Can I charge my laptop battery at 20V and 5A if the specs state 19.5V and 3.33A?

[Silver_Is_Money]

My HP Pavilion's battery charging requires a wall wort that delivers 19.5 VDC and is capable of 3.33A, but what would happen if I supply it with 20VDC and up to 5.4A (from a 13.6V when full to 12.8V when 80% discharged, and 8Ah rated LiFePO4 battery, via a boost converter that outputs at 20VDC).  I know that the laptop was tested to draw 3.33A max, and will therefore likely at most times draw far less than 3.33A, and that the 5A capability on the supply side is therefore not an issue of concern, but my concern is specifically in regard to the 20 VDC. 

[p.larner]

give it a try,just let us know when the fires out!.

[langwadt]

it'll most likely work just fine with 20V, why don't you change the boost converter to ooutput 19.5V?  a single resistor is probably what it takes

[Silver_Is_Money]

give it a try,just let us know when the fires out!.

I just measured the voltage on my nominally 19.5V wall wort that came with the laptop, and it's 19.87V.

[Infraviolet]

I've heard rumours that 19.5V in a lot of equipment is effectively a way to relabel a 20V device, either because there are resistive losses expected in the cabling, or because some countries put extra bureaucracy around the sale of items with >=20V subsystems (considering it an intermediate category the way 48V is often seen as the upper end of "low" voltages and 50V sometimes considered the lowest "high voltage" for which shocks become possible) so by setting a 20V tolerant system up to work with 19.5V+/-some_percentage manufacturers can sidestep this. I'd be very surprised if anything made for 19.5V wasn't tolerant for voltages +/- several volts from this, though wouldn't be confident introducing more voltage to an expensive device than I'd measured it to use unless I'd reverse engineered its power input schematic to be sure nothing in there was absurdly tightly spec'd.

[Veteran68]

If the original laptop charger is rated at 19.5V, then 20V should be fine and well within the design tolerances of the laptop and charging circuit.

The supply's current capacity is irrelevant as long as it's enough for the circuit's demand -- the battery/BMS circuit will only pull the current it's designed for. If it's designed to pull 3A max, then you could use a 10A supply and it will still only pull 3A. Unless there's a short to ground somewhere in the path, but you'll find out pretty quickly if that's the case, as the laptop won't operate and either the supply itself or some part of the circuit will likely burn out.

[zilp]

I guess it should be pointed out that nothing inside the laptop is running directly off the voltage that you plug into it. The computer part runs at 5 V and less, so there is a (switching) regulator in between the power input and all of that, and the battery, of course, needs a controlled charging current, so there needs to be a (switching) regulator in between the power input and the battery, too.

So, this isn't about "is there some component inside the laptop that can't deal with 2.5 % higher voltage?" (which there might be ... but even that isn't really all that likely), it's about "are the input regulators specced with extremely tight margins?" ... and no, they most certainly are not. That would be the dumbest thing ever, because there would be no advantage to doing so, and a significant risk for the manufacturer, not only due to warranty claims from mixed-up power supplies (whether they are legally responsible or not, customers *will* try to have that fixed under warranty), but also because of higher susceptibility to transients, power supply tolerances, and to otherwise benign faults (think: power supply filtering cap fails, regulation overshoots, and instead of having to replace the faulty power supply, you now have to replace the laptop ...).

So ... well, never say never, I guess, you never know for sure how dumb some device has been designed, but really, I would be surprised if a laptop specified for a 19.5 V supply wouldn't work just fine with a 24 V supply.

[Veteran68]

I guess it should be pointed out that nothing inside the laptop is running directly off the voltage that you plug into it. The computer part runs at 5 V and less, so there is a (switching) regulator in between the power input and all of that, and the battery, of course, needs a controlled charging current, so there needs to be a (switching) regulator in between the power input and the battery, too.

While it is true that in modern laptops there are many LV regulated power rails derived from the input main power rail (e.g. 0.8V, 1V2, 3V3, 5V), in some laptops the full input voltage/current is supplied to the battery. Some laptops have lower voltage batteries and step down the input to something like 15V, 12V or 8V for charging, while others do take the full 19-20V into the battery. I can't swear to it but I'm pretty sure not all use a separate buck regulator to "cap" the battery voltage. More likely, I'd expect either the BMS, or the main input MOSFETs before it even gets to the BMS, would monitor the input voltage and shutdown in an unsafe over-voltage scenario.

