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:

Battery charge time is simply how long it would take for a battery to be fully charged after getting fully discharged.

When not fully discharged, battery charge time is the time it will take a rechargeable battery to get a full charge from its current state of charge.

How to Use Our Battery Charge Time Calculator

  • Enter your battery capacity in the corresponding field.
  • From the options (Ah, mAh, Wh, and kWh) by the right of the battery capacity field, choose the appropriate battery capacity unit. Assuming your battery’s capacity is 15 Ah, you’ll choose Ah from the options.
  • Enter the battery charger current or wattage in the appropriate field.
  • Choose the appropriate charge current unit from the options to the right of the charge current input field. When working with charge current, you can choose either mA or A, depending on the charging current unit stated on the charger. But when working with charge power/wattage, choose W.
  • Pick your battery type from the list: Different rechargeable batteries have different electrochemistry, and charge/discharge cycles, amongst other features. These features affect the way the battery charges and, consequently, the charge time. For instance, lead acid batteries have a 50% charge/discharge cycle. So, when estimating their charge time with their battery capacity, we’ll only consider 50% of the capacity. Instances like this are why it is vital to specify the battery type.
  • Specify the battery’s state of charge: This is optional (but if left blank, the battery charge time calculator will assume the battery is fully discharged – at 0%). Since charge time varies with the state of charge, specifying the state of charge is pretty useful when you want to know long it will take to charge a battery from its current state. Basically, if your battery’s state of charge is 50%, it will have a longer charge time than when it is at 80%.
  • Finally, click on the “Calculate” button to get your result from the battery charge time calculator.

How to Calculate Charging Time Using Battery Capacity and Battery Charging Current

We can calculate battery charging time using battery capacity and charge current. All we’ll do is divide battery capacity by the battery charger current:

charge time = battery capacity ÷ charger current

When battery capacity is in watts-hour (Wh), we’ll divide it by charger power/wattage:

charge time (h) = battery capacity (Wh) ÷ charger power/wattage (W)

When the capacity of the battery pack is in amp-hours (Ah), we’ll divide by charger current in amps (A):

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

When the capacity is in milliamp-hours (mAh), we’ll divide by charger current in milliamps (mA):

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

This method will give us a rough estimate of how long it will take to charge rechargeable batteries; it is not the most accurate.

In real life, various events happen while charging a battery, and those events affect battery charging time. This formula does not factor in those events, hence the low accuracy.

Still, dividing battery capacity by charger current is a quick and easy way to tell charge time.

Example 1:

A 50 Ah car battery is connected to a charger whose charge rate current is 6 amps. If this rechargeable battery is completely depleted, how long will it take the battery to get a full charge?

The battery will get a full charge in:

50/6 = 8.3 hours

Example 2:

A 720 Wh, 12 V car battery is connected to a charger whose charge rate current is 6 amps. If the battery is at 0%, how long will it take before it gets a full charge?

In this example, the battery capacity is in Wh while the charge rate current is in amps (A). So, we cannot use both units together.

However, since we know the battery voltage, we can convert Wh to Ah. After that, we’ll divide Ah by the charge rate current to get the battery charging time.

Converting 720 Wh to Ah:

Ah = 720/12 = 60 Ah

Estimating battery charging time using battery Ah and charge rate current:

the battery charging time = 60/6 = 10 hours

How to Calculate Charging Time Using Battery Capacity, Battery Charging Current, and Charge Efficiency

If we want a more accurate estimation of a battery’s charge time, we can include charge efficiency in the previous formula (battery capacity and charging current formula):

battery charge time = battery capacity ÷ (charge efficiency x charge current)

As with before, when the battery size is in Ah, the charge current should be in A. Then when it is in mAh, the charge current should be in mA. However, if it is in Wh, we’ll use charge wattage (W).

Charge efficiency is largely dependent on battery type. For one, lithium-ion batteries have the highest charge efficiencies (around 95%) with lead acid batteries coming after them. So, when estimating battery charge time while considering charge efficiency loss, the battery type is important.

