How power consumption impacts user experience

14
Nov. 2018

Battery capacity is finite. As a result, the average power consumption determines how often the user has to replace batteries. Since manufacturers want battery replacement to be as infrequent as possible, they seek to lower power consumption. Lowering power consumption limits the functionality that a device could have.

Let’s run some numbers:

A high-end AA alkaline battery provides 2500 mAh under optimal load. This translates to 3.75 WH for a single battery. If a device has 8 AA batteries – which is probably the higher end of what’s acceptable – the total power available for the device is 30 WH.

If you assume the customer is not using a high-end battery or using a rechargeable battery, that total available capacity is lower. If you are interested in more detail, RightBattery.com has lots of data on various types of batteries.

If you are willing to use two D batteries, the total available power is about 40 WH. Using two D batteries would, of course, make the device larger and heavier.

Let’s analyze how much operating time 30 WH would give you for various average power consumption.

Average consumption (mWH) Expected operating time
days Months Years
1          1,250            41.7              3.5
2             625            20.8              1.7
3             417    13.9      1.2
5             250        8.3           0.7
10        125           4.2           0.3
20    63           2.1           0.2
50             25           0.8           0.1

You can see that even under optimal load conditions, and even when using 8 high-end AA batteries, the time between battery replacements becomes shorter than 6 months when the average power consumption is about 6 or 7 mW.

Of course not all devices will want 8 AA batteries. An Arlo Pro video camera, for instance, accepts four CR123A battery. This Lithium battery has the capacity of 4.5 WH at most, so a total of about 18 WH. High power consumption is related to video streaming. Making some assumptions about power consumption of the camera:

  • If the camera averages 10 seconds/day of video streaming, the battery would last for 4 months.
  • With an average of 6 minutes/day of video streaming, the battery would last about 2 weeks
  • With an average of 1 hour/day of video streaming, the battery would last about 1 day.

Would you be happy replacing 4 CR123A batteries in a camera every 2 weeks? I know I wouldn’t be!

Thus, power consumption determines the frequency of battery replacements, which in turn impacts user experience.

But what if a vendor wants to create a device with an average power consumption of 10 mW, or 50mW or 100mW?

In that case, manufactures have to use a power cord. But a power cord negatively impacts the user experience in a different way: need to be close to a power outlet, more difficult to move the device, sometimes need to hide the power cord.

This is where long-range wireless power comes in. The ability to deliver 300 mW or even 3 Watts at a distance can revolutionize power delivery and the user experience. With wireless power, you can get the convenience of wire-free devices but add features and usability that is similar to wired devices.

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