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Looking at using the charger for laptop A to charge laptop B results in a lot of different recommendations, so I am not sure what to believe in. Especially I find that many websites give recommendations without explaining the science behind it, making it hard for me to discern the validity.

So, my question: When and why is it safe to use a specific laptop charger for another laptop?

Toke Faurby
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  • This question is too broad. In terms of safety, it is only save to use the charger that belongs to the laptop, because thats the only combination that has to be designed and tested by the manufacturer. – Stefan Wyss Jun 03 '18 at 13:25
  • Possible duplicate of [Choosing power supply, how to get the voltage and current ratings?](https://electronics.stackexchange.com/questions/34745/choosing-power-supply-how-to-get-the-voltage-and-current-ratings) – τεκ Jun 03 '18 at 13:32
  • *When and why is it safe* "Safe" isn't very specific. I'd say if noting explodes, causes harm or fire, it is safe. But using an adapter that damages and breaks your laptop you might not think of as "safe", yet it is safe in my definition. What you're after is "what works". Since you have to ask, you simply should **not** use any adapter other than the one the manufacturer recommends. Ignore that advice and you risk damaging laptop and/or adapter. Although I personally do what I just told you not to do, as an EE I know what I'm doing and it is **my risk**. – Bimpelrekkie Jun 03 '18 at 13:51

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If you use the correct terminology much confusion can be dispelled.

The devices you are referring to are not "chargers". They are laptop power supplies.

The "charger" is part of the laptop and is integrated with the battery management system. This means that, in general, it should be safe to power a laptop from a power supply with the correct DC voltage output and adequate current rating (greater than or equal to the laptop's requirement).

The problem arises in that many manufacturers protect the power supplies by adding a third pole to the connector to signal the power supply's rating to the laptop. There are various schemes such as a fixed voltage or a serial transmission over the extra wire to the laptop.

enter image description here

Figure 1. A random web image showing power connector pinout for Lenovo laptops using the yellow rectangular plug. The resistor, which is in the power supply, will be used by the laptop charging circuitry to determine the power capability of the PSU. If it meets or exceeds the power requirement of the laptop then charging will be enabled. Otherwise the laptop will run on "mains" only. Image source: Lenovo power supply plug configuration on YouTube.

In some cases - I'm familiar with the older Dell laptops - the laptop could be powered by any of their power supplies with the right connector but would only charge when they identified that the power supply was capable.

When and why is it safe to use a specific laptop charger for another laptop?

  • When the voltage is right.
  • When the PSU can supply adequate current.
  • Optionally, when the handshaking is correct.

This varies from device to device so you will have to expand your knowledge for the brand and models in question. You will also need to be content in the knowledge that you are voiding any warranty still left on the laptop or power-supply.

Transistor
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Laptop chargers have standardized pretty much to 19.5V for historical reasons but generally have a wide operating voltage range for operation. So called Universal Chargers with selectable voltages , once common are no longer stocked in some big box stores. The 3.5mm barrel is most common and suits this category.

Proprietary chargers still exist but consumers prefer standardization. So this is the trend I see. Of course there will always be monster in micro exceptions.

So if Dell is now 20V , you can add 25% V tolerance and use any wattage supply bigger than you need, that never gets used up.

Most SMPS designs have a broad sweet spot for efficiency and a 50% input voltage tolerance range. Since MOBO accepts such a wide range to convert to BMS charging and logic voltages 5V ,3V3 , 1V etc it runs with/without a battery off charger OR from the battery without charger equally well.

Tony Stewart EE75
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