Calculating Battery Life: Is 0.05 Hours Correct?

In summary, to determine the approximate battery life for an application, you can use the battery's rating of 12V 7Ah and the power of the application (in this case, 1700W). Using the formula P=V^2/R, you can calculate the current requirement and then use the formula time = energy/power to calculate the battery life. It is important to also consider the internal resistance of the battery and its potential safety hazards when drawing high currents. Alternatively, you can directly use the energy balance method to calculate the battery life.
  • #1
JD88
110
0
I am trying to determine how long a particular battery will last for my application.

These numbers are just for an example.

Battery stats:
12 V 7 Ah

Application
1700 W

Power=Current*Voltage
1700W = Current * 12V
Current=141.7A

Time = 7Ah / 141.7A
Time = 0.05 hours

Is this correct, or are there other factors that I must consider?
 
Engineering news on Phys.org
  • #2
JD88 said:
Time = 7Ah / 141.7A
Time = 0.05 hours

Is this correct, or are there other factors that I must consider?
That the wires would melt and the battery would explode if you could pull 140A from it.

Batteries have an internal resistance due to the chemistry - think of it as a small resistor in series with the battery.
As you draw a current, the resistance creates a voltage difference and so the voltage coming out of the battery drops, the current flowing through the resistance also generates heat in the battery.
For a lead acid battery this resistance is very low so you can get a very large current from a lead acid battery - but this doesn't mean that it is safe.
 
  • #3
I don't care about the numbers from my example. Just the procedure. Is that how I would go about determining how long the battery would last?
 
  • #4
The battery rating, 12V 7Ah, means that 7 amps can be drawn from the battery at 12 volts for an hour, this is its energy content.

For an appliance rated at 1700W, you need to calculate its resistance at its rated voltage, ie use P=V^2/R. Calculate the current requirement. Now 7 amps for 1 hr = x amps for y hrs!
Or
Directly use energy balance.
 
Last edited:
  • #5
Sorry your method has confused me.

An example. A lithium ion battery rated at 4.4Ah powers a 20W bulb at 14.4V

20W/14.4V = 1.39A 4.4Ah/1.39A = 3.17hrs

These are the specifications of my mountain bike lights. They do run for approximatley 3.2 hours before the world goes dark and I am walking back to my car blind. Its dark in Thetford Forest when moon is new.

No resistance needed to work this out. Also if you follow your resistance method the current works out to be 141.7A. The same as JD88's answer. So your method is just a more complicated way of getting to same answer!

JD88 you are right!
 
  • #6
The amp hours is the total energy available in the battery.

7 Ahr *(3600s/hr) = 25200 Asec

power = energy / time

time = energy/power

time = 25200 Asec / 1700 W

time = 14 Sec
 

1. How do you calculate battery life?

Battery life can be calculated by dividing the battery's capacity (in ampere-hours) by the current draw of the device (in amperes).

2. Is 0.05 hours a correct battery life calculation?

It depends on the capacity of the battery and the current draw of the device. 0.05 hours is equivalent to 3 minutes, so it may be a correct calculation for small devices with low current draw.

3. What factors affect battery life?

The capacity of the battery, the current draw of the device, and the type of battery chemistry all affect battery life. Other factors include temperature, age of the battery, and usage patterns.

4. How accurate are battery life calculations?

Battery life calculations can provide a rough estimate, but they may not be completely accurate due to factors such as variations in battery capacity and changes in current draw over time. It's best to use them as a guide and monitor actual battery usage for more accuracy.

5. How can I prolong battery life?

To prolong battery life, you can reduce the current draw of your device by turning off unnecessary features or using power-saving modes. You can also use batteries with higher capacities or switch to more energy-efficient battery chemistries.

Similar threads

  • Electrical Engineering
Replies
26
Views
2K
  • Electrical Engineering
Replies
14
Views
2K
  • Electrical Engineering
Replies
15
Views
1K
Replies
7
Views
2K
  • General Engineering
Replies
17
Views
2K
Replies
8
Views
934
  • Electrical Engineering
Replies
2
Views
1K
  • Other Physics Topics
Replies
10
Views
2K
Replies
13
Views
1K
  • Mechanical Engineering
2
Replies
39
Views
4K
Back
Top