Difference between revisions of "Pinebook Pro power and charging"

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m (→‎Type-C current limit: mention that current higher than 2.5 A won't be reported in sysfs but still can be used)
(→‎Charging indicator LED: Expanded a bit)
 
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Pinebook Pro external power and charging circuity is quite
The [[Pinebook Pro]] external power and charging circuity is quite rudimentary, and hence quirky. This article aims to explain all the fine points so that the behaviour could be understood and dealt with.
rudimentary, and hence quirky. This article aims to explain all the
fine points so that the behaviour could be understood and dealt with.


== Monitoring and control ==
== Monitoring and control ==
=== Overview ===
=== Overview ===
No control of charging is possible other than by physically plugging
and unplugging the chargers.


Software monitoring is also quite limited, one can check whether a
No control of charging is possible other than by physically plugging and unplugging a charger.
charger is connected (in
<code>/sys/class/power_supply/dc-charger/online</code> and
<code>/sys/class/power_supply/tcpm-source-psy-4-0022/online</code>)
and see the current battery voltage (in
<code>/sys/class/power_supply/cw2015-battery/voltage_now</code>).


The [https://cdn.datasheetspdf.com/pdf-down/C/W/2/CW2015-Cellwise.pdf CW2015] battery monitoring IC only measures the voltage and tries to
Software monitoring is also quite limited, one can check whether a charger is connected (in <code>/sys/class/power_supply/dc-charger/online</code> and <code>/sys/class/power_supply/tcpm-source-psy-4-0022/online</code>) and see the current battery voltage (in <code>/sys/class/power_supply/cw2015-battery/voltage_now</code>).
guessimate the State Of Charge and Remaining Run Time. The last value
 
(along with the ''nominal'' battery capacity) is also used by the
The [https://cdn.datasheetspdf.com/pdf-down/C/W/2/CW2015-Cellwise.pdf CW2015] battery monitoring IC only measures the voltage and tries to estimate the State Of Charge and Remaining Run Time. The last value (along with the ''nominal'' battery capacity) is also used by the kernel driver to ''compute'' the current. The estimations might be relatively accurate under certain conditions but you can not really know if they're met with your laptop load at any given moment, so the only value provided that can be trusted is the voltage.
kernel driver to ''compute'' the current. The estimations might be
relatively accurate under certain conditions but you can not really
know if they're met with your laptop load at any given moment, so the
only value provided that can be trusted is the voltage.


=== Charging indicator LED ===
=== Charging indicator LED ===
There is a red LED near the barrel socket that's connected
There is a red LED near the barrel socket that's connected directly to the [https://www.ti.com/lit/ds/symlink/bq24171.pdf?ts=1607068456825&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FBQ24171 BQ24171] battery charging IC.
directly to the
[https://www.ti.com/lit/ds/symlink/bq24171.pdf?ts=1607068456825&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FBQ24171 BQ24171] battery charging IC.


It can indicate one of the three states:
It can indicate one of the three states:
Line 32: Line 17:
# LED ''on'' means the charger is supplying current to the battery and the system;
# LED ''on'' means the charger is supplying current to the battery and the system;
# LED ''off'' means the charger is turned off, and the whole system is powered from the battery;
# LED ''off'' means the charger is turned off, and the whole system is powered from the battery;
# LED ''blinking with 0.5&nbsp;Hz frequency'' signals some hardware error: typically [[#Battery temperature fix|battery over-temperature protection]] or input under-voltage; in this case the charger is also off, and the system is powered from the battery all the time.
# LED ''blinking with 0.5&nbsp;Hz frequency'' signals some hardware error: typically [[#Battery temperature fix|battery over-temperature protection]] or input under-voltage (from a [[:File:Pbp_charger_top.jpg|failed charger]]); in this case the charger is also off, and the system is powered from the battery all the time.


