Laptop battery charging circuit
1. Some theory
1.1. Introduction
Almost every laptop has a charging circuit of some sort, that does a few things:


Manages switching between AC adapter and battery power
Manages battery charging and generates power rail to charge battery
There are 2 different designs to handle switching between AC adapter and battery power
and feeding the system:


Hybrid power boost (HPB) and traditional circuits without power boost (see
Warning in HPB section)
Narrow VDC (NVDC)
There are also 2 different designs to handle battery charging rail generation:


Buck converter
Buck-Boost converter
Almost all laptops without USB-C charging capabilities use a buck converter. Most of
these use HPB, except Apple laptops which always use NVDC. A bunch of non-Apple
ultrabooks use NVDC as well.
Almost all laptops (incl. Apple) with USB-C charging capabilities use NVDC with a buckboost converter.
Some netbooks or entry-level laptops (especially ARM platforms and some Atom
platforms) can have a circuit different to what is presented here, using a PMIC that
handles almost every power-related features. Since these aren't common and highly
depend on the board and the ICs used, they are not covered here.
For MacBooks without USB-C you have some more details on the charger IC
here: https://logi.wiki/index.php/ISL6258_...roubleshooting
1.2. Hybrid power boost design
In a Hybrid Power Boost configuration, the synchronous buck converter runs as a
normal buck converter when the adapter provides power to the system and is charging
the battery. When the adapter power is not sufficient, the synchronous buck converter
runs in reverse to boost the battery voltage to around 20V. Thus the battery
supplements the adapter whenever the adapter power is not sufficient. This requires
no circuit changes from a traditional adapter.
The change is required in the control circuit (the battery charger controller). The
advantage of this system over a traditional charger is that the battery is able to assist
the adapter during turbo workloads. This system has the disadvantage that the light
load efficiency is pretty low as it is difficult to achieve high light load efficiency at high
input voltage.
In this design, the system is fed from the AC adapter, and a MOSFET (we will call it
battery-to-system MOSFET) allows the current to flow from the battery to the system
when on battery only, while prevent current from flowing directly from the AC adapter
to the battery when AC adapter is plugged in.
Therefore, when on AC adapter, the main power rail voltage will be the voltage
provided by the AC adapter. In general this voltage is between 19V and 20V. A few
machines use 12V or 16V.
When on battery, the main power rail voltage will be the voltage provided by the
battery. This depends on the number of cells in series in the battery and the state of
charge, but it is lower than 19V. (see Battery voltage section)
Warning: Hybrid power boost provides an additional feature where the battery can
provide additional power to the system even when running on AC adapter. Older
designs used the same circuit but the charger IC was not able to provide this feature.
The presence or abscence of this feature is irrelevant to this article, so the older
designs will fall under the hybrid power boost term as well.
1.3. Narrow VDC design
This figure shows the Narrow VDC (NVDC) topology. Here, the system bus (Vsys) is
not connected directly to the adapter. It is connected to the output of the buck
converter. Hence, NVDC operates only as a buck converter, both when NVDC charges
the battery and when the battery supplements the adapter and provides power to the
system. NVDC implementation reduces the switch-over period between the charging
mode and the hybrid power mode. NVDC implementation allows the system to
minimize the period of overloading the input power source when CPU is in Turbo Boost
mode.
The advantage of using the NVDC system is that the overall system efficiency is better
compared to the Hybrid Power Boost (HPB) charger. The system can be designed for a
smaller voltage rating since the system has a lower Vin. The disadvantage is that the
charger components’ size and power dissipation increases.
In this design, the system is fed directly from the battery charging rail. The voltage on
the main power rail is always close to the voltage on the battery. (see Battery voltage
section)
1.4. Buck converter
This works the same as any single-phase synchronous buck converter. There's one highside MOSFET and one low-side MOSFET turning on and off alternatively to step down the
voltage from the input, for example from 19V to 12.6V.
(from bq24715 datasheet)
1.5. Buck-Boost converter
A synchronous 4-switch buck-boost converter uses a first set of high-side and low-side
MOSFETs before the inductor. They can act as a regular buck converter to step down the
voltage coming from the AC adapter. There's a second set of MOSFETs after the
inductor, they are used as a boost converter to step up the voltage coming from the AC
adapter.
That way, the machine can take 5V to 20V from an USB-C charger and still be able to
charge the battery. Of course, with only 5V as the input, the power will be lower than
with 20V, so the battery will charge slower and the laptop may not turn on without
battery or when the battery is discharged.
(From ISL9238 datasheet)
1.6. Battery voltage
Laptops battery packs are based around lithium cells.
These are either Li-ion cells, round cylinder similar to your alkaline battery but in a
18650 package, or LiPo cells, flat rectangular cell like in a smartphone.
Li-ion cells typically have a nominal voltage of 3.6V to 3.7V, LiPo cells typically have a
nominal voltage of 3.7V to 3.85V. It varies depending on the brand and series of the cell.
The battery voltage and by extension the voltage used to charge the battery depends
directly on how many cells there are in series inside the battery pack.
Battery packs can be described with a "xSyP" number, where y is the number of cells in
a group connected in parallel, and x is the number of groups connected in series.
All cells in parallel in a group will have the same voltage across them. Putting groups in
series will sum their voltages.
For example a 3S2P pack with 11.1V nominal voltage contains 6 cells, 3 groups in series
of 2 cells in parallel, for a total voltage across the pack of 3×3.7V = 11.1V.
The number of cells put in parallel in a group helps increasing the capacity, but it doesn't
change the voltage. We are interested in the voltage here, so we will ignore the cells in
parallel.
The "nominal voltage" represents the voltage around which the cell is throughout most
of its discharge, it should be what you are able to measure on the pack when it's
charged to around 50%.
When charged fully, the voltage will be higher, when completely discharged, the voltage
will be lower.
Charging voltage will be a bit higher than the fully charged voltage of the pack. For
example, an 11.55V pack (3S of 3.85V nominal) can in general be charged at 13.1V
(4.35V per cell). An 11.1V pack (3S of 3.7V nominal) can in general be charged at 12.6V
(4.2V per cell). A 10.8V pack (3S of 3.6V nominal) can in general be charged at 12.3V
(4.1V per cell).
This of course highly depends on the exact type of cells used, and using a voltage that's
too high for a given cell type can at best damage the cell, at worst be a safety threat.
2. Troubleshooting charging circuit
2.1. No power
First thing to check if the machine is not powering on is if the main power rail is present.
If it is present, then in general the problem is not in the charging circuit. You can still
check "Shorted DC-in MOSFET" and "Missing voltage on the charger IC" as they could
still be a source of trouble in some cases.
To be able to check the main power rail, you have to identify if you have an HPB or an
NVDC configuration.

