How To Calculate Battery Run Time

There are just too many questions you will wonder when designing your device with a battery inside it.

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The boss just want a cheap and small battery with it, but with no further information on how long does the end customer wants, how small can it be.

in this article, we will show you:

1 How to Calculate the Run Time of a Specific Battery?
2 How to Calculate Battery Capacity ?
3 Battery Capacity Calculator (Instant Calculation Tool)
4 Battery Run Time Calculator
5 How to convert Watts to Amps or Amps to Watts or Volts to Watts

Ready for your battery design?

Let&#;s go.

In the ideal/theoretical case, the time would be Time(H) = Capacity(Ah)/Current(A).

If the capacity is given in amp-hours and current in amps, time will be in hours (charging or discharging).

Feel Confused ?

So how to calculate how long a battery will last?

Throw away how long will a battery last calculator, and let&#;s see an actual case, 12V 10 Ah lithium battery delivering 1A, would last 10 hours. Or if delivering 10A, it would last for only 1 hour, or if delivering 5A, it would last only for 2 hours.

In other words, you can have &#;any time&#; as long as when you multiply it by the current, you get 10Ah (the battery capacity).

It is that simple.

so no more confusion on how to calculate battery life.

For a mAh(2.5Ah) battery with a device that draws 500mA(0.5A) you have:

2.5Ah/0.5A=5 Hours

For a 36V Li-ion Battery with capacity of 24Ah rechargeable battery delivering 2A current, then you can constantly use your battery for:

24Ah/2A=12 Hours

One more example: rechargeable 24V 10Ah lithium battery, it delivery 10A current, then you can constantly use 1 hour (10Ah/10A=1h)

for a 24V 5Ah lithium battery with 2.5A output, its working time is about 2 hours&#;5Ah/2.5A=2h)

Please take note that most batteries, especially those with circuits, will not work down to 0 Volts as a power supply (if it goes to zero, it will have shorter battery life, or even become dead battery if not charged in time), that&#;s to say,your circuit will stop working at a set voltage before the battery is fully drained.

see below dischaging chart

it will not go to zero(totally empty)

We, therefore, will need to times 0.8-0.9 for the calculation: just take samsung inr 25r as an example

that&#;s 2.5Ah/0.5A*0.9=4.5 Hours

What if you know Watts only, you will notice that every device use watt to determine it&#;s main specifications.

5 Watt bulb,

20W Laptop,

100W Motor,

200W Solar Street Light

Just name a few.

In Theory, that&#;s:

Discharging Time=Battery Capacity*Battery Volt/Device Watt.

Say, SAMSUNG INR 50E 5AH*3.7V/10 Watts = 1.85 hrs

With 90% Power efficiency for Li-ion/LiPo batteries. Then

Discharging Time=Battery Capacity * Battery Volt*0.9 / Device Watt

5Ah*3.7V*0.9/10W = 1.66 hours

Let&#;s explain with more examples:

for a mAh 3.7v battery to power a 3.7V 10W digital device, how to calculate the running time?

for 3.7V 10W device&#;working current would be 10÷3.7 = 2.A = .7 mA

In theory that&#;s: mAh ÷ .7 mA = 0.666 h = 40 min

In reality that&#;s: mAh ÷ .7 mA*0.9 = 0.599h = 36 min

Quick Notes: 1A=mA (mA is current, mAh is Capacity)

Or you can use 3.7V*1.8Ah(mAh)*0.9/10W=0.599h=36min

Another example: 12V 60Ah li-ion battery pack to power 220V 100W light

Working time: 12V*60Ah*0.9/100W=6.48 H

How to Calculate Battery Capacity ?

Things would be quite complicated as the battery would be in different shape(Curved shape, Round,Rectangular etc ) and different discharging current(5C, 30C, or even up to 100C) or even temperature requirement(like low temperature battery).

feel frustrated?

