How To Calculate Battery Run Time - DNK Power

How To Calculate Battery Run Time

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

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 lithium battery range 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 18650 2500mAh(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 inr18650 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 INR21700 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 1800mAh 3.7v 18650 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 = 2.7027A = 2702.7 mA In theory that’s: 1800mAh ÷ 2702.7 mA = 0.666 h = 40 min In reality that’s: 1800mAh ÷ 2702.7 mA*0.9 = 0.599h = 36 min

Quick Notes: 1A=1000mA (mA is current, mAh is Capacity)

Or you can use 3.7V*1.8Ah(1800mAh)*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