Elementary OS claiming my battery is seven percent yet mag-safe charger says it's full

Question

Just installed Elementary OS on my Macbook pro (2010) with the battery on full charge. I boot up elementary and suddenly I'm told it's on 7%. Granted, my battery is horrendously weak (1hr to reach 0% on macOS), but the charger is always connected and indicating that the battery is at 100%. When I remove the charger though it legitimately seems like the battery is 7% since it decreases every 3 mins (as seen on MacOS), and once it reached zero it requested that I remove the installation device and press enter (consequently shutting off the mac). Without plugging in the charger I boot the device (this time to macOS) and it tells me the battery is at 33%??? Multiple google searches got me nowhere. Anyone has any idea what's wrong?

Answer 1

There isn't anything "wrong", per se. This is just a matter of elementary OS using different math from Apple. Let's look at how these two OSes can show different numbers for the same hardware.

_{Note: As I'm not sure whether you're using a 13" or 15" MacBook Pro, I'll use the battery specifications for the 15" model from 2010.}

First, let's take a look at what sort of statistics your battery has to offer with upower:

upower -i `upower -e | grep 'BAT'`

This will give you something like the following:

native-path:          BAT0
  vendor:               SMP
  model:                45N1149
  serial:               3126
  power supply:         yes
  updated:              Thu Aug  5 04:14:40 2021 (106 seconds ago)
  has history:          yes
  has statistics:       yes
  battery
    present:             yes
    rechargeable:        yes
    state:               fully-charged
    warning-level:       none
    energy:              43.93 Wh
    energy-empty:        0 Wh
    energy-full:         43.96 Wh
    energy-full-design:  56.16 Wh
    energy-rate:         38.691 W
    voltage:             12.197 V
    percentage:          99%
    capacity:            78.2764%
    technology:          lithium-ion
    icon-name:          'battery-full-charged-symbolic'

_{Note: This battery output is from a Lenovo Thinkpad W541. Yours may look different.}

When macOS is determining your battery life, it looks at the energy and energy-full values to determine how much life remains, while elementary OS (and many other Linux derivatives) look at energy and energy-full-design. With this in mind, let's do some math.

A 2010-era 15" MacBook Pro notebook usually ships from the factory with a 63.5Wh battery. 7% of that would be about 4.45Wh.

OS	Calculation		Value
MacOS	energy / energy-full	4.45 / 4.45	100%
elementary OS	energy / energy-full-design	4.45 / 63.5	7%

For many Debian-based Linux distributions, the "critical battery shutdown" takes place at 2%. That works out to about 1.27Wh.

OS	Calculation		Value
MacOS	energy / energy-full	1.27 / 4.45	28.539%
elementary OS	energy / energy-full-design	1.27 / 63.5	2%

If we take into account the energy rebound that takes place after a battery stops discharging momentarily (as well as Apple's propensity to use round, "people friendly" numbers whenever possible), we can consider the 33% value that macOS reports as a rounding error.