All other kinds of blinking really indicate the charger getting turned
All other kinds of blinking really indicate the charger getting turned on and off, this happens when BQ24171 detects ''battery full'' condition, disables the charger, the system starts drawing current from the battery, the voltage quickly drops, and the charger is enabled again to compensate for the discharge. The blinking frequency would depend on the current system load, battery temperature, and the backlight level (as the backlight power source adds up to ~70&nbsp;mV ripple to the voltage monitoring net). This "trickle-charging" is harmful for lithium batteries, but no workaround is possible other than fully disconnecting the external power source, and it's not clear whether that would do more good than harm.
on and off, this happens when BQ24171 detects ''battery full''
 
condition, disables the charger, the system starts drawing current
Another observed behavior is that the status LED blinks randomly, from time to time and unrelated to the system load, especially when the screen brightness is cranked to the maximum, and the battery isn't fully charged.  This has been attributed to some strange feedback that the BQ24171 receives and becomes confused, but further analysis is required.
from the battery, the voltage quickly drops, and the charger is
enabled again to compensate for the discharge. The blinking frequency
would depend on the current system load, battery temperature, and the
backlight level (as the backlight power source adds up to ~70&nbsp;mV
ripple to the voltage monitoring net). This "trickle-charging" is
harmful for Li-based batteries, but no workaround is possible other
than fully disconnecting the external power source, and it's not clear
whether that would do more good than harm.


=== Monitoring currents ===
=== Monitoring currents ===
The charging IC uses two measurement shunt resistors: ''R37'' for
input current, and ''R43'' for battery current, both
0.010&nbsp;Ohm. They're easily accessible for external equipment after
removing the RF shield on the mainboard, and one can use a
battery-powered voltmeter or a differential probe to properly measure
the real current at any given moment. Do ''not'' connect non-isolated
oscilloscope ground clip to them, that might damage the
equipment.


With the external chargers disconnected the system is powered by the
The charging IC uses two measurement shunt resistors: ''R37'' for input current, and ''R43'' for battery current, both 0.010&nbsp;Ohm. They're easily accessible for external equipment after removing the RF shield on the mainboard, and one can use a battery-powered voltmeter or a differential probe to properly measure the real current at any given moment. Do ''not'' connect non-isolated oscilloscope ground clip to them, that might damage the equipment.
battery, so measuring voltage on ''R43'' (along with the battery
 
voltage at about the same moment) can be used to learn the system
With the external chargers disconnected the system is powered by the battery, so measuring voltage on ''R43'' (along with the battery voltage at about the same moment) can be used to learn the system power consumption under different software loads.
power consumption under different software loads.


== Charging ==
== Charging ==
=== Overview ===
=== Overview ===
[[File:pbp-charging-simplified.png|thumb|Pinebook Pro simplified charging schematics]]
[[File:pbp-charging-simplified.png|thumb|Pinebook Pro simplified charging schematics]]
When an external charger is connected, the battery charging process is
automatically activated, it doesn't depend on any software
interactions and works all the same even with the main SoC powered
down. The system automatically chooses between the barrel socket
(limiting current draw to 3&nbsp;A) and Type-C source (limited to
2.5&nbsp;A), with the former preferred when both are connected at the
same time (but the current limit is enforced as if Type-C was used).


The maximum charging current under normal conditions is limited to
When an external charger is connected, the battery charging process is automatically activated, it doesn't depend on any software interactions and works all the same even with the main SoC powered down. The system automatically chooses between the barrel socket (limiting current draw to 3&nbsp;A) and Type-C source (limited to 2.5&nbsp;A), with the former preferred when both are connected at the same time (but the current limit is enforced as if Type-C was used).
2.75&nbsp;A and the voltage to 4.35&nbsp;V. Battery temperature
affects these values, and if the measuring is [[#Battery temperature fix|done properly]] the charge is fully suspended under 0&nbsp;°C or above
60&nbsp;°C, maximum current halved below 10&nbsp;°C, maximum voltage
reduced to 4.24&nbsp;V above 45&nbsp;°C and to 4.19&nbsp;V above 50&nbsp;°C.


The charging process automatically terminates when the voltage reaches
The maximum charging current under normal conditions is limited to 2.75&nbsp;A and the voltage to 4.35&nbsp;V. Battery temperature affects these values, and if the measuring is [[#Battery temperature fix|done properly]] the charge is fully suspended under 0&nbsp;°C or above 60&nbsp;°C, maximum current halved below 10&nbsp;°C, maximum voltage reduced to 4.24&nbsp;V above 45&nbsp;°C and to 4.19&nbsp;V above 50&nbsp;°C.
the recharge threshold (upper limit - 0.1&nbsp;V) ''and'' the current
 
falls below 275&nbsp;mA. However, this also stops supplying external
The charging process automatically terminates when the voltage reaches the recharge threshold (upper limit - 0.1&nbsp;V) ''and'' the current falls below 275&nbsp;mA. However, this also stops supplying external power to the system, so if it's running the battery voltage almost immediately drops below the recharge threshold, and the charging is turned on again.
power to the system, so if it's running the battery voltage almost
immediately drops below the recharge threshold, and the charging is
turned on again.


