If the 12v battery runs down, then computer won't work and your car won't start without some sort of boost to boot up computer. Since computer needs about 10 amps, even AA battery pack will be enough as long as it has 12 volts. In normal use computer will not allow traction battery to go dead. If the car sits unused for very long time (my guess would be at least 3 months) then you would need to call tow truck and get Toyota recharge traction battery. But I would imagine if the traction battery was fully charged and in good shape then even after full year it should have enough charge to start the ICE. If the traction battery don't hold the charge then something is wrong.