We here have 2 ways for you to get the battery capaicty.

let&#;s start from simple ways

1 Get Battery Capacity Based on Size

if size are given or can be checked by a ruler, we can then get the battery capacity. The size of Lithium polymer battery has a great impact on its capacity, that&#;s the thickness, width, and length of a battery. The material and Production technology will have an effect on the lipo cell capacity.

The actual capacity will be very complex, but luckily we have a simple and fast calculate formula.

Capacity= Thickness*Width*Length*K

K= mah/mm³, which is a parameter that ranges from 0.07-0.12. for a general calculation, we will set it to 0.1

take battery cell for example (T=10mm, W=34mm, L=50mm)

that&#;s 10*34*50*0.1=, in reality, it&#;s about mAh or more.

what about (also called LP)?

that&#;s 6*34*50*0.1= in actual it will be about mAh

2 Get Battery Capacity by Energy Density

The lithium battery usually comes with a pouch or cylindrical form.

in cylindrical form there is capacity listed for each size:

like for lithium battery: mAh, mAh,mAh, mAh, mAh

for lifepo4 battery:mAh, mAh,mAh

PS: quick notes: if space allows, cylindrical battery cells will always be your first consideration on the table. They are on the shelf and therefore has short lead time and small MOQ.

so we can quickly get battery capacity(max capacity) based on battery energy density. for lithium battery it would be 100-265 Wh/kg or 250-670 Wh/L.

Let&#;s say what would be the battery capacity at 11.1V 7kgs

it would be 7kgs*265 wh/kgs=Wh

so that&#;s Wh/ 11.1V=167Ah

Impotant note: above are max capacity as there is BMS and wiring cables included for a battery pack.

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How long will your battery last? find out with our easy-to-use battery runtime calculator.

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Calculator Assumptions

• This calculator will consider the efficiency of an inverter (90%) and the efficiency of the battery discharge (lead acid: 85%, Lithium: 95%).

Limitations Of This Calculator

• Please note that the calculator doesn&#;t include Peukert&#;s law, temperature, and battery age in its calculations, which can affect the battery&#;s discharge time. But it can give you a close estimated runtime.

How To Use Our Battery Runtime Calculator?

1. Enter battery capacity in amp-hours (Ah): If the battery capacity is mentioned in watt-hours (Wh), Divide the watt-hours by battery voltage (V) to find out the battery capacity in Ah.

2. Enter your battery voltage (V): Do you have a 12v, 24, or 48v battery? For a 12v battery, ENTER 12.

3. Select your battery type: For lead acid, sealed, flooded, AGM, and Gel batteries select &#;Lead-acid&#; and for LiFePO4, LiPo, and Li-ion battery types select &#;Lithium&#;.

4. Enter your battery&#;s state of charge (SoC): SoC of a battery refers to the amount of charge it has relative to its total capacity. A fully charged battery will have 100% SoC.

5. Enter your battery&#;s recommended depth of discharge (DoD) limit: Battery depth of discharge (DoD) measures the used capacity of your battery from its total capacity.

Lead-acid, AGM, sealed, flooded, and Gel batteries should not be discharged below 50%, while only lithium (LiFePO4, LiPo, and Li-ion) batteries can be safely depleted to 100%. Ask your manufacturer or have a look on at your battery&#;s specs sheet for a more accurate value.

6. Is your load connected through an inverter? Select yes, if you&#;re using an inverter to run the AC appliances. Or select no, if the appliance is directly connected to the battery without an inverter (usually not recommended).

7. Enter total output load in watts: If your appliance has an output load mentioned in amps, convert it into watts by multiplying the amps by the given volts of the appliance.

2 Ways To Calculate Battery Runtime

Ready for calculation? Let&#;s dive in! I&#;ll share 2 methods to estimate battery life from basic (least accurate) to advanced (most accurate).

