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I want to charge my laptop with DIY power-bank which has variable voltage output. Laptop peak consumption is 65 W and the power-bank has plenty of power to deliver it as it was previously used as ebike battery able to deliver 1000 W.

My laptop's original wall-charger has 19.5 V output so I set the same voltage on powerbank, but when I connect the powerbank to laptop it doesn't show charging indicator and percentages on laptop battery doesn't increase, but they doesn't drop either.

It seems that the power-bank keeps the laptop battery charged to the point when it was connected to laptop, but it doesn't activate the charging mode, even though it outputs the same voltage as the original laptop charger.

What can be a problem and how to fix it?

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  • \$\begingroup\$ What brand and model of laptop? Does the connector have a third pole for communication between the power supply and the laptop so that the laptop knows how much current it can safely draw from the PSU? Most do. \$\endgroup\$
    – Transistor
    Apr 22, 2023 at 18:02
  • \$\begingroup\$ Oh, and the charger is just a constant-voltage power supply. The charger (charge management) is inside the laptop. (Phones have a similar setup.) \$\endgroup\$
    – Transistor
    Apr 22, 2023 at 18:04
  • \$\begingroup\$ Are they both USB-C? \$\endgroup\$
    – Ste Kulov
    Apr 22, 2023 at 18:11
  • \$\begingroup\$ It is a dell Vostro 3500 series laptop and it has a standard circular laptop charging input 4.5x3mm, I think it has only plus and minus pole as it is only one pin in the middle of the jack which is positive and on the outside is negative . @Transistor \$\endgroup\$ Apr 22, 2023 at 18:12
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    \$\begingroup\$ The older Dells and other brands had a larger plug and there was a third contact inside the sleeve. Modern PSUs could be superimposing data on the DC lines too. \$\endgroup\$
    – Transistor
    Apr 22, 2023 at 18:55

3 Answers 3

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  1. There IS a third contact, but you're unaware of it:

enter image description here

(The image is taken from https://www.flipkart.com/dell-vostro-3500-65w-original-65-w-adapter/p/itmeeh2xshm4x4yb and modified)

The outside ring is the negative supply contact, the inner ring is positive supply contact, and the middle pin is the communication contact.
So, a power supply needs to communicate to the laptop how much power is available so that the laptop can draw as much as it needs. Otherwise, only the laptop's DC-DC converters used for running laptop circuits besides battery charging will operate until the input voltage drops too low.
You could fix the situation by getting the communication IC from another charger or elsewhere. It is typically a 3-pin device (TO-92 case, like a transistor) which is programmed with adapter info.
There should be more about it on the Internet.

  1. For the longest Li-Ion battery life, keeping its charge between 30% and 80% will give you about 2.5 times more cycles, approximately, and I'm not sure how many times longer lasting battery.
    What I would suggest is that you charge the battery to no more than 80% with the original adapter, and then use your battery bank or another adapter to supply the laptop so that its battery doesn't continue charging over 80%.
    This is actually a useful situation for prolonging a laptop's battery life, and it just gave me an idea, since my own laptop is always plugged-in and charging, and I was upset that there was no ready solution to limit the battery charging levels in the laptop.
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  • \$\begingroup\$ thanks for so detailed explanation. Hvala. \$\endgroup\$ Apr 22, 2023 at 19:31
  • \$\begingroup\$ Edin Fifić - Hi, Please remember the site rule which requires that when a post includes content (e.g. text, image, photo etc.) copied or adapted from elsewhere, that original content must be correctly referenced. The source webpage or PDF etc. should be linked as a minimum (references for books / articles should include title, author(s), publisher, edition, page numbers etc.). Please can you edit your answer to include the required source link for the original image? Thanks. \$\endgroup\$
    – SamGibson
    Apr 22, 2023 at 19:36
  • \$\begingroup\$ @SamGibson Sure, but the image is likely from a temporary page which may be gone because it's an online marketplace. \$\endgroup\$ Apr 22, 2023 at 19:39
  • \$\begingroup\$ @EdinFifić - Thanks, I understand that it's not your responsibility if the source page disappears in future. \$\endgroup\$
    – SamGibson
    Apr 22, 2023 at 19:44
  • \$\begingroup\$ @SamGibson Understood, thank you. \$\endgroup\$ Apr 22, 2023 at 19:50
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You can't simply apply voltage to laptop and expect it to charge.

