Why my laptop’s fan keeps running with low CPU usage?

March 19, 2013 By | 2 Comments

Have you ever wondered why your laptop’s fan is running loudly despite of relatively low CPU usage (like 10 to 15%)? Or your laptop can operate on battery only for couple of hours with such low CPU usage?

If so, this posting answers why it happens and tell you what you can do to improve the situation.

CPU Usage number can be misleading

In the past, most laptops were equipped with single-core single-threaded CPUs and their structure was something like Figure 1. When program runs, most components in the system turned active as shown in Figure 2 (colored components are active). Once it is finished, the entire system returns to idle (power saving) state. So, the power consumed by the laptop was determined primarily by how long they become active. Hence, the relationship between CPU usage and power consumption was almost linear and looked like Figure 3.

Fig 1. Single CPU system configuration
1. Single CPU system configuration
Fig 2. 100% busy single CPU system
2. 100% busy single CPU system
Fig 3. CPU usage vs. power consumption
3. CPU usage vs. power consumption

However, if your laptop is new, it probably has more than one CPU core and each core may support multi threading (also known as Intel Hyper-Threading Technology). For example, dual-core multi-threading enabled CPU is configured as shown in Figure 4. Because all CPU cores/threads share the rest of the system, only one active CPU core/thread will keep most of the system active (in other words, high power consumption state) as shown in Figure 5. This means, if you are running single-threaded application and it keeps one CPU thread busy 100%, total CPU usage is still 25%, but most of the components in your laptop are active and consuming very high power already. By increasing workload, more cores/threads will become active, but the power consumption increases only gradually because additional cores/threads are relatively small part of the system (Figure 6~8). As a result, the relationship between CPU usage and power consumption is not linear and looks like Figure 9. Power consumption increases very rapidly until the first core/thread’s usage reaches 100%. In this example, it is when total CPU use reaches 25%.

If you compare Figure 2 and Figure 5, you will notice that they are very similar situations (one core or thread is running 100%). In fact, they consume similar amount of power. The only difference is, Figure 2 was called “100% CPU used”, and people accepted short battery life in the past. But, Figure 5 is now considered “only 25% CPU used” and people tend to think “CPU is not so busy, and it should not consume much power”.

Fig 4. Dual-core multi-threaded
4. Dual-core multi-threaded
CPU configuration
Fig 5. 25% busy dual-core
5. 25% busy dual-core
multi-threaded system
Fig 6. 50% busy dual-core
6. 50% busy dual-core
multi-threaded system
Fig 7. 75% busy dual-core
7. 75% busy dual-core
multi-threaded system
Fig 8. 100% busy dual-core
8. 100% busy dual-core
multi-threaded system
Fig 9. CPU usage vs. Power Consumption
9. CPU usage vs. Power Consumption

So, what can you do?

If most of the programs you run are (usually, most programs except for heavy games and image/video processing programs are single-threaded), you need to anticipate the CPU usage impact to the battery life can be aware that power consumption increases very rapidly as CPU usage number increases. Just a couple of percentage of CPU usage increase can easily decrease more than one hour of battery life. To maximize your laptop’s battery life, you need to carefully monitor CPU usage and make sure that unnecessary programs are not wasting CPU cycles.

You can monitor program activities with standard tools like Windows Task Manager, but they only tells you current snapshot. You will not know what happened while you are not watching at the tool window. Battery Life Maximizer shows the cumulative CPU time usage by each program. This will make it easy for you to find the program that is wasting CPU time on your laptop.


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Comments (2)

  1. Steve Cataldi

    Your explanation is way over my head. Right now my fan is running great,. When I have multiple programs it goes faster,. Anti-virus, Facebook etc. I heard to click on ctrl, shift, & esc. but it’s over my head. My lap top is clean. Thank you.

    • Luculent Systems

      Hello Steve,

      Thank you for your question. This article is targeted to technical audience and maybe too technical for ordinary people.
      If you are annoyed by the fan, please try the following:

      1. Start Task Manager by right click on the task bar and selecting “TaskManager” or “Start Task Manager”.
      2. If you are using Windows 10 and see “More details” at the bottom of the Task Manager window, click it.
      3. Select “Processes” tab
      4. Click “CPU” column in the header and sort the programs by their CPU usage
      5. Check if any program is using more than 10% CPU usage.

      Please make sure that the high CPU usage programs are doing “meaningful job” (what is meaningful is very subjective and only YOU can determine that). If not, your options are:

      • Manually close the program (not just minimize the window, but quit the program).
      • Update to the latest version if it is possible (it could be a bug of the program).
      • If the program is frequently wasting CPU for no good reason, consider switching to other similar products.

      Hope this helps.

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