I recharged the mouse, but:

$ solaar show
Unifying Receiver
  Device path  : /dev/hidraw0
  USB id       : 046d:c52b
  Serial       : 23FF9429
    Firmware   : 12.10.B0032
    Bootloader : 02.15
    Other      : AA.AA
  Has 3 paired device(s) out of a maximum of 6.
  Notifications: wireless (0x000100)
  Device activity counters: 1=70, 2=105

  1: Wireless Keyboard K270(unifying)
     Codename     : K270(unifying)
     Kind         : keyboard
     Wireless PID : 4003
     Protocol     : HID++ 2.0
     Polling rate : 20 ms (50Hz)
     Serial number: 1A395FC1
          Firmware: RQK 35.00.B0017
     The power switch is located on the top case.
     Supports 12 HID++ 2.0 features:
         0: ROOT                   {0000}   
         1: FEATURE SET            {0001}   
         2: DEVICE FW VERSION      {0003}   
         3: DEVICE NAME            {0005}   
         4: BATTERY STATUS         {1000}   
         5: unknown:1820           {1820}   hidden
         6: REPROG CONTROLS        {1B00}   
         7: WIRELESS DEVICE STATUS {1D4B}   
         8: unknown:1DF0           {1DF0}   hidden
         9: unknown:1DF3           {1DF3}   hidden
        10: ENCRYPTION             {4100}   
        11: KEYBOARD LAYOUT        {4520}   
     Has 8 reprogrammable keys:
         0: Play/Pause                 => Play/Pause                    nonstandard
         1: Mute                       => Mute                          nonstandard
         2: Volume Down                => Volume Down                   nonstandard
         3: Volume Up                  => Volume Up                     nonstandard
         4: MY HOME                    => HomePage                      nonstandard, reprogrammable
         5: Mail                       => Email                         nonstandard, reprogrammable
         6: SLEEP                      => Sleep                         nonstandard, reprogrammable
         7: Calculator                 => Calculator                    nonstandard, reprogrammable
     Battery: 70%, discharging.

  2: Wireless Mouse MX Master
     Codename     : MX Master
     Kind         : mouse
     Wireless PID : 4071
     Protocol     : HID++ 4.5
     Polling rate : 8 ms (125Hz)
     Serial number: F8EBBFD1
        Bootloader: BOT 56.10.B0005
          Firmware: MPM 12.10.B0005
          Firmware: MPM 12.10.B0005
             Other: 
     The power switch is located on the base.
     Supports 32 HID++ 2.0 features:
         0: ROOT                   {0000}   
         1: FEATURE SET            {0001}   
         2: DEVICE FW VERSION      {0003}   
         3: DEVICE NAME            {0005}   
         4: WIRELESS DEVICE STATUS {1D4B}   
         5: RESET                  {0020}   
         6: unknown:0021           {0021}   
         7: BATTERY STATUS         {1000}   
         8: unknown:1806           {1806}   internal, hidden
         9: CHANGE HOST            {1814}   
        10: REPROG CONTROLS V4     {1B04}   
        11: ADJUSTABLE DPI         {2201}   
        12: VERTICAL SCROLLING     {2100}   
            Roller type: 3G
            Ratchet per turn: 24
            Scroll lines: 0
        13: SMART SHIFT            {2110}   
        14: HIRES WHEEL            {2121}   
            Multiplier: 8
            Has invert
              Normal wheel motion
            Has ratchet switch
              Normal wheel mode
            High resolution mode
            HID notification
        15: GESTURE 2              {6501}   
        16: unknown:00C2           {00C2}   
        17: unknown:1813           {1813}   internal, hidden
        18: unknown:1830           {1830}   internal, hidden
        19: unknown:1890           {1890}   internal, hidden
        20: unknown:1891           {1891}   internal, hidden
        21: unknown:18A1           {18A1}   internal, hidden
        22: unknown:18C0           {18C0}   internal, hidden
        23: unknown:1DF3           {1DF3}   internal, hidden
        24: unknown:1E00           {1E00}   hidden
        25: unknown:1EB0           {1EB0}   internal, hidden
        26: unknown:1803           {1803}   internal, hidden
        27: unknown:1861           {1861}   internal, hidden
        28: unknown:9001           {9001}   internal, hidden
        29: unknown:9200           {9200}   internal, hidden
        30: unknown:9202           {9202}   internal, hidden
        31: unknown:1805           {1805}   internal, hidden
     Has 8 reprogrammable keys:
         0: LEFT CLICK                , default: LeftClick                   => LEFT CLICK                
             mse, pos:0, group:1, gmask:1
         1: RIGHT CLICK               , default: RightClick                  => RIGHT CLICK               
             mse, pos:0, group:1, gmask:1
         2: MIDDLE BUTTON             , default: MiddleMouseButton           => MIDDLE BUTTON             
             mse, reprogrammable, divertable, pos:0, group:3, gmask:7
         3: BACK AS BUTTON 4          , default: BackEx                      => BACK AS BUTTON 4          
             mse, reprogrammable, divertable, pos:0, group:2, gmask:3
         4: FORWARD AS BUTTON 5       , default: BrowserForwardEx            => FORWARD AS BUTTON 5       
             mse, reprogrammable, divertable, pos:0, group:2, gmask:3
         5: unknown:00C3              , default: unknown:00A9                => unknown:00C3              
             mse, reprogrammable, divertable, pos:0, group:3, gmask:7
         6: unknown:00C4              , default: unknown:009D                => unknown:00C4              
             mse, reprogrammable, divertable, pos:0, group:3, gmask:7
         7: unknown:00D7              , default: unknown:00B4                => unknown:00D7              
             divertable, virtual, pos:0, group:4, gmask:0
     Battery: 90%, discharging.

  3: Performance Mouse MX
     Codename     : Performance MX
     Kind         : mouse
     Wireless PID : 101A
     Protocol     : HID++ 1.0
     Polling rate : 8 ms (125Hz)
     Serial number: 767493F0
     The power switch is located on the base.
     Battery: unknown (device is offline).


$ for DEVICE in `upower -e | grep hidpp`; do upower -i ${DEVICE} ; done
  native-path:          hidpp_battery_2
  model:                Wireless Mouse MX Master
  serial:               4071-f8-eb-bf-d1
  power supply:         no
  updated:              Fri 13 Nov 2020 15:45:35 CET (79 seconds ago)
  has history:          yes
  has statistics:       yes
  mouse
    present:             yes
    rechargeable:        yes
    state:               fully-charged
    warning-level:       none
    battery-level:       full
    percentage:          100% (should be ignored)
    icon-name:          'battery-full-charged-symbolic'

  native-path:          hidpp_battery_3
  model:                Wireless Keyboard K270
  serial:               4003-1a-39-5f-c1
  power supply:         no
  updated:              Fri 13 Nov 2020 15:45:47 CET (67 seconds ago)
  has history:          yes
  has statistics:       yes
  keyboard
    present:             yes
    rechargeable:        yes
    state:               discharging
    warning-level:       none
    battery-level:       normal
    percentage:          55% (should be ignored)
    icon-name:          'battery-low-symbolic'