Formula 1

Formula: Battery capacity in Ah ÷ Output amps (A)

Or, Battery runtime = Battery capacity in watt-hours (Wh) ÷ Total Output in watts

Accuracy: Lowest

Dividing the battery capacity (in amp-hours &#; Ah, or milliamp-hours &#; mAh) by the output load (in amps &#; A, or milliamps &#; mAh) is the least accurate way to calculate the battery runtime.

Because it doesn&#;t take into account for battery&#;s discharge efficiency rate, recommended depth of discharge, and state of charge.

Mostly, the battery capacities are mentioned in amp-hours (Ah) but our appliances are in watts. Which makes it confusing in the first place. You can convert the battery capacity in watt-hours or the appliance input capacity into watt-hours to make it work.

Battery capacity in watt-hours = Battery Ah × Battery voltage

AdvertisementsOutput load in load in amps = Load in watts ÷ volts

Example

Let&#;s say you have:

• Battery capacity: 50Ah
• Output load: 10A

To calculate 50ah battery lifetime using this formula, divide 50ah by 10a.50ah ÷ 10a = 5 hrs

According to this formula, a 50-ah battery will run a 10-amp load for 5 hours.

Formula 2

Formula: Battery runtime = (Battery Ah × Battery volts × discharge efficiency × DoD limit × SoC × inverter efficiency) ÷ load

Accuracy: Highest

This formula takes into account for battery&#;s discharge efficiency rate, recommended depth of discharge, and state of charge.

Based on directscience.com data:

• Lead-acid batteries discharge efficiency &#; 80 &#; 85%
• Lithium-ion batteries discharge efficiency &#; 90 &#; 95%

Example

Let&#;s continue with the previous example and find out the most accurate runtime estimate.

Let&#;s say you have:

• Battery capacity: 50Ah
• Battery voltage: 12V
• Battery type: Lead acid
• Battery SoC: 100% (fully charged)
• DoD limit: 50%
• Running load through an inverter? Yes (inverter efficiency &#; 90%)
• Output load: 120 watts (10 amps @ 12v)

Now let&#;s put this info into our 2nd formula. (50ah × 12v × 85% × 50% × 100% × 90%) ÷ 120 watt (229.5) ÷ 120 watt = 1.9 hours

Turns out, in actuality a 50ah battery will run a 10A load for about 1.9 hours.

Why None Of These Methods Guarantee 100% Accuracy?

Battery runtime formulas aren&#;t fully accurate due to the complexity of battery discharge, as there are other factors that can impact discharge time. Which are&#;

1. Batteries Discharge Unevenly

If you discharge a battery too quickly, it won&#;t provide as much energy as the label claims, known as Peukert&#;s law. This is because more energy turns into heat instead of powering devices when discharged quickly.

The effect of Peukert&#;s law on Lead-acid vs. Lithium: Lead acid batteries lose energy faster when discharged quickly, while lithium batteries can be discharged at up to 50% of their capacity without losing power.

Here&#;s an example of how discharge time affects the usable capacity of a 100ah lead acid battery.

2. Battery Life Cycles Matter

Batteries can only be charged and discharged for a limited number of times, which is called the life cycle.

Lead-acid batteries last for a few hundred cycles if they are maintained properly. Lithium batteries can last for thousands of cycles. But as batteries are used and charged more, they hold less charge capacity.

After about 500 cycles, a lead-acid battery will lose about 20% of its capacity, while a lithium battery will 20% of its capacity after about cycles. Check your battery&#;s data sheet for more accurate numbers.

3. Effect Of Temperature On Batteries Performance

Temperature affects how well your battery works. Usually, batteries work best when it&#;s between 68°F and 77°F (20°C to 25°C).

A higher or lower temperature (than what&#;s recommended) will affect the battery&#;s performance. A rule of thumb is &#;an increase or decrease in temperature to 77ºF or 25º C can reduce battery performance by 50%&#;.

To get the most out of your battery, keep it at the right temperature by using a cooling system or putting it in a spot where it won&#;t get too hot or too cold.