With an HPB configuration you have to check the voltage after the 2nd DC-in
MOSFET and after the current sensing resistor for AC adapter power input. You
are supposed to find the AC adapter voltage.


(from Quanta X63 schematics)
With an NVDC configuration, you are supposed to find the battery voltage (see
Battery voltage section):
o If it's a buck converter you have to check the voltage after the buck
converter inductor and after the fuse if it exists


(from Apple 820-00165 schematics)
If it's a buck-boost converter you have to check the voltage at the output of the
2nd set of MOSFETs and after the fuse if it exists.

o
(from Apple 820-00239 schematics)
If you confirmed your main power rail is missing, there are several possibilities
2.1.1. Burnt fuse
You found voltage before the fuse (after the buck converter output inductor or buckboost 2nd set of MOSFETs), but not after it: fuse is blown. Replace it and go to the next
section "Short to ground on main power rail". Don't apply power before resolving the
short to ground.
2.1.2. Short to ground on main power rail
Most common issue in this case is a short to ground on main power rail caused by a
capacitor, and IC, a MOSFET…
In this case you have to measure *exact* resistance to ground on the main power rail
(same point as previously). If it is lower than several kohms there's a short to ground.
As indicated in the "Begginer's guidelines: what not to do" section, DO NOT inject 19V.
Check high-side MOSFETs for short, easiest way is to place one probe on the main power
rail and the other probe on each large inductor of the board. If one is lower than 1 ohm,
there's a possibility of high-side MOSFET short. If not, you can start injecting voltage *at
1V*, and double check none of the large BGA is warming up. If 1V is not enough, you
can start increasing voltage, after making sure none of the large BGA is warming up
again.
Once you find the bad component, remove it, confirm short to ground is gone, and
replaced it. Don't apply power to the board if the component is removed without being
sure that it'll not cause any harm.
2.1.3. Shorted DC-in MOSFET
This is the second most common issue. What is troubling for beginners here is that the
MOSFET that is blocking the current is *not* the MOSFET that has failed.
What happens here is that the MOSFET that shorts is the one with the body diode in
forward (in the illustration below, the 2nd one, note that they can be exchanged), not
the one with the body diode in reverse.
Effectively, the shorted MOSFET will have drain/source/gate connected together. Both
DC-in MOSFET gates are also connected together. And Both DC-in MOSFET sources are
connected together. So with gate/source of DC-in MOSFET in forward bias shorted, it
also causes gate/source of the DC-in MOSFET in reverse bias to be shorted. This MOSFET
is therefore turned off permanently, not letting the current through.
The other MOSFET could also be shorted, in the case of an HPB topology the main power
rail will be present since current will flow through the shorted MOSFET and through the
body diode of the other MOSFET, but some signal from the charger IC might be missing
causing the platform not to turn on. In an NVDC topology the main power might be
missing.
Simply check resistance between drain/source/gate of both DC-in MOSFETs to confirm
they are not shorted. It should show at least several kohms.
(from Quanta X63 schematics)
2.1.4. Burnt current sensing lines
The charger IC also monitors the current flowing from the AC adapter, and from/to the
battery to prevent overcurrent conditions. It can happen that the current sensing circuits
get burnt, either the current sensing resistors themselves or the resistors that connect
them to the current sensing amplifier inputs of the charger IC.
Easiest way is to measure resistance directly across the pins of the current sensing
amplifier inputs of the charger IC. The measured resistance should be the sum of the
current sensing resistors, and the resistors that connect it to the charger IC. This also
accounts for a possible break in the line that could be caused by liquid damage for
example.
(from Quanta X63 schematics)
2.1.5. Missing voltage on the charger IC
Charger IC requires several voltages to work.
Exact names depend on the charger IC used, but there is at least:



