Aaruni's Blog v 3.0

Claiming Things

In November 2020, I was able to get my hands on a System76 Lemur Pro (lemp9). The thin and light laptop boasts a phenomenal battery life of 10+ hours of light typing work (at the expense of underwhelming performance, even when plugged into the wall). Recently, in a private group, when discussing laptop battery life, I pointed out that you don’t necessarily need ARM silicon or macOS to get more than a couple hours of battery, and that my laptop with intel and Linux can still deliver around 9 hours of battery life when doing light work. My claims were ridiculed and I was asked to do a “real test” of battery life to substantiate my claims.

Specification

My laptop, now almost 5 years old, doesn’t have the pristine battery it once did. But still, for the sake of hard facts, I agreed to the test anyway. Before I go on to describe the test and result, I should mention (and learn to look up) the actual battery specifications. Fortunately for me, Linux makes this easy.

Apparently, the battery I have was designed to have a capacity of almost 74 Wh, but after almost half a decade of abuse, and countless discharge cycles, current capacity is only 55 Wh, a reduction of more than 25%. Here’s the command to check battery statistics, along with its full output.

$ upower -i /org/freedesktop/UPower/devices/battery_BAT0
  native-path:          BAT0
  vendor:               Notebook
  model:                BAT
  serial:               0001
  power supply:         yes
  updated:              Monday 26 May 2025 05:39:40 AM (24 seconds ago)
  has history:          yes
  has statistics:       yes
  battery
    present:             yes
    rechargeable:        yes
    state:               charging
    warning-level:       none
    energy:              25.4639 Wh
    energy-empty:        0 Wh
    energy-full:         55.055 Wh
    energy-full-design:  73.92 Wh
    energy-rate:         19.3347 W
    voltage:             8.271 V
    charge-cycles:       N/A
    time to full:        1.5 hours
    percentage:          46%
    capacity:            74.4792%
    technology:          lithium-ion
    charge-start-threshold:        75%
    charge-end-threshold:          80%
    charge-threshold-supported:    yes
    icon-name:          'battery-good-charging-symbolic'
  History (charge):
    1748230780	46.000	charging
  History (rate):
    1748230780	19.335	charging
    1748230750	19.350	charging
    1748230720	19.335	charging

Testing Methodology

My testing methodology was straightforward, even if a bit primitive. I let the laptop charge up as much as my patience allowed, and then let her rip. As a workload, I put on a 1080p HEVC stream over the wifi, to simulate an updating screen, along with video decode and network access. To record battery levels, I wrote a loop to log the current battery level and the current time to a file every 30 seconds.

Results

We started with 95% battery, at 11.40. We reached 90% battery at 11.59 (5% in 20 minutes). 75% battery was remaining at 13.02 (15% in 62 minutes). Battery levels reached 50% at 14.44 (25% in 102 minutes). Battery fell to 20% at 16.33 (30% in 109 minutes). This is the point where the laptop alerts you of a low battery. The last recorded entry was 0.5% at 17.52 (19.5% in 89 minutes). The battery gave up less than 30 seconds later.

Overall, we used 95% battery (52.3 Wh) in 6 hours 12 minutes, giving us an average consumption of 8.43 Watts. If we had a full battery, at pristine capacity, the same test would have given us (assumming the same average consumption) almost 9 hours of battery life! Here’s a graph of the battery runtime over the course of this test. Its interesting to note that from around full, until 20%, the graph is more or less a straight line, and then it suddenly plummets. I don’t know enough about battery technology to interpret this behaviour, but it certainly looks interesting.