Charge HP 440 G3 Probook via a 12V car battery

[Silver_Is_Money]

I need to be able to operate my Probook laptop in the field (literally), and I have a small LiFePO4 12V 8Ah battery that I will be boosting to 19.5V via a boost converter.  The 4.5mm x 3mm plug-in jack on my Model 440 G3 Probook has a white wire (+), a black wire (-), and also a blue wire that connects to the center pin.  I've measured the DC voltage across the white and black at 19.87V for my AC to DC wall wort. And I've measured the voltage across the blue and black wires at 19.15V.  It appears that the blue center pin wire initiates battery charging on my laptop.  Can I simply link the white and blue wires and send 19.5V through both the white and blue wires whereby to initiate battery charging when using the LiFePO4 battery in the field?  Or alternately can I incorporate a simple voltage divider circuit via utilizing a ~390 Ohm  Resistor and an ~10 KOhm resistor and feeding the blue wire with ~19.2V via this means whereby to initiate the charging of the laptops 14.8V internal battery?  Please send the reply to my email.

[Jupitero]

Hello.

I would approach this by getting a HP compatible laptop charger that you can plug into the 12V car auxiliary power outlet ("cigarette lighter") typically found in most cars. Connect the 12V battery to a car aux power receptacle part. This way you can also charge the laptop in different vehicles and I think it is also a simpler solution.

[Silver_Is_Money]

Thank you kindly!  That will work for the car and laptop, and I'll search to see if such a device exists.  But I also need field use where my car will not be near.  That's why I mentioned the LiFePO4 battery within the body of my post.

[Jupitero]

Hello.

What I meant was to buy a car cigarette light charger female socket part and wiring that to the battery. That way you can easily switch between the car cigarette outlet and your battery. See image:

[Silver_Is_Money]

I just measured my wall wort with no load and then when plugged into my Powerbook 440 G3 when it's battery was fully charged and this is what I discovered:

1) Wall Wort with no load:
-------------------------------------------------------------------
Voltage on white wire (inside wall) of jack = 19.87V
Voltage on blue center 'Smart' pin wire of jack = 19.15V

2) Wall Wort when plugged into the Powerbook, and when it's battery is fully charged:
--------------------------------------------------------------------
Voltage on white wire (inside wall) of jack = 19.87V
Voltage on blue center 'Smart' pin wire of jack = 2.77V

[Silver_Is_Money]

I just took the next logical step. My logic was that if when the battery is full, the "Smart Pin" (blue wire/center pin) on the HP approved power adapter (wall wort) is low in voltage at 2.77V (such as it is), then perhaps when the battery is nearly fully drained and the charger is plugged in again, this so-called "Smart Pin" will recognize the need and rise in voltage to somewhere closer to the 19.15V that it reads when not under any load (IE, not plugged into the computer and charging the battery"). So I aggressively drove the battery on my Probook down until I received a low battery warning. Then I plugged the power adapter into the Probook and initiated a recharge from the depths of extreme battery depletion. When I put the multimeter on the white wire to the jack (from the power adapter/wall wort) it read 19.87V (as it always does). Then when I moved the probe to the blue wire I was stunned to see that it read only 2.87V while in the depths of rapid recharging a nearly depleted battery (vs. 2.77V when the battery was completely charged earlier today). Thus the "Smart Pin" no longer seems smart to me at all. But rather the smarts that determine battery charge initiation and termination are internal to the computer. And now I know that my voltage divider feeding the blue wire (center "Smart"pin) must reduce 19.87V to around 2.77V to 2.87V. Bingo! A two resistor arrangement is the answer. The small voltage on the center pin only appears to tell the computer that the voltage source (wall wort) is acceptable. So now I can move to powering and recharging my computer via a voltage boosted LiFePO4 battery instead of the wall wort, and ready it for field use.

[Silver_Is_Money]

I built a prototype of the the two resistor voltage divider circuit using 30 KOhm and 5 KOhm resistors. When fed via my LiFePO4 battery along with my boost converter ramping it up to 19.87V, the divider voltage (voltage between the resistors) to the blue wire (center "Smart" pin) read 2.85V. Very close to predicted...

The HP Probook440 G3's battery was at 78%, so I shut it down, plugged it into my battery circuit, and then turned the computer back on. It booted right up and went directly into battery charging mode. I watch it slowly charge the battery from 78% to 100%, and then the "battery is fully charged" message appeared. Just as if the computer was on the standard HP 120VAC to 19.5VDC (actually 19.87VDC) power adapter. When I disconnect the barrel jack a message now pops up and says "battery discharging from 100%", and when I plug it back in this message vanishes.

BTW, I'm running Linux Mint 21.3.

So far this project seems to be a whopping success!