Not sure about how the negative reading is happening - sounds like a user issue w/o clarification. If the battery is older, replace it - voltage doesn’t always mean current is available. Many modern car electrical systems will give weird readings when the battery gets weak. If the car can be started/jump started, and seems to run OK, then I would assume the battery is low on vitamin Ampere.
Brand doesn’t matter, they all pretty much operate the same way. I have a 2004 Expedition that had this issue. Turns out that the rear windshield wiper unit was not returning to the “park” mode because the unit was broken. By not being in park mode, it was running power to the unit to try to get it to park. This killed the battery overnight. Had to replace the unit. Once that was done, no more problems. Son is driving it, truck has 180,000 on it, drives like new.
Did you discover what the problem was?
Shorted fuel pump relay running pump with ignition off? Bad ignition switch staying “hot” in the off position?
Diodes in alternator.
Hope this gets to everyone, the heater/ac blower was hard-wired into the battery. I really feel dumb. I still hate fords, but at least I did not have to roll around in the mud.
Thank you to everyone for the responses. God Bless.
Keith