For those who don't know , "repair mode" on a "smart charger" is not some high tech thing. It is just like having a dumb charger hooked on in the first place.
From Hagarty https://www.hagerty.com/media/maintenance-and-tech/wrenchin-wednesday-outsmarting-the-smart-battery-charger/
While you’ll often find that the battery is as dead as Zed and will need to be replaced, there’s an off-chance that the sulfate layer on the battery’s lead plates can be removed with a long recharge. If your “smart” charger doesn’t already have a desulfate mode, which bypasses some of these safety measures and charges the battery in waves to attempt to break-off and dissolve the lead-sulfur crust that have built up on the battery’s plates, you can use a helper battery to trick the charger into activating and providing its full juice — which will hopefully start the desulfation process inside the battery.
In other words, a dumb charger is doing the exact same thing that a "smart" charger does when it in "repair" or "desulfate" mode. And is why dumb chargers can revive dead batteries