DCIN/VCC: input voltage from the AC adapter, should be close to AC adapter
voltage
ACIN/ACDET: voltage divider from the AC adapter to detect a correct voltage
range. Exact required voltage depends on the charger IC but it often needs to be
higher than 2.4V or 2.6V
REGN/VDD: internal LDO output in general between 5V and 6V, input taken from
DCIN/VCC.
These signals should come up once the previous signals are good and the previous
problems have been excluded:


ACOK: asserts that the input voltage is good, for P-channel MOSFETs it often
drives their gates directly
ACDRV: only present with N-channel MOSFETs, drives their gate, should be 6V
above input voltage so in general around 25V
2.1.6. Damaged passive component
This will be harder to spot, but depending on the circuit a missing/bad resistor or bad
capacitor could be the culprit. Visual inspection is the key here. This is very rare though,
except in case of liquid damage. If there is liquid damage, follow the visual hints.
One possible issue would be the gate resistors for the DC-in MOSFET, so you can always
check them if you're in doubt.
2.1.7. Dead charger IC
If you excluded all the previous possibilities, the most probable is that the charger IC
itself is dead. Replace it and see if the behaviour changes.
2.2. Battery not charging
First thing is to try another battery. Then try another battery. Finally try another battery.
99% of battery problems are the battery itself, even "new", market is filled with garbage
batteries. A "new" battery should come charged at around 50%, at worst between 20%
and 80%, and the machine should turn on from it. Some batteries need to be
"kickstarted" by plugging in the AC adapter for a short while to turn them on.
Once again it is important to know if you have an NVDC or an HPB topology.
Battery has to be detected to start charging.
2.2.1. Battery not detected
Issue on charging circuit SMBUS.
Check voltage and diode mode to ground on charging circuit SMBUS SDA and SCL lines
(from Quanta X63 schematics)
2.2.2. Low voltage on battery charging rail
If it's an NVDC topology, the machine wouldn't turn on with a too low voltage on the
battery charging rail. However with NVDC there is a charging MOSFET between the buck
converter output and the battery power rail that could be the culprit or its gate resistor.
If it's HPB topology, the battery is in general connected directly to the buck converter
output. A bad or deep discharged battery will pull down the output. Let the battery
charge overnight and see if it recovers, if not, replace the battery. Without battery, in
HPB tology the output of the battery charging rail will often be low.
DC-in MOSFET or current sensing could be the problem, see "No power section".
Some charger IC have a "CELL" pin to control the default voltage to apply to a battery
relative to its number of cells in series. Wrong setting can cause wrong voltage.
2.2.3. High voltage on battery charging rail
It could be a shorted high-side MOSFET on the buck converter for the battery charging
rail.
In the case of HPB toplogy, it can also be a shorted battery-to-system MOSFET.
(from Quanta X63 schematics)
Some charger IC have a "CELL" pin to control the default voltage to apply to a battery
relative to its number of cells in series. Wrong setting can cause wrong voltage.