Mileage Drop

I have a 2015 with 86k. I was getting in the high 30s when hot and low when cold. I changed my oil and within a day my mileage dropped to 16. I was and have been getting the service high voltage error code which I have been clearing out. The fluids are fine. I also replaced the original 12v battery after the mileage dropped. Any advice would be appreciated and unfortunately I disposed of the old oil so I can’t pour it back lol !