So ... well, never say never, I guess, you never know for sure how dumb some device has been designed, but really, I would be surprised if a laptop specified for a 19.5 V supply wouldn't work just fine with a 24 V supply.

24V would likely be fine for many laptops, I agree. I wouldn't want to make an overly-broad assumption though, nor even if it functioned would I want to apply that much over-voltage over an extended period of time.

[langwadt]

in some laptops the full input voltage/current is supplied to the battery.

I find that hard to believe

[Veteran68]

in some laptops the full input voltage/current is supplied to the battery.

I find that hard to believe

While I wouldn't call it common, out of the hundreds of laptop repair videos I've probably watched, I've seen it. Not sure how I'd go back and find them now, but will post if I do.

Most laptop batteries are going to be somewhere around 11-15V, but there are exceptions.

[Silver_Is_Money]

I can boost the voltage from my LiFePO4 battery (13.6V when fully charged, and 12.8V at 80% discharged) up to 19.5V instead of 20V, but how can I buck the current of this 8 Ah battery down to 3.33A max while I'm boosting it to 19.5V?  Resistors?

[Doctorandus_P]

Best I know wallwart voltages for laptops are vendor buy-in issue. It started long ago where the input voltage had quite narrow margins to be accepted as "valid", and as far as I know it was only a DRM thing, It would be quite easy to make a laptop charge from anything between 12V and 30V. The laptop already has built in SMPS circuits, to the only change is probably the difference between a buck or a sepic converter.

But it's not completely bollocks either. If the laptop tries to pull more current out of a wall wart then it can deliver, strange things can happen. Some will go into an overload or hickup mode, others will just blow a fuses or release the magic smoke.

Manufacturers have no incentive to make laptops with "universal" inputs. They much rather sell you double marked up spare power supplies instead.

EU started harmonization in phone chargers long ago (15 years?) and lately USB-C with built in power management and delivery is gaining traction.

[Silver_Is_Money]

I just watched a YouTube video of a gent powering his laptop off of a car battery with only the assistance of a boost converter.  No smoke.

https://youtu.be/oxy10TYGh5A

[IanB]

but how can I buck the current of this 8 Ah battery down to 3.33A max while I'm boosting it to 19.5V?

You can't, and you don't need to. You cannot fix both current and voltage at the same time. If you fix the voltage at 19.5 V, the laptop will decide the current. You have no control over it. Just make sure the supply is good for at least 3.33 A and you are golden.

[Psi]

I personally wouldn't think twice about applying 20V to something designed for 19.5V if that thing has an internal switchmode system. I would just do it without thinking.
It's close enough.

[kjr18]

I think there is one more thing to consider. What type of plug does this laptop charger has? Simple 5.5/2.5 mm jack or some other with third contact? While 20V are close enough to work here without problems you might encounter problems with those pesky "smart" power supplies that use third pin for some type of power supply identification, and maybe limit power drawn from power supply or not work at all.

[radiolistener]

My HP Pavilion's battery charging requires a wall wort that delivers 19.5 VDC and is capable of 3.33A, but what would happen if I supply it with 20VDC and up to 5.4A

I think it should works fine. 20V is about 2.6% overvoltage for 19.5V input which should be within tolerance.
In addition there is also some voltage drop on the wire between charger and notebook. So the actual voltage on notebook input will be close to the expected 19.5V.

[Silver_Is_Money]

I think there is one more thing to consider. What type of plug does this laptop charger has? Simple 5.5/2.5 mm jack or some other with third contact? While 20V are close enough to work here without problems you might encounter problems with those pesky "smart" power supplies that use third pin for some type of power supply identification, and maybe limit power drawn from power supply or not work at all.

My HP Pavilion laptop's 4.5mm x 3 mm power jack looks like this.  Given that there are three wires, how should I connect it to my boost converter?

[radiolistener]

Just make sure that you're using reliable power supply which can handle max possible current with no issue.
Also make sure that you're using thick enough cable and good connectors which can handle at least 4 Amps with no significant voltage drop and heating at 65 Watt. 
And don't forgot to double check polarity before connection...

