Warning: Attempt this at your own peril.
While busy with class this past term, I accidentally let the battery in my Nikon Z9 run down. Usually, batteries are only mostly dead, and will charge back up. This one was beyond mostly dead. Nikon’s charger failed to recognize it at all, and it will blink furiously to let you know that you should immediately go to their website and purchase a new battery for $249. I had already done this a few months prior, and pulled out a spare (still in the box) only to find it, too, was entirely DOA – refusing to charge.
Most, if not all, Li-Ion chargers look for a small charge coming from the battery before it will recognize them. This helps prevent fires and electrocution from people doing dumb things, like putting the wrong battery on the charger, or sticking their tongue on the contacts Christmas Story style. The Nikon MH-33, however, seems to be designed for one purpose: to sell more batteries. My wife’s Canon, on the other hand, happily charges batteries that have been sitting dead in her drawer for years without any issues. Nikon seems to deliberately be built with poor tolerances for the range of voltages that it will recognize on, with a minimum somewhere around 7.5v. If your battery is anything resembling “pretty dead”, you’re stuck buying new overpriced batteries. Or are you? If you can get the charger to recognize the battery, it can often be revived.