=== Example run and charge time calculations ===
=== Example run and charge time calculations ===
<!-- 19:25 < PaulFertser> So my first quick measurements on the shunt:
<!-- 19:25 < PaulFertser> So my first quick measurements on the shunt: with display off and system idle: 1.7 A @3.78V = 6.46 W; with display on backlight at 0: 7.03 W; with backlight at 4095: 10.51 W; with backlight at 3700: 9.64 W. with performance CPU governor. -->
with display off and system idle: 1.7 A @3.78V = 6.46 W; with display
 
on backlight at 0: 7.03 W; with backlight at 4095: 10.51 W; with
Assuming a fully charged 9600&nbsp;mAh battery and an idle system using ''performance'' cpufreq governor with backlight at 3700/4095 consuming 9.6&nbsp;W we can expect
backlight at 3700: 9.64 W. with performance CPU governor. -->
Assuming a fully charged 9600&nbsp;mAh battery and an idle system
using ''performance'' cpufreq governor with backlight at 3700/4095  
consuming 9.6&nbsp;W we can expect


<code>9.6 Ah * 3.8 V / 9.6 W = 3.8 h</code>
<code>9.6 Ah * 3.8 V / 9.6 W = 3.8 h</code>
Line 98: Line 48:
so that gives 3.8 hours of run time.
so that gives 3.8 hours of run time.


If the same battery is empty and a barrel plug charger is
If the same battery is empty and a barrel plug charger is connected while system has the same load it will need
connected while system has the same load it will need


<code>9.6 Ah * 3.8 V / ((3 A * 5 V * 0.9 - 9.6 W) * 0.95) = 9.85 h</code>
<code>9.6 Ah * 3.8 V / ((3 A * 5 V * 0.9 - 9.6 W) * 0.95) = 9.85 h</code>


that is 9.85 hours of charging from zero to full, assuming 0.9 DC-DC
that is 9.85 hours of charging from zero to full, assuming 0.9 DC-DC conversion efficacy and 0.95 charging efficacy.
conversion efficacy and 0.95 charging efficacy.


Removing the system load reduces the time to
Removing the system load reduces the time to
Line 110: Line 58:
<code>9.6 Ah * 3.8 V / (2.75 A * 3.8 V * 0.95) = 3.67 h</code>
<code>9.6 Ah * 3.8 V / (2.75 A * 3.8 V * 0.95) = 3.67 h</code>


so if you need to fully charge the battery, e.g. before a trip, the
so if you need to fully charge the battery, e.g. before a trip, the fastest and most reliable way is to power down (not suspend) the system, leave the device with the charger connected for a few hours, upside down for better cooling, and wait for the red LED on the side to turn off.
fastest and most reliable way is to power down (not suspend) the system, leave the device with the charger connected
for a few hours, upside down for better cooling, and wait for the red
LED on the side to turn off.


=== Working without battery ===
=== Working without battery ===
With the battery disconnected the charger isn't going to turn on, and
With the battery disconnected the charger isn't going to turn on, and the system won't be getting any power from the external source. That's why PBP has additional bypass cable that allows connecting external power directly to the system power bus. Of course it should be kept disconnected when the battery is present to avoid excess voltage overcharging and destroying the battery. It's also recommended to add additional insulation to the cable connectors, as they expose battery and charger positive terminals on bare metal, and should never be accidentally connected to ground.  
the system won't be getting any power from the external source. That's
why PBP has additional bypass cable that allows connecting external
power directly to the system power bus. Of course it should be kept
disconnected when the battery is present to avoid excess voltage
overcharging and destroying the battery. It's also recommended to add
additional insulation to the cable connectors, as they expose battery
and charger positive terminals on bare metal, and should never be
accidentally connected to ground.  


