Nikon d850 battery dead

The newer batteries are designed to permanently brick if you remove the cells through a nonresettable BMS flag, or they dump the data when you remove power with volatile storage; it depends, but it's generally done with a tamper flag to shut the whole thing down. There's not much we can do to rebuild them because of that, quite frankly.

You will need to replace the battery; look for the EN-EL15c or the older equivalents; Nikon took notes from Canon and upgraded existing packs while keeping support for the legacy DSLRs for the mirrorless cameras so the EL15c is likely an upgraded pack. If you can find genuine NOS EN-EL15 batteries, then you'll be fine since the D850 will be fine on the "original" version of the packs but the newer cameras either take them or reject them as they aren't always equipped with the authentication ICs on the 15c or it's a different signature then the Z mirrorless bodies expect; even if original. If it's genuine and rejects these cameras, the chip isn't there or the signature is "wrong" being older.

The main issue with NOS is making sure it's genuine (I have run into issues with "NOS" counterfeit LP-E6 batteries) but even with the risk I buy them from reputable markets like Amazon where I can reject them and easily send the packs back. The reason NOS LP-E6's are cheap is the reduced burst rate on the RF bodies; they don't want them unless they are cheap or it's all they can get; the E6NH is the "correct" battery for the RF bodies, for instance. The other reason they are cheap is they're mostly going to be "gray market" NOS; my NOS LP-E6 came from Japan which was obvious by the documentation.

WARNING FOR GRAY MARKET NOS: Nikon WILL NOT REPAIR, REPLACE OR SUPPORT ANYTHING GRAY MARKET IN A OUTSIDE MARKET so you'll just have to buy a new battery with Nikon if you pick up a genuine NOS gray market EN-EL15/15b.
Example of the "gray market": I buy a Canon R6 with 500 clicks from Japan for a lot less than I'd buy it for in the US, with the understanding it has an English/Japanese language lock which can be neutralized if the imported camera works with Tornado EOS. With SOME cameras, the batteries cannot come to the destination country due to shipping restrictions as well (if I had this happen I would expect compensation before shipping so I can get one while I wait); most come over under UN3841.
The other issue is chargers: Most chargers from countries like Japan use a cord so the manufacturers can adapt it more easily on the in-box one, but fix the plug on US in-box chargers. This can be fixed with a good power cord (which I have enough that have procreated over time this is a literal non-issue). As long as the charger just needs to be 100-240V, I'm good.

The major downside on why I don't think buying gray market is for everyone is repair issues: IF the camera has an issue, it isn't going to be "covered" by Canon USA at their expense; repairs come at cost, potentially even Canon US repair programs but those may still get a pass. The same downsides apply to accessories and lenses. As long as the discount is solid ($500+ on a prosumer/pro body) I have no qualms about buying from countries like Japan in the US and shipping my lens or camera here. The risks mean I can't say in a good concise gray market should be encouraged; if you accept the risks, buy your gear this way as long as it's significantly cheaper. Buying from the gray market is a "you do you" thing.