Charge Threshold on a Laptop

Question

I have a feature called charge threshold in my laptop (It's pretty common for laptops to have them these days), it's like you set a limit to how much your battery gets charged once that limit is reached the adaptor turns cuts the supply and only resumes to charge again when the battery is reaches below some set limit. What I am interested in how does it actually works because when I switch off the charger and switch it back on what it does is it holds the charge of the battery if it hasn't reached the upper limit, ie. doesn't let the battery charge anymore or deplete.

So is it that the laptop is actually running on the charger? (As an charging adaptor can supply different power as per the demand) I looked for the specifications of my adaptor and it says it does operate at 65W (20V-3.25A), 45W (15V-3A), 18W (9V-2A), 10W (5V-2A). So can it run on the adaptor completely? I've uploaded an image in which it clearly shows that the battery is indeed inactive.

Or is it just trickle charging the battery? It might be the case that the software isn't able to refresh as quickly as the battery switches between charging and discharging states. Also if the device was powered solely by charger then how is the extra current managed when I put it in sleep?

But then I happen to have a older laptop (8yrs old) which now runs solely on the charging adaptor as it's battery has lost the capacity to hold any charge, which I can easily put in sleep while on charge, the point I'm trying to make is that this is possible if the manufacturers intended. But then would love to know how.

Answer 1

One of the most enduring myths about electricity is that it works like water flowing through a pipe*. The power adapter isn't like a water pump that needs to be regulated depending on how much water is being consumed. Rather, it just provides a steady voltage up to a certain maximum current, and the internal laptop charge circuitry decides how much current to draw from the adapter and send to the battery.

This is why it's important to get an adapter that's rated at or above the maximum current draw of the laptop. If you get one that's undersized, it will overheat from the laptop's draw, and could burn out or even start a fire.

So, yes, just like a desktop computer, a laptop can run just fine from its power adapter. In fact, laptops that have an externally-accessible battery will usually run just fine even if the battery is completely removed**. The charging circuitry will notice that the battery isn't there, and respond by not sending any current to the battery connectors.

*Yes, the water analogy can be useful in certain circumstances, which is how the myth got started, but in some ways electricity works very differently.

**Laptops with internal batteries tend to not let you do that. Depending on your view of computer manufacturers, you may view this as either a safety mechanism (so the laptop won't run if something's not hooked up right) or a money-making mechanism (forcing you to buy one of their batteries if you want to use your computer), but the point is that it's an artificial restriction, imposed by design, rather than something that happens "naturally".