[Silver_Is_Money]

Errata correction: I've been referring to my HP laptop as a Pavilion model throughout this thread due to brain fade, but it's actually a HP ProBook.  That changes nothing much...

[kjr18]

Well, those laptops use plugs and sockets with three contacts: Outer shell is negative, inner shell is positive and pin in middle is typically some kind of sense pin, probably used to detect power supply type or current capacity. I just checked my charger (only 45W) and I have negative on outside, positive 19,87V inside and about 15V on inner pin.

[Silver_Is_Money]

Well, those laptops use plugs and sockets with three contacts: Outer shell is negative, inner shell is positive and pin in middle is typically some kind of sense pin, probably used to detect power supply type or current capacity. I just checked my charger (only 45W) and I have negative on outside, positive 19,87V inside and about 15V on inner pin.

It appears that the blue wire on the 4.5mm-3mm jack is the center pin.  The black wire is negative (outer jack wall/shell), and the white wire is positive (inner jack wall/shell).  Voltage measurements are as follow:

For my original HP wall wort:
---------------------------------------
Across the black wire and the white wire my volt meter reads 19.88V when common on the multi-meter is on the outside shell, and red is touched to the inside.
Across the black wire (outer shell) and blue wire (where blue = center pin) my volt meter reads 19.15V (best I can tell)

For my replacement wall wort (which just arrived yesterday):
--------------------------------------------------------------------------------
Across black and white (jack shell outer and shell inner) my volt meter reads 19.98V
Across black and blue (the center pin) my volt meter reads 19.30V (best I can tell)

Both wall worts seem to charge the battery of the old HP ProBook just fine.  My intent is to use the new wall wort for my typical home computing use, and to connect the jack and three wire set from my old/original wall wort to my boost converter and power it via a small 12.8V to 13.6V 8Ah LiFePO4 battery for field use.  But the lower voltage on the center pin has presented me with a setback that wasn't anticipated.  A quandary in need of guidance.

[MrAl]

Well, those laptops use plugs and sockets with three contacts: Outer shell is negative, inner shell is positive and pin in middle is typically some kind of sense pin, probably used to detect power supply type or current capacity. I just checked my charger (only 45W) and I have negative on outside, positive 19,87V inside and about 15V on inner pin.

It appears that the blue wire on the 4.5mm-3mm jack is the center pin.  The black wire is negative (outer jack wall/shell), and the white wire is positive (inner jack wall/shell).  Voltage measurements are as follow:

For my original HP wall wort:
---------------------------------------
Across the black wire and the white wire my volt meter reads 19.88V when common on the multi-meter is on the outside shell, and red is touched to the inside.
Across the black wire (outer shell) and blue wire (where blue = center pin) my volt meter reads 19.15V (best I can tell)

For my replacement wall wort (which just arrived yesterday):
--------------------------------------------------------------------------------
Across black and white (jack shell outer and shell inner) my volt meter reads 19.98V
Across black and blue (the center pin) my volt meter reads 19.30V (best I can tell)

Both wall worts seem to charge the battery of the old HP ProBook just fine.  My intent is to use the new wall wort for my typical home computing use, and to connect the jack and three wire set from my old/original wall wort to my boost converter and power it via a small 12.8V to 13.6V 8Ah LiFePO4 battery for field use.  But the lower voltage on the center pin has presented me with a setback that wasn't anticipated.  A quandary in need of guidance.

How are you connecting the blue wire with your new wall wart?
Is it left open?

[Silver_Is_Money]

How are you connecting the blue wire with your new wall wart?
Is it left open?

The blue wire feeds the center pin with 19.30V on my new wall wort, and on my old (now disassembled) wall wort the same is 19.15V.  Will two 10Kohm resistors in series drop ~19.9V to ~19.3V?  If so I can come off of the positive output of the boost converter with two leads, one at ~19.9V for the jacks white wire, and one (with the two resistors in series) going to the jacks center pin via the blue wire at ~19.2V to 19.3V.  The center pin seems to (as has already been stated) be an HP planted tricky sort of safety to verify that one of their brand of wall worts is being used.

[soldar]

Dell laptops use a similar system where the central pin uses SMbus
https://en.wikipedia.org/wiki/System_Management_Bus
to communicate with the battery pack.