== Hardware modifications ==
== Hardware modifications ==
=== Type-C current limit ===
=== Type-C current limit ===
Since there's no software control over the input current limit
{{Warning|The 0.5&nbsp;A difference described in this section is there to carve out some power for a USB-C dock connected to the Pinebook Pro's USB-C port.  This is actually against the USB Power Delivery specification, but it leaves some power to the USB-C dock, which it requires to power itself and any devices connected to it. Thus, the procedure described in this section will most probably make using USB-C docks unreliable or even impossible, leaving the USB-C port usable for connecting only USB-C chargers or bus-powered USB-C devices.}}
unmodified PBP always tries to draw up to 2.5&nbsp;A from a Type-C
charger.


It's recommended to manually check
Since there's no software control over the input current limit unmodified PBP always tries to draw up to 2.5&nbsp;A from a Type-C charger.
<code>/sys/class/power_supply/tcpm-source-psy-4-0022/current_max</code>
for all the chargers you're using. When the value is lower than
2.5&nbsp;A you shouldn't use that charger with PBP as it would get
overloaded, running out of specs.


If all of the chargers you want to use can supply 3&nbsp;A or more ''at 5&nbsp;V'' (the sysfs file will still report 2.5&nbsp;A so check the official charger specs and/or label) consider lifting the limit to make it even with the
It's recommended to manually check <code>/sys/class/power_supply/tcpm-source-psy-4-0022/current_max</code> for all the chargers you're using. When the value is lower than 2.5&nbsp;A you shouldn't use that charger with PBP as it would get overloaded, running out of specs.
barrel plug charger. For that remove the ''R148'' resistor on the
[https://wiki.pine64.org/images/b/b7/Pinebookpro-v2.1-bottom-ref.pdf bottom layer] of the mainboard.


The easiest way is to use a soldering iron tip big enough to hold a
If all of the chargers you want to use can supply 3&nbsp;A or more ''at 5&nbsp;V'' (the sysfs file will still report 2.5&nbsp;A so check the official charger specs and/or label) consider lifting the limit to make it even with the barrel plug charger. For that remove the ''R148'' resistor on the [https://wiki.pine64.org/images/b/b7/Pinebookpro-v2.1-bottom-ref.pdf bottom layer] of the mainboard.
1&nbsp;mm drop of an SnPb solder (it mixes with Pb-free nicely and
 
lowers the melting point) to heat both sides of the resistor at once
The easiest way is to use a soldering iron tip big enough to hold a 1&nbsp;mm drop of an SnPb solder (it mixes with Pb-free nicely and lowers the melting point) to heat both sides of the resistor at once and lift it off.
and lift it off.


=== Battery temperature fix ===
=== Battery temperature fix ===
To ensure safe operation the charger IC is constantly monitoring the
{{Warning|The procedure described in this section alters the operating parameters of the lithium battery built into the Pinebokk Pro, which may be unsafe, and in extreme conditions may even introduce a fire hazard.  Use the described procedure at your own risk.  Additional verfication of the described procedure is currently pending.}}
battery temperature with the sensor integrated inside the pack. The
 
thermistor used is a 103AT NTC but the corresponding circuity on PBP
To ensure safe operation the charger IC is constantly monitoring the battery temperature with the sensor integrated inside the pack. The thermistor used is a 103AT NTC but the corresponding circuity on PBP mainboard was calculated for some other type. This results in the charger IC detecting 45&nbsp;°C when the battery is in fact at just 35&nbsp;°C, and 60&nbsp;°C when the battery is at 46&nbsp;°C. It's easy to hit this threshold with heavy CPU or GPU loads as the metal back cover heats up from the SoC and slightly warms up the battery. Under these conditions the charging is suspended (with charging LED signalling a hardware issue), and the intensive tasks are continued on battery power alone, heating it up even more.
mainboard was calculated for some other type. This results in the
 
charger IC detecting 45&nbsp;°C when the battery is in
To fix this issue the resistor divider needs to be replaced to match the datasheet recommended values. For that one needs to change two 0402 resistors on the bottom side of the mainboard: use 2.2&nbsp;kOhm 1&nbsp;% for ''R52'' (instead of 4.4&nbsp;kOhm installed by the factory), note it's the one closer to the board edge; and 6.8&nbsp;kOhm 1&nbsp;% for ''R54'' (30&nbsp;kOhm from the factory).
fact at just 35&nbsp;°C, and 60&nbsp;°C when the battery is at 46&nbsp;°C. It's
 
easy to hit this threshold with heavy CPU or GPU loads as the metal
If your local hackspace doesn't have suitable resistors consider getting a sample book from e.g. Aliexpress, it should cost less than 15&nbsp;USD including shipping.
back cover heats up from the SoC and slightly warms up the battery. Under these conditions the charging is suspended (with charging
LED signalling a hardware issue), and the intensive tasks are
continued on battery power alone, heating it up even more.


