unable to find the battery's voltage, which you need to convert milliamp hours to watt hours.

You know the charger's output voltage is 5 volts, so you settle on amp hours for battery capacity and amps for charge rate.

With that decided, you first divide watts by volts to get your charging current in amps.

10W ÷ 5V = 2A

Next, you convert battery capacity from milliamp hours to amp hours by dividing milliamp hours by 1000.

3000mAh ÷ 1000 = 3Ah

Now you have your battery capacity and charging current in 'matching' units. Finally, you divide battery capacity by charging current to get charge time.

3Ah ÷ 2A = 1.5 hrs

In this example, your estimated battery charging time is 1.5 hours.

Formula 2

Formula: charge time = battery capacity ÷ (charge current × charge efficiency)

Accuracy: Medium

Complexity: Medium

No battery charges and discharges with 100% efficiency. Some of the energy will be lost due to inefficiencies during the charging process.

This formula builds on the previous one by factoring in charge/discharge efficiency, which differs based on battery type.

Different battery types have different charging efficiencies

Here are efficiency ranges of the main types of rechargeable batteries (source):

  • Lithium: 90-95%
  • Lead acid: 80-85%
  • NiCd: 70-85%
  • NiMH: 70-85%

Note: Real-world charge efficiency is not fixed and varies throughout the charging process based on a number of factors, including charge rate and battery state of charge. The faster the charge, typically the less efficient it is.

Example 1: Lead Acid Battery

Let's assume you have the following setup:

  • Battery capacity: 100Ah
  • Charging current: 10A
  • Battery type: Lead acid

To calculate charging time using Formula 2, first you must pick a charge efficiency value for your battery. Lead acid batteries typically have energy efficiencies of around 80-85%. You're charging your battery at 0.1C rate, which isn't that fast, so you assume the efficiency will be around 85%.

With an efficiency percentage picked, you just need to plug the values in to the formula.

100Ah ÷ (10A × 85%) = 100Ah ÷ 8.5A = 11.76 hrs

In this example, your estimated charge time is 11.76 hours.

Recall, that, using Formula 1, we estimated the charge time for this setup to be 10 hours. Just by taking into account charge efficiency our time estimate increased by nearly 2 hours.

Example 2: LiFePO4 Battery

Let's assume you again have the following setup:

  • Battery capacity: 1200Wh
  • Charging rate: 150W
  • Battery type: Lithium (LiFePO4)

Based on your battery being a lithium battery and the charge rate being relatively slow, you assume a charge efficiency of 95%. With that, you can plug your values into Formula 2.

1200Wh ÷ (150W × 95%) = 1200Wh ÷ 142.5W = 8.42 hrs

In this example, your estimated charge time is 8.42 hours.

Using Formula 1, we estimated this same setup to have a charge time of 8 hours. Because lithium batteries are more efficient, factoring in charge efficiency doesn't affect our estimate as much as it did with a lead acid battery.

Example 3: Lithium Ion Battery

Again, let's revisit the same setup as before:

  • Battery capacity: 3000mAh
  • Charging rate: 10W
  • Charging voltage: 5V
  • Battery type: Lithium (Li-ion)

First, you need to assume a charge efficiency. Based on the battery being a lithium battery and the charge rate being relatively fast, you assume the charge efficiency is 90%.

As before, you need to 'match' units, so you first convert the charging current to amps.

10W ÷ 5V = 2A

Then you convert the battery's capacity from milliamp hours to amp hours.

3000mAh ÷ 1000 = 3Ah

With similar units, you can now plug everything into the formula to calculate charge time.

3Ah ÷ (2A × 90%) = 3Ah ÷ 1.8A = 1.67 hours

In this example, your estimated charge time is 1.67 hours.

Formula 3

Formula: charge time = (battery capacity × depth of discharge) ÷ (charge current × charge efficiency)

Accuracy: Highest

Complexity: Highest

The 2 formulas above assume that your battery is completely dead. In technical terms, this is expressed by saying the battery is at 100% depth of discharge (DoD). You can also describe it as 0% state of charge (SoC).

