My car has to sit for a year, will a dead battery have any harmful effects on any of the electronics? 2007 Mustang GT 4.6L.
After a modern car sits for about thirty days the battery will discharge. This is normal due to the parasitic draw caused by the control modules and computers inside the vehicle.
A deeply discharged battery can be damaged to the point where it will no longer be able to hold a full charge and will go dead especially in colder weather due to the lack of driving or storage.
The best way to prevent the battery from discharging is to get yourself a battery maintenance charger or battery tender. This device can be bought in auto parts stores or you can have a wide variety of choices from Amazon.com or Northerntools.com
A battery tender will maintain a battery voltage and will stop charging when a charge is no longer needed. When the battery is at a point where the tender senses a charge is needed it will supply a low state of charge with low amperage to maintain the full charge with now damage to the battery then it will turn off at the set point programmed in the tender to avoid overcharging and or damage to the battery.
Do not get a trickle charger; this will supply a constant voltage and can dry out the battery damaging it and could possible cause the battery to explode from the gasses that are built up inside the battery.