To fix this issue the resistor divider needs to be replaced to match
the datasheet recommended values. For that one needs to change two
0402 resistors on the bottom side of the mainboard: use 2.2&nbsp;kOhm
1&nbsp;% for ''R52'' (instead of 4.4&nbsp;kOhm installed by the factory),
note it's the one closer to the board edge; and 6.8&nbsp;kOhm 1&nbsp;% for ''R54''
(30&nbsp;kOhm from the factory).


If your local hackspace doesn't have suitable resistors consider
[[Category:Pinebook Pro]]
getting a sample book from e.g. Aliexpress, it should cost less than
15&nbsp;USD including shipping.

Latest revision as of 22:28, 8 November 2023

The Pinebook Pro external power and charging circuity is quite rudimentary, and hence quirky. This article aims to explain all the fine points so that the behaviour could be understood and dealt with.

Monitoring and control

Overview

No control of charging is possible other than by physically plugging and unplugging a charger.

Software monitoring is also quite limited, one can check whether a charger is connected (in /sys/class/power_supply/dc-charger/online and /sys/class/power_supply/tcpm-source-psy-4-0022/online) and see the current battery voltage (in /sys/class/power_supply/cw2015-battery/voltage_now).

The CW2015 battery monitoring IC only measures the voltage and tries to estimate the State Of Charge and Remaining Run Time. The last value (along with the nominal battery capacity) is also used by the kernel driver to compute the current. The estimations might be relatively accurate under certain conditions but you can not really know if they're met with your laptop load at any given moment, so the only value provided that can be trusted is the voltage.

Charging indicator LED

There is a red LED near the barrel socket that's connected directly to the BQ24171 battery charging IC.

It can indicate one of the three states:

  1. LED on means the charger is supplying current to the battery and the system;
  2. LED off means the charger is turned off, and the whole system is powered from the battery;
  3. LED blinking with 0.5 Hz frequency signals some hardware error: typically battery over-temperature protection or input under-voltage (from a failed charger); in this case the charger is also off, and the system is powered from the battery all the time.

All other kinds of blinking really indicate the charger getting turned on and off, this happens when BQ24171 detects battery full condition, disables the charger, the system starts drawing current from the battery, the voltage quickly drops, and the charger is enabled again to compensate for the discharge. The blinking frequency would depend on the current system load, battery temperature, and the backlight level (as the backlight power source adds up to ~70 mV ripple to the voltage monitoring net). This "trickle-charging" is harmful for lithium batteries, but no workaround is possible other than fully disconnecting the external power source, and it's not clear whether that would do more good than harm.

Another observed behavior is that the status LED blinks randomly, from time to time and unrelated to the system load, especially when the screen brightness is cranked to the maximum, and the battery isn't fully charged. This has been attributed to some strange feedback that the BQ24171 receives and becomes confused, but further analysis is required.

Monitoring currents

The charging IC uses two measurement shunt resistors: R37 for input current, and R43 for battery current, both 0.010 Ohm. They're easily accessible for external equipment after removing the RF shield on the mainboard, and one can use a battery-powered voltmeter or a differential probe to properly measure the real current at any given moment. Do not connect non-isolated oscilloscope ground clip to them, that might damage the equipment.

With the external chargers disconnected the system is powered by the battery, so measuring voltage on R43 (along with the battery voltage at about the same moment) can be used to learn the system power consumption under different software loads.

Charging

Overview

Pinebook Pro simplified charging schematics

When an external charger is connected, the battery charging process is automatically activated, it doesn't depend on any software interactions and works all the same even with the main SoC powered down. The system automatically chooses between the barrel socket (limiting current draw to 3 A) and Type-C source (limited to 2.5 A), with the former preferred when both are connected at the same time (but the current limit is enforced as if Type-C was used).

The maximum charging current under normal conditions is limited to 2.75 A and the voltage to 4.35 V. Battery temperature affects these values, and if the measuring is done properly the charge is fully suspended under 0 °C or above 60 °C, maximum current halved below 10 °C, maximum voltage reduced to 4.24 V above 45 °C and to 4.19 V above 50 °C.

