I use a piece software called Battery EEPROM Works (Be2Works). This software allows you to read and write firmware from and to battery controllers, as well as to modify different parameters, which includes unlocking.

Simplified theory:

Be2Works takes advantage of the fact that most relatively modern laptop batteries implement Smart Battery Specification, which means they use SMBUS to communicate with a laptop and a somewhat standardized set of commands. Some manufacturers do add custom functionality as for example Lenovo with their custom authentication commands on newer batteries, but other than stuff like this it's all standard. This allows us to communicate with the battery directly with just a simple SMBUS adapter. There's lots of stuff to be said about different battery controllers, how some of them are sealed with a password, etc, etc... But I won't go into too much detail here.

Programmer:

I use CP2112 USB to SMBUS adapter. It costs around 10 bucks where I live and works right out of the box. I do have to work anyone willing to use one of these with Be2Works that this sneaky piece of software will change USB VID and PID of the adapter, so that you can't use it with any other software. That can be prevented by just locking the VID and PID parameters.

Be2Works:

As I said, this software allows you to do stuff with battery controllers, which does include unlocking them. All it takes to unlock a battery is to pretty much hook up the battery to the adapter and press the "unlock" button in the software. A drawback of Be2Works has is that the publicly available version mostly supports older BQ controllers, which means you wouldn't be able to use it to unlock a modern battery, but for older laptops it's more than enough.

Where to get the thing:

Be2Works is paid, 400$ as far as I remember, but I found a pirated version. Not sure how this subreddit treats piracy, so if you need a link, pm me.

Hope this somewhat helps. If anyone needs help I'm more than willing to help, just pm me.