Laptop Battery Testing
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APPENDIX GUIDE A-4
Understanding Laptop Batteries
The rechargeable batteries that power today's laptops are much more complex than the batteries typically found in other consumer electronic devices. They use sophisticated chemistry and control circuitry to maximize their runtime and longevity across years of use and potentially hundreds of charge/discharge cycles. Understanding their behavior - especially ones that have seen significant use - can be difficult but is very important for our Laptop Build processes. This guide is intended to help you measure and interpret a battery's vital signs and determine how much useful capacity remains.
Checking the State of a Battery
☐ To check and monitor the state of the battery, open a terminal and type:
watch -n3 -d cat /proc/acpi/battery/*/*
- The window should now display many lines of information about the battery that refresh every 3 seconds or so. The most important lines to look for are shown below with values taken from a healthy battery as an example. Other lines have been intentionally left out of this example and replaced with a -, but will appear in real-world use. Real-world values may be significantly different, use different units, or be missing entirely.
- design capacity: 5200 mAh last full capacity: 4331 mAh - design voltage: 11100 mV - charging state: charging present rate: 1549 mA remaining capacity: 3679 mAh present voltage: 12358 mV
Interpreting Battery State
In this example we can see from the difference between the
last full capacity and
design capacity that the battery has seen some use but still has about 83% of its original capacity. If the
last full capacity is less than 30% of the
design capacity, the battery may end up being too worn out to function correctly.
We can also see from the
charging state and
present rate that it is charging at a healthy rate, and we would expect the value for
remaining capacity to increase steadily until it is at or near the value for
last full capacity. A fully discharged battery will typically charge at several hundred mA at first, then faster (1000 - 3500 mA), then slower and slower as it approaches its
last full capacity; when it is fully charged the
charging state should change to
charged and the
present rate should be at about
You may also have noticed that the
present voltage is actually somewhat higher than the
design voltage; this is normal for a healthy battery. When the battery is discharging the value will usually drop initially then settle in very near or a bit below its
design voltage. If the
present voltage continues to drop well below the
design voltage as it discharges, it may suddenly shut off entirely even when the
remaining capacity is high; this type of failure usually indicates a worn out or damaged battery.
If a battery has not been charged or used for a long time (like 6 months or more), its present voltage may be significantly below its design voltage, and won't charge normally until it "pre-charges" its voltage back to healthy levels. When this is the case, the
charging statewill read "
charging" and the
present ratewill be much lower than usual (e.g. <400 mA) or even read 0 mA, but the
present voltagewill slowly increase. When the
present voltagegets within about 1500 mV of its
design voltage, you should notice the
present rateincrease significantly and the
remaining capacitybegin to increase as well. The battery should then more or less charge normally.
Battery Charging and Testing
Confirm the Battery is Fully Charged
☐ Set the laptop on the testing shelf and make sure it is plugged in to a power outlet.
☐ To monitor the state of the battery, open a terminal and type:
watch -n3 -d cat /proc/acpi/battery/*/*
- If the battery seems like it is healthy and charging normally, check the Start box next to Battery Charge on the Keeper Label and let it finish charging.
Test Battery Life
☐ When the battery is fully charged, open a terminal and run:
☐ Open a second Terminal (hint: press Super + T to quickly open a new terminal window) and run:
while : ; do stress-test ; sleep 300 ; done
☐ Disconnect the AC adapter from the power outlet, but keep it with the laptop for later.
☐ Get a Battery Testing label from the wall next to the testing shelf and stick it to the palm rest of the laptop.
How it works
The laptop will run 3 minutes of
stress-test, then 5 minutes at idle, in a loop until it runs out of power and shuts off. For every minute the laptop is on during battery testing, a line of text is logged in a file called
bat_mon. When battery test is run again, the script checks for the
bat_monfile and counts up the number of lines of text in the file, then outputs the number as the number of minutes the battery lasted.