Formula 3 incorporates DoD to let you estimate charging time regardless of how charged your battery is.

Example 1: 50% DoD

Let's revisit this setup, but this time assume our lead acid battery has a 50% DoD. (Most lead acid batteries should only be discharged to 50% at most to preserve battery life.)

  • Battery capacity: 100Ah
  • Charging current: 10A
  • Battery type: Lead acid
  • Battery DoD: 50%

As before, let's assume a charging efficiency of 85%.

We have all the info we need, so we just plug the numbers into Formula 3.

(100Ah × 50%) ÷ (10A × 85%) = 50Ah ÷ 8.5A = 5.88 hrs

In this example, your battery's estimated charge time is 5.88 hours.

Example 2: 80% DoD

For this example, imagine you have the following setup:

  • Battery capacity: 1200Wh
  • Charging rate: 150W
  • Battery type: Lithium (LiFePO4)
  • Battery DoD: 80%

As before, we'll assume that the charging efficiency is 95%.

With that in mind, here's the calculation you'd do to calculate charge time.

(1200Wh × 80%) ÷ (150W × 95%) = 960Wh ÷ 142.5W = 6.74 hrs

In this example, it will take about 6.7 hours to fully charge your battery from 80% DoD.

Example 3: 95% DoD

Let's say your phone battery is at 5%, meaning it's at a 95% depth of discharge. And your phone battery and charger have the following specs:

  • Battery capacity: 3000mAh
  • Charging rate: 10W
  • Charging voltage: 5V
  • Battery type: Lithium (Li-ion)
  • Battery DoD: 95%

As before, we need to convert capacity and charge rate to similar units. Let's first convert battery capacity to amp hours.

3000mAh ÷ 1000 = 3Ah

Next, let's convert charge current to amps.

10W ÷ 5V = 2A

Because the charge C-rate is relatively high, we'll again assume a charging efficiency of 90% and then plug everything into Formula 3.

(3Ah × 95%) ÷ (2A × 90%) = 2.85Ah ÷ 1.8A = 1.58 hrs

Your phone battery will take about 1.6 hours to charge from 5% to full.

Why None of These Formulas Is Perfectly Accurate

None of these battery charge time formulas captures the real-life complexity of battery charging. Here are some more factors that affect charging time:

  • Your battery may be powering something. If it is, some of the charge current will be siphoned off to continue powering that device. The more power the device is using, the longer it will take for your battery to charge fully.
  • Battery chargers aren't always outputting their max charge rate. Many battery chargers employ charging algorithms that adjust the charging current and voltage based on how charged the battery is. For example, some battery chargers slow the charge rate down drastically once the battery reaches around 70-80% charged. These charging algorithms vary based on charger and battery type.
  • Batteries lose capacity as they age. An older battery will have less capacity than an identical new battery. Your 100Ah LiFePO4 battery may have only have around 85Ah capacity after 1000 cycles. And the rates at which batteries age depend on a number of factors.
  • Lithium batteries have a Battery Management System (BMS). Besides consuming a modest amount of power, the BMS can adjust the charging current to protect the battery and optimize its lifespan. iPhones have a feature called Optimized Battery Charging that delays charging the phone's battery past 80% until you need to use it.
  • Lead acid battery chargers usually have a timed absorption stage. After being charged to around 70-80%, many lead acid battery chargers (and solar charge controllers) enter a timed "absorption" stage for the remainder of the charge cycle that is necessary for the health of the battery. It's usually a fixed 2-3 hours, regardless of how big your battery is, or how fast your charger.

In short, batteries are wildly complex, and accurately calculating battery charge time is no easy task. It goes without saying that any charge time you calculate using the above formulas -- or our battery charge time calculator -- should be viewed as an estimate.

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Alex Beale
Alex Beale
Hi, I'm Alex. I’m a DIY solar power enthusiast on a journey to learn how to solar power anything. Footprint Hero is where I’m sharing what I learn – as well as the (many) mistakes I’m making along the way.