Example 1:

How long will it take a 100 Ah lithium-ion battery at 0% to fully charge when it is connected to a charger whose charge rate is 7 A? [Assume charging efficiency of the battery is 93%]

Since we know the battery capacity, charging current, and charge efficiency, this example is straightforward. The battery will get a full charge in:

= 100 ÷ (93% x 7) = 100 ÷ (0.93 x 7) = 100 ÷ 6.51 = 15.4 hours

If you try this example out with our battery charge time calculator, you should get a similar result.

Example 2:

A 1000 Wh lead-acid battery has an average efficiency loss of 18%. If it was at 0% when we started charging it with a 110 W charger, what would be this battery’s charge time?

Since the battery’s efficiency loss is 12%, its charge efficiency would be = 100 – 18 = 82%

Now that we know the battery’s charge efficiency, we can calculate charge time:

= 1000 ÷ (82% x 110) = 1000 ÷ (0.82 x 110) = 1000 ÷ 90.2 = 11.1 hours

Why Does Charge Inefficiency Make the Estimation a Bit More Accurate?

The charging and discharging process of any battery experiences some efficiency loss. In other words, batteries neither charge nor discharge with 100% efficiency. There will always be some power loss due to heat and other inefficient processes.

By factoring efficiency loss into our calculation, we’ll calculate charge time with a value closer to the actual charge current.

How to Calculate Charging Time Using Battery Capacity, Depth of Charge, Charge Efficiency, and Battery Charging Current

For even higher accuracy, we can incorporate depth of charge (DoD) into the formula from the previous section.

battery charge time = (DoD x battery capacity) ÷ (charge efficiency x charging current)

DoD is pretty much the inverse of state of charge (SoC). Its a value that tells us the percentage of the total battery capacity that has been discharged.

DoD = 100 – SoC

Going by the formula above, if your battery level is at 30%, your DoD would be 70%. Then if the battery is at 65%, the DoD would be 35%.

Example 1:

A 300 Ah lithium battery at 40% is being charged with a charging current of 10 A. How long will it take to fully charge the battery? [Assume the charging efficiency of the battery is 94%]

We know the battery’s SoC, 40%. So, we can calculate its DoD:

Battery’s DoD = 100 – 40 = 60%

Now that we know the DoD, we can calculate the charging time:

Charging time for the battery at 40% = (60% x 300) ÷ (94% x 10)

= (0.6 x 300) ÷ (0.94 x 10) = 180 ÷ 9.4 = 19.15 hours

Why Does Depth of Discharge Make the Estimation Even More Accurate?

DoD makes the estimation even more accurate because it tells us how much of the battery the charger is trying to replenish.

In the previous methods/formulas, the assumption was that the battery had been completely depleted. So, the charger was trying to replenish the whole battery capacity. But by adding DoD to the formula, we know exactly how much of the battery capacity the charger will replenish. Hence, the higher accuracy.

Factors that Affect the Battery Charging Process

These are some of the factors that affect how fast batteries charge:

  • Charger’s Charging Algorithm
  • Battery Charging Algorithm
  • Battery Load
  • Drop in Battery Capacity

Charger’s Charging Algorithm

Some battery chargers have special charging algorithms that see their charge rate current change in response to the battery’s charge level. A common instance of this is when the charging current of some chargers drops when the battery gets to 80%.

Battery’s Charging Algorithm

Certain battery types have charging algorithms that alter the charging rate once they get to a certain level of charge.

Lead-acid batteries, for instance, go into the absorption phase after the bulk phase gets them to around 80%. In the absorption phase, the battery’s input current is reduced, slowing the charging rate. The absorption phase of lead-acid batteries can last for around 2 hours to 2 hours 30 minutes.

Battery Load

If there’s a load on a battery while it’s charging, some of the charging currents will go into powering that load. As such, the battery’s input current would be lower, and the charging time would be longer.

Drop in Battery Capacity

As batteries age, they lose ions, and their electrodes degrade. Those two phenomena and many others cause batteries to experience a progressive drop in capacity. Of course, with a lower capacity, aging batteries charge faster since they only have to replenish a fraction of the battery’s original capacity.

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