Section 9: Battery Management

The Big Picture: A Fuel Tank That Slowly Shrinks

A laptop battery is like a fuel tank, but one that gets a little smaller every time you use it. Windows cannot make the tank bigger, but it can read the gauge, tell you how much is left and how healthy the tank is, and avoid habits that wear it out faster.

9.1 Battery Reporting to the OS

What it is: A laptop battery is not just cells. It is a pack with a small computer-like board, and that board tells Windows about its state.

Figure 34.2 — The inside of a laptop battery pack, on squared paper with a matchstick for scale. Look for the six purple cylindrical cells (printed "Li-ion" and carrying a safety warning) in two rows of three, the thin circuit board with a gold-pin connector along the bottom, and the orange flexible strips that join the cells. Photo: Lead holder, CC BY-SA 3.0, via Wikimedia Commons.

The data link lets the laptop's firmware and Windows read the battery status. In Device Manager (Chapter 14) you will see two items under Batteries: Microsoft ACPI-Compliant Control Method Battery (the battery, read through ACPI tables from Section 1) and Microsoft AC Adapter.

What Windows can learn:

On desktop PCs there is no main battery, so many of these do not apply.

9.2 Battery Health and Wear Awareness

What it is: All Li-ion batteries age. Each use and each hot day takes a little capacity away permanently.

Battery health  =  Full charge capacity  /  Design capacity

Example:  new pack designed for 50,000 mWh
          now holds only 40,000 mWh when full
          health = 40,000 / 50,000 = 80%

A health of 100% means "like new". A battery under about 80% is often called worn: you still use it, but it empties noticeably faster.

What wears a battery faster (and how to slow it):

Charge limits. Many PC makers (not Windows itself) add a feature in their own app or in the firmware setup (Chapter 5) that stops charging at, say, 80%. Names differ, such as conservation mode or battery care. Windows may also show a "battery health" or "smart charging" style option on some PCs (hedge: names and availability differ by maker and version).

9.3 Seeing Health With powercfg /batteryreport

What it is: powercfg /batteryreport writes an HTML report about the battery. It is read-only for your system settings; it only creates a file.

powercfg /batteryreport

It prints where it saved the file, normally battery-report.html in the folder where you ran the command. Open it in a browser. Look for:

Hedge: it works on laptops and tablets with a battery. On a desktop it says no battery was found. Some PCs report some fields as blank.

9.4 Battery Saver and Background Drain

Battery saver is a Windows feature in Settings → System → Power & battery (Chapter 16). When switched on (manually, or automatically at a chosen level such as 20%), Windows reduces screen brightness and limits some background activity, so the charge lasts longer.

Windows also shows a battery usage by app view in Settings, so you can see which apps used most.

9.5 A Different Battery: The CMOS Coin Cell

Do not mix up the main battery with the small coin cell on the motherboard. They have different jobs.

Figure 34.3 — The small CR2032 coin battery on a motherboard, shown in Chapter 4. It only keeps the clock and firmware settings alive when the PC has no other power. Photo: Arnulf zu Linden, CC BY-SA 3.0, cropped, via Wikimedia Commons.

9.6 Safety: A Swollen Battery

Safety: If a laptop's battery swells, so the case bulges, the trackpad lifts or the lid will not close flat, stop using it. Do not charge it, puncture it or squeeze it. Unplug the PC, power it off and have the battery replaced by a repair shop. A damaged Li-ion battery can overheat or catch fire. The safety warning printed on the cells in Figure 34.2 is there for this reason.

Summary