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.
Troubleshooting charging circuit
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.
With an NVDC configuration, you are supposed to find the battery voltage (see Battery voltage section):
If it’s a buck converter you have to check the voltage after the buck converter inductor and after the fuse if it exists.
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.
If you confirmed your main power rail is missing, there are several possibilities
Burnt fuse
You found voltage before the fuse (after the buck converter output inductor or buck-boost 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.
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.
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.
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.
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.
CMSRC (present only on some charger IC paired with N-channel MOSFETs): voltage reference/source taken in between the 2 DC-in MOSFETs to generate N-channel DC-in MOSFET gate drive voltage (ACDRV). Should be close to AC adapter voltage.
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 DC-in MOSFETs it often drives their gates directly and is active low. For N-channel DC-in MOSFETs it should be active high and may be pulled-up externally to a 3.3V rail.
ACDRV: only present with N-channel MOSFETs, drives their gate, should be 6V above input voltage so in general around 25V
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, as well as the resistor connected to CMSRC pin when it is present, so you can always check them if you’re in doubt.
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.
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.
Some laptops with non-removable battery (newer Acer) have a battery enable/disable switch. Back cover must be properly assembled otherwise the battery won’t work.
Some of the above points at 2.1 No Power can also apply to battery charging so you should check them as well (in particular 2.1.3. Shorted DC-in MOSFET).Once again it is important to know if you have an NVDC or an HPB topology.Battery has to be detected to start charging.
AC adapter not detected, wrong AC adapter
Laptops with AC adapter identification circuits such as HP and Dell won’t charge the battery if the AC adapter is not properly recognized or of the incorrect power rating.For HP, the laptop must be able to turn on from AC adapter only. For Dell the AC adapter must be recognized properly in the BIOS setup with the correct power rating.
Battery not detected
Issue on charging circuit SMBUS.Check voltage and diode mode to ground on charging circuit SMBUS SDA and SCL lines
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.
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.
Parts of charging circuit:
SDA AND SCL ,CURRENT SENSING AMPLIFIER, DC-IN MOSFETS, BATTERY CHARGING MOSFETS,