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Comment by brynet | Hacker News Reader

> This is interesting, am I correct in thinking this a feature implemented by Apple

Yes, battery charge control is a hardware(/firmware) feature supported on other modern laptops as well, such as the Lenovo ThinkPads, but it's not a standard so it requires explicit driver and OS support.

OpenBSD recently added support for this as well for both of these implementations (Apple silicon and ThinkPads).

https://marc.info/?l=openbsd-cvs&m=168436150408382&w=2

https://marc.info/?l=openbsd-cvs&m=168458409622780&w=2

https://marc.info/?l=openbsd-cvs&m=168521616605492&w=2

I know certain Android/Samsung phones support this as well, not sure about iOS/macOS.

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I am on macos 13.3.1 and have noticed this feature for at least a couple of months, maybe longer. It says Charging On Hold (Rarely Used On Battery)

I can never get that to activate, even though I use my MBP 14" 100% at home...

It’s in settings under battery

In my experience for it to take effect you need either a predictable window of plugged in time (it will delay charging until it needs to to hit 100% by when you normally unplug) or just to use your compute almost exclusively plugged in (at which point it will start letting the battery drift down to 80%). If you regularly unplug to work on the couch or whatever you may not ever see the effect directly unless you wake up an hour early to check the charge state of your laptop.

Optimized Battery charging has been available since Mac OS 11. Works on intel MacBooks too.

https://www.macworld.com/article/235001/macos-big-surs-batte...

I really want to try this out on my work ThinkPad since it stays plugged in effectively all the time. It requires a kernel module though, I'd need to get that approved first and I haven't got around to it.

Naïve me doesn't understand why it can't be done from userspace, but I'm sure there is a good reason...

If your laptop firmware doesn't support it, here is one trick if you have a removable battery:

Put a piece of paper over one of the batteries middle contacts. That will make the firmware think the battery is overheating. It will then refuse to charge, but will still happily discharge.

You can do that to keep your battery at 80% while still on AC power. Handy if you operate from AC power 99% of the time, yet don't want your battery to die from being stored at 100% charge and hot for many years.

> Put a piece of paper over one of the batteries middle contacts.

Bad, very bad idea if you don't know what you are doing - depending on where the "smarts" in the BMS are, you may damage your battery or make your BMS think the pack is broken or prevent your BMS from recognizing a charge state mismatch (and in the worst case, a cell going undervoltage or reverse polarity) as you have a good chance that you cut off one of the cell balancer contacts. This trick only works with removable phone batteries.

Would an engineer really make a consumer design where a sliding contact failing open led to a safety incident?

Every laptop since the 90's has the balancing and protection circuitry inside the battery, not inside the laptop.

Indeed, at worst you'll get a "no battery installed" and the laptop might not even try to discharge it.

The comment you're replying to is just paranoic FUD. Which removable laptop battery has external balancing contacts!?!? They're all +/-/data.

I'm a corporate firefighter assistant. If there's one thing people should not be messing around with, it's Li-Ion batteries. These things are dangerous enough even without people messing around with their control and management systems.

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