How Long Does A 12v Battery Last?

Here&#;s a chart on how long 12v different amp-hour (Ah) batteries will last running a 10-watt load.

Table 1: how long will 12v battery last?

Summary

• A 12v battery will last anywhere between 5-20 hours while running a load.

How Long Will A 24v Battery Last?

Here&#;s a chart on how long a 24v different capacity lead acid and lithium (LiFePO4) battery will last running a 100 watts AC load.

Table 2: how long will 24v battery last?

SummaryAdvertisements

• 24v lead-acid battery will last between 10 to 30 hours while running a 100-watt AC load.
• 24v Lithium (LiFePO4) battery will last between 20 to 80 hours while running a 100-watt AC load.

How Long Will A 48V Battery Last?

Here&#;s a chart on how long will 48v different amp-hours (Ah) battery will last on a 500-watt load.

Table 3: how long will a 48v battery last?

SummaryAdvertisements

• 48v lead acid battery will last anywhere between 4 hours to 22 hours while running a 500-watt load.
• 48v lithium battery will last anywhere between 8 hours to 50 hours while running a 500-watt load.

How Long 70ah Battery Last?

Table 4: how long will 70ah battery last?

SummaryAdvertisements

• 70ah battery will last anywhere between 15 hours to 15 minutes running different appliances.

Full article: How Long Will 70Ah Battery Last? (In Real World Conditions)

How Long Will 110ah Battery Last?

Here are charts on how long will a 12v 110ah lead acid and lithium battery will last on load.

12v 110ah Lead Acid BatteryAdvertisements

Table 5: how long will 110ah lead acid battery last?

Summary

• 12v 110ah lead-acid battery with a 50% depth of discharge limit will last between 10 hours to 36 minutes.

12v 110ah Lithium BatteryAdvertisements

Table 6: how long will 110ah lithium battery last?

Summary

• 12v 110ah lithium (LiFePO4) battery with a 100% depth of discharge limit will last between 23 hours to 1 hour.

How Long Will 300ah Battery Last?

Table 7: how long will 300ah battery last?

SummaryAdvertisements

• A 12v 300ah lead acid battery will last anywhere between 28 hours to 20 minutes.

How Long Will 600ah Battery Last?

Here are charts on how long will a 12v 600ah lead acid and lithium battery will last on load.

12v 600ah Lead Acid Battery

Table 8: how long will 600ah lead acid battery last?

Summary

• A 12v 600ah lead acid battery will last anywhere between 50 hours to 50 minutes running different watt appliances.

12v 600ah Lithium Battery

Table 9: how long will 600ah lithium battery last?

Summary

• A 12v 600ah lithium battery will last anywhere between 120 hours to 2 hours running different-watt appliances.

How Long Will A 150ah Battery Last?

Here are charts on how long a 12v 150ah lead acid and lithium battery will last on load.

12v 150ah Lead Acid Battery

Table 10: how long will 150ah lead acid battery last?

Summary

• 12v 150ah lead acid battery will last anywhere between 14 hours to 1 hour running different watt appliances.

12v 150ah Lithium (LiFePO4) Battery

Table 11: how long will 150ah lithium battery last?

Summary

12v 150ah lithium battery will last anywhere between 30 to 2 hours running different watt appliances.

How Long Will 200ah Battery Last?

Table 12: how long will 200ah battery last?

Summary

• 12v 200ah lead acid battery will last anywhere between 15 hours to 40 minutes running different appliances.
• 12v 200ah lithium battery will last anywhere between 34 hours to 1 hour running different appliances.

Conclusion

Calculating battery runtime is a complex process, and there is no one-size-fits-all formula. The accuracy of the results depends on several factors, including battery age, temperature, load, and other external factors.

While our calculator and mentioned formulas can provide a rough estimate, they should not be solely relied upon. It is essential to understand the limitations of these methods and factor in additional variables to ensure that the battery runtime is accurately estimated for the intended use case.

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