Most laptops have some communication mechanism to determine how much power can be drawn from the supply.

Without this communication it will refuse to draw any current from source.

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    \$\begingroup\$ I think most will power the laptop but the charge management will refuse to charge the battery. I may be out of touch though! \$\endgroup\$
    – Transistor
    Apr 22, 2023 at 18:07
  • \$\begingroup\$ I think you are right because it shows some tiny drop on diy powerbank voltage after a hour of watching youtube, and the laptops battery stays at the same percentage. @Transistor \$\endgroup\$ Apr 22, 2023 at 18:17
  • \$\begingroup\$ @Justme how can I make sure that that communication mechanism is present in charging jack. I figured out it has only two poles by looking at the jack, so there is no way to communicate the other states, but I guess I am wrong. \$\endgroup\$ Apr 22, 2023 at 18:21
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    \$\begingroup\$ @Markonioninioni If charging works, the laptop should show "connected to mains power", because if there's no communication (which is feasible when it's using one of the round 2-pin jacks) it cannot distinguish between your external battery and an actual PSU. \$\endgroup\$
    – PMF
    Apr 22, 2023 at 18:39
  • \$\begingroup\$ @PMF so I guess that's why it draws power from the diy powerbank and keeps laptop battery at the same percentage, but it doesn't charge it. Are there any drawbacks using it this way? \$\endgroup\$ Apr 22, 2023 at 18:43
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So the downside to using your laptop the way it current is just using the load to power the device and is isolated from the battery is that if you have your battery at 20% and want to use your laptop that day without any AC charging around you will HAVE TO bring out the power bank otherwise you'll have very little time left. Hopefully your laptop should start with a completely empty battery but can't guarantee that.

I doubt this is damaging the charging circuit inside the laptop but checking it for heat buildup will give you a clue. Probably some kind of mosfet switch or similar saying "No communication line, no power comes here because I know youre not using a real dell power supply"

A lot of people prefer USB C PD 3.0. They put a small 20V trigger module into their laptop in place of the barrel jack (or next to it) and somehow attach that to the points where the barrel jack power starts on the PCB that has the charging circuit. There might be an easy way to trick that third communication line with just a resistor or shorting it to something, do a bit of Googling and see if its possible.

USB C PD gives you lots of power bank options. I love to build power banks but only half the kits I'm currently looking at offer a DC barrel input/output and they are usually more expensive but if you already have one then maybe PD isn't the way for now. In the future your next laptop will surely be USB C PD so keep that in mind. There are some great boards and some TERRIBLE boards that output enough power for a laptop

IP2368-one of the best. 4 3.7V cells in series can be boosted to give you 100W PD at 20V 5A. Less than 4S I think only gives you 60W. You can go up to 6S but there is a component to replace near the input that can't handle more than 18V. So 2S to 6S, 3.7V cells or 3.2V cells. Lots of flexibility using the resistors to change stuff. Get the IP2368 mini model that comes in a metal box and has an XT30 PCB mounted. Far superior to the regular board and only $21 shipped to the USA

IP5389-I'm not sure if this one I trust. The board I tried with this IC only put out 60W until the board died but I used a TZT board that was kinda flimsy and I think the 5389 might have potential.

120W Bidirectional Full Protocol Fast Charging blah blah blah-sold by Feb 22 Store and now many others for $17 to $20. 20V 6A max output another 4S configuration board. Only takes 3.7V lithium ion, only 4S.

SW2303-Garbage. The blue board versions can't do 100W ever. I don't touch boards with the SW2303 IC anymore.

SW3516- 60W PD and many other protocols. Buck only so you must provide over 20V if you want 20V 3A for your max PD power.

Then Feb 22 Store has a bunch of DIY kits with various IC's. READ the listing first to see what IC's are being used. Their classic 100W powerbank with the clear acrylic chell to see the cells uses the IP5389 as its main discharge board. You can pay money to have more added up to 2. I've never seen the IP5389 put out 20V 5A but I did have one of my subscribers on YouTube get close. Another guy I met on YouTube couldn't get his to charge his Mac.

My Youtube is Eric Klein and I have a video going over all the boards I like and what not

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