The charging process automatically terminates when the voltage reaches the recharge threshold (upper limit - 0.1 V) and the current falls below 275 mA. However, this also stops supplying external power to the system, so if it's running the battery voltage almost immediately drops below the recharge threshold, and the charging is turned on again.

Example run and charge time calculations

Assuming a fully charged 9600 mAh battery and an idle system using performance cpufreq governor with backlight at 3700/4095 consuming 9.6 W we can expect

9.6 Ah * 3.8 V / 9.6 W = 3.8 h

so that gives 3.8 hours of run time.

If the same battery is empty and a barrel plug charger is connected while system has the same load it will need

9.6 Ah * 3.8 V / ((3 A * 5 V * 0.9 - 9.6 W) * 0.95) = 9.85 h

that is 9.85 hours of charging from zero to full, assuming 0.9 DC-DC conversion efficacy and 0.95 charging efficacy.

Removing the system load reduces the time to

9.6 Ah * 3.8 V / (2.75 A * 3.8 V * 0.95) = 3.67 h

so if you need to fully charge the battery, e.g. before a trip, the fastest and most reliable way is to power down (not suspend) the system, leave the device with the charger connected for a few hours, upside down for better cooling, and wait for the red LED on the side to turn off.

Working without battery

With the battery disconnected the charger isn't going to turn on, and the system won't be getting any power from the external source. That's why PBP has additional bypass cable that allows connecting external power directly to the system power bus. Of course it should be kept disconnected when the battery is present to avoid excess voltage overcharging and destroying the battery. It's also recommended to add additional insulation to the cable connectors, as they expose battery and charger positive terminals on bare metal, and should never be accidentally connected to ground.

Hardware modifications

Type-C current limit

Warning: The 0.5 A difference described in this section is there to carve out some power for a USB-C dock connected to the Pinebook Pro's USB-C port. This is actually against the USB Power Delivery specification, but it leaves some power to the USB-C dock, which it requires to power itself and any devices connected to it. Thus, the procedure described in this section will most probably make using USB-C docks unreliable or even impossible, leaving the USB-C port usable for connecting only USB-C chargers or bus-powered USB-C devices.

Since there's no software control over the input current limit unmodified PBP always tries to draw up to 2.5 A from a Type-C charger.

It's recommended to manually check /sys/class/power_supply/tcpm-source-psy-4-0022/current_max for all the chargers you're using. When the value is lower than 2.5 A you shouldn't use that charger with PBP as it would get overloaded, running out of specs.

If all of the chargers you want to use can supply 3 A or more at 5 V (the sysfs file will still report 2.5 A so check the official charger specs and/or label) consider lifting the limit to make it even with the barrel plug charger. For that remove the R148 resistor on the bottom layer of the mainboard.

The easiest way is to use a soldering iron tip big enough to hold a 1 mm drop of an SnPb solder (it mixes with Pb-free nicely and lowers the melting point) to heat both sides of the resistor at once and lift it off.

Battery temperature fix

Warning: The procedure described in this section alters the operating parameters of the lithium battery built into the Pinebokk Pro, which may be unsafe, and in extreme conditions may even introduce a fire hazard. Use the described procedure at your own risk. Additional verfication of the described procedure is currently pending.

To ensure safe operation the charger IC is constantly monitoring the battery temperature with the sensor integrated inside the pack. The thermistor used is a 103AT NTC but the corresponding circuity on PBP mainboard was calculated for some other type. This results in the charger IC detecting 45 °C when the battery is in fact at just 35 °C, and 60 °C when the battery is at 46 °C. It's easy to hit this threshold with heavy CPU or GPU loads as the metal back cover heats up from the SoC and slightly warms up the battery. Under these conditions the charging is suspended (with charging LED signalling a hardware issue), and the intensive tasks are continued on battery power alone, heating it up even more.

To fix this issue the resistor divider needs to be replaced to match the datasheet recommended values. For that one needs to change two 0402 resistors on the bottom side of the mainboard: use 2.2 kOhm 1 % for R52 (instead of 4.4 kOhm installed by the factory), note it's the one closer to the board edge; and 6.8 kOhm 1 % for R54 (30 kOhm from the factory).

If your local hackspace doesn't have suitable resistors consider getting a sample book from e.g. Aliexpress, it should